Monday, October 4, 2010

Goldilocks Planet


By now you have probably heard media reports of a 'Goldilocks' planet orbiting Gliese 581, the seventh sixth planet found in the retinue of this dim red star.

Mass media hype aside, this discovery is both important and unsurprising. Important because this is the first known planet apart from Earth itself that orbits entirely inside its parent star's habitable zone, and so could potentially harbor life without broiling or freezing it. Unsurprising because nearly 500 extrasolar planets have now been discovered, and sooner or later one was going to turn up in the right orbit.

All we know about Gliese 581g are its orbit, at about 0.15 AU from its parent star, and its approximate mass, about 3-4 times Earth's. We do not even know for sure that it is 'a planet' rather than, say, two planetary-mass bodies orbiting each other. We know nothing about its composition, such as whether there is water vapor in its atmosphere, let alone liquid water on its surface. We know only that liquid water could exist at that distance from the star, unless the planet has an intense greenhouse atmosphere or some other complication.

But let the speculation begin, as naturally it already has. If it is a single body it should be tide-locked to Gliese 581. This used to be a deal breaker, but current thinking is that atmospheric heat transfer is ample to keep the air from freezing out on the nightside.

If the planet has extensive uplands and limited water, the water might (my own speculation) form vast ice sheets on the nightside instead of pooling as oceans. On the other hand, the general feeling seems to be that big planets will have more water, while the heavier gravity should make a rocky surface flatter - how much so is above my pay grade to estimate. But this may well be a waterworld, or even a 'water giant' with a hydrosphere thousands of kilometers deep instead of Earth's thin muddy film of liquid.

As Arthur Clarke said of Jupiter, when it was thought to possibly have a deep hydrosphere, think of the fishing.


I would not rush out to put a colony on Gliese 581g in my setting. It is probably not a world for us. (If any worlds are 'for us' beyond the one we evolved on and any we may one day terraform.) But we are free to imagine a golden-red glint of sun across a very distant sea.


Image source.

107 comments:

Milo said...

This paper gives a nice discussion of the issues faced by such a world - be sure the read the last paragraph ;)

As far as the search for habitable planets goes, I am much more interested in finding alien life forms than in finding someplace to plop down a colony. As such, the tantalizing promise yet vague weirdness of a twilight planet actually appeals to me - I'm looking for aliens, after all, i.e. creatures that evolved in an environment different from us.

If Gliese 581g doesn't have life, then there are other opportunities for life in the system - Gliese 581c and Gliese 581d have both been suggested as potentially habitable, though these are somewhat unlikely (currently believed to be too Venusian/Martian). All three of these planets are also pretty heavy, meaning they have the potential to support a moon of significant size (not that our equipment would be capable of detecting it currently), which would be tidally locked to the planet rather than the star and so would have the advantage of a "normal" (from our perspective - given the abundance of red dwarfs, it's possible that tidally locked planets are actually the most common lifebearing worlds in the universe) day/night cycle.

Jim Baerg said...

Once again Poul Anderson deserves a kudo or two for his world building, in this case the planet Ikrananka in the novella _The Trouble Twisters_ (published in the early 1960s). In the story Ikrananka orbits a K0 dwarf at about 1/3 AU. It's tide locked to its star & is quite habitable, though much of its water is frozen out on the dark side.

Milo said...

Incidentally, saying "nearly 500 extrasolar planets have now been discovered, and sooner or later one was going to turn up in the right orbit" is not really fair, since most planets we've found are Hot Jupiters. This is (probably) not because those are more common, but because those are easier to detect with equipment not sensitive enough to find anything else. So locating anything this small is still something to write home about - when you consider how few small planets we know of (note Gliese 581g is only third on that list! - and also note that one of the two smaller planets orbits a pulsar, and the other also orbits Gliese 581, being more or less that star's Mercury), then a potentially habitable one is actually pretty remarkable.

Finding this many planets in one star system is also the exception rather than the rule. Most stars have maybe one wacky Hot Jupiter, and you'll be impressed if you manage to find a second one. Gliese 581 has an entire solar system of much more familiar-looking planets (seven found so far!). In terms of mirroring our solar system, Gliese 581 remains - for now - nearly unique, and definitely the most spectacular.

Tony said...

Re: low proportion of earthlike planets among those so far discovered:

The Kepler mission, if the proportions among candidates in the preliminary results hold up, will change that, at least in terms of planetary mass. (And likely, if our theories of planet formation are correct, in terms of orbital position as well.)

Re: Uniqueness of the Gliese 581 system in terms of number of planets:

Almost certainly an artifact of the small mass of the primary, relative to a lot of stars we've looked at. Smaller planets have a detectable effect on a red dwarf where they wouldn't on a larger star.

Anonymous said...

GL 581g sounds like one of the most exotic locals Human's have discovered to date. This world would be amazing to visit; I'm not sure about starting a colony there, due to the high gravity, but it would be truely astounding to explore! Even if it took 80- 100 years to reach it, it would be worth it, just for the alien vistas.

Ferrell

Kaleb said...

Interesting. I wonder what type of fish could be caught there, and what it would take to get me and my fishing gear there.

Sean said...

I was using Gliese 581d for a setting in a short story I've been working on - damn astronomers making matters more complicated than need be.

Milo said...

Kaleb: Amusingly, some kind of "fish" is actually pretty likely. On Earth, most fish tend to broadly stick to their ancestral body plan, and a very closely fish-like shape has evolved independantly multiple times (in actual fish, ichtyosaurs, and dolphins), complete with similar streamlining, tail, dorsal fin, etc. when these had nothing to derive from, suggesting that this shape is strongly favored for movement in an aquatic environment. As such, I expect the shape has a pretty good chances of convergently evolving on any world that has life - even if said life is based on ammonia or methane rather than water, since they all still form seas (which is the point).

The caveat here is that as far as I know the fish-like shape is only found in vertebrates, so it may not actually be that strongly favored unless you have at least that much in common with fish to begin with. It's anyone's guess whether the features that define our vertebrates will be found much elsewhere, given that vertebrates only evolved once here - but were pretty successful when they did, growling larger than anything else and domination ecosystems.

In any case these "fish" would not necessarily have other things in common with us - they might be covered in something other than scales, for example (dolphins do fine without those), their respiration organs will not necessarily be on the neck or look like gills we know, they may have entirely different reproductive systems, etc.

Milo said...

"Growling larger"? How did that typo happen? And that should be "dominating", not "domination". :(

Thucydides said...

"Hot Jupiter" solar systems probably don't have other small planets; I'd guess the gravitational effects of the massive hot Jupiter would have disrupted planetary formation as it spiraled in. A space faring civilization will probably find lots of asteroids and a huge deep space halo of debris surrounding the star.

Small planets orbiting red dwarfs are an interesting idea; many SF writers have portrayed them as having a thin habitable zone surrounding the twilight zone, with a hot pole and a frozen cold pole. Even heat flow from the atmosphere and possibly oceans would not change the fundamentals; the hot pole will be a desert and the cold pole will still be an arctic waste, but the habitable zone can be widely expanded.

Far future humans considering settling such planets can add platoons of solettas to light up the planet, or even applying gravitational torque through asteroid flypasts or other more advanced means to get the planet rotating. This sort of geoengineering assumes either the planet is not already inhabited, or that future humans have different value systems than we do today.

Dex said...

It is important to remember that Gliese 581g may actually be Gliese 581g #1, #2, etc. This leads to interesting possibilities.

Once you separate the mass into at least two large bodies, the mechanics of the system become much more complicated and interesting. Adjust the radius of the world(s) up or down based on what we do know and it is possible to arrive at paired worlds that both have gravity between Mars and a slightly heavier Earth.

Milo said...

Incidentally, Gliese 581 actually has a Hot Neptune (Gliese 581b) between its pseudo-Mercury (Gliese 581e) and its pseudo-Venus (Gliese 581c). (Don't take the analogy too far - said "pseudo-Mercury" is incredibly small for an extrasolar planet, but is still 1.7 Earth masses.)

So Hot Neptunes / Hot Jupiters don't seem to be a complete dealbreaker for smaller planets. They must be able to peacefully insert themselves somehow.

Also, remember that when gas giants migrate in and intrude on preexisting planets' orbits, those planets have to go somewhere. Quite often, they'll merrily continue orbiting as the gas giant's moon. This moon could still support life. (Titan, whether it has life or not, is a good proof of concept for most of the conditions necessary for life.) Such captured ex-planets are likely to orbit retrograde, if captured by a gas giant that migrated inward (rather than outward) - like Neptune's moon Triton, which is indeed believed to have been acquired this way.

Milo said...

Rex: Gliese 581g might be a binary planet, but it's considerably more likely that it's a single planet with one or more moons that only contribute a small fraction of the system's total mass.

Anyway, there's no real need to split it up. At 3.1 Earth masses, it is almost certainly a rocky planet, and its gravity will only be mildly harsh even to humans - and quite comfortable to the locals. High gravity isn't a barrier to life forming, and can actually help - gravity helps with holding a thick atmosphere, which besides the obvious uses, would also help to spread heat across the surface of the planet to broaden the habitable zone as Thucydides describes it.

Rick said...

Welcome to another new commenter!

Milo is correct that the number of currently known planets says nothing about how common or uncommon earthlike planets are, for whatever standard of 'earthlike.'

What we have is not a statistical sample of planets, but a sample of planets that are easy to find with our current techniques.

One interesting bias is time. If the planet searchers are observing a lot of 'normal Jupiters' out there, in classic near circular orbits at a few AU, these will only have had time to make about one orbit since the precision searches began. But you need at least one full orbit to 'call' a detection. So they are still in the process of being discovered (or not).


Whether vertebrate equivalents commonly evolve is an interesting question. First you need complex multicellular, etc. But my understanding is that exoskeletons are only suited to insect sized or crab sized organisms. If large size is adaptive, some equivalent to a skeleton might develop, even if it follows a different path than however bones arose.

In that case I would indeed expect fish-shaped creatures, because that is the efficient shape for prolonged, high speed underwater motion. It 'evolved' one other time on Earth - torpedoes and subs have much the same configuration.

Thucydides said...

I'm thinking of a "Hot Jupiter" which forms early in the life of a solar system. So far as i understand the theory, the forming protoplanet is exchanging momentum with the gas, dust and other protoplanets, and because of the disparity in mass, the Hot Jupiter is flinging mass outwards while being dragged inwards.

An already formed planet may be captured (or shot into a highly elliptical orbit around the primary), but for much of the mass of the solar system, it will remain dissacociated, and probably end up in the exosolar Oort cloud.

Since we really don't know much about how planetary systems are formed, I will stand by saying this is only a guess, which will be resolved by future observations.

Large free flying observatory mirrors can be the size of earthly continents, and at that size, should be able to resolve features on the surface of the exoplanets. Astronomy in the late 21rst century will be pretty exciting.

jnutley said...

Dex makes a good point. All these worlds around Gliese 581 are gravitational vectors from the wobble of the primary. If astronomers with our current tech were in that system and observed Sol, they'd have no evidence to indicate that Sol c was actually Sol c1 and Sol c2. Break Gliese 581g into a primary and a satellite and it is no longer tidally locked.

Look at:

http://en.wikipedia.org/wiki/Theia_(planet)#Theia

and the paper linked at:

http://arxiv.org/abs/astro-ph/0405372

Arguing from the Copernican principal, other systems will likely form large impacters at the lagrange points with their stars and prime planets. Can't know how often they form, or how often they collide to create stable planet/moon systems without more data, but Earth is in Sol's habitable zone, and Gliese 581g is in Gliese 581's habitable zone, so the parallel is there.

Milo said...

Submarines look much less fishlike to me than ichtyosaurs. I don't think it's fair to call them fishlike anymore than airplanes are birdlike. (The real birdlike vehicles would be ornithopters, which we've managed to build but aren't really all that useful. Biomechanics and human engineering impose different evolutionary circumstances.)

Milo said...

Jnutley:

"Break Gliese 581g into a primary and a satellite and it is no longer tidally locked."

Only if it's a very large satellite. I don't think something weighing 1.23% of the primary's mass, like Earth's moon does, would be sufficient to override the star's pull. And Earth's moon is unusually large.

Of course the satellite would not be tidally locked (to the sun). And you can still find life there.


"Arguing from the Copernican principle, other systems will likely form large impacters at the Lagrange points with their stars and prime planets."

Some other systems will. The fact that it only happened once in ours, despite several rocky planets (one of which is nearly Earth-sized), suggests it won't be too common. It also won't be exceedingly rare, but I think it'll still be a mildly notable exception, rather than the rule.


"but Earth is in Sol's habitable zone, and Gliese 581g is in Gliese 581's habitable zone, so the parallel is there."

I see no reason why being in the habitable zone would meaningfully increase or decrease a planet's chances of experiencing a Theia-style collision.

jnutley said...

Rick said:

"But my understanding is that exoskeletons are only suited to insect sized or crab sized organisms."

Please consider the invertebrates of the Coal Age (Carboniferous):

http://en.wikipedia.org/wiki/Arthropleura

the practical size of an invertebrate would appear to be proportional to the concentration of oxygen (in the case of Earth). An ecosystem relying on methane or fluorine might also allow large invertebrates at high concentrations.

Milo said...

Actually, exoskeletons are only inadequate on land (and even there, there can be workarounds, and the limit isn't as harsh as sometimes perceived - look at these). In water, buoyancy means you're nearly weightless (Earth life has approximately the same density as water, plus or minus some variation based on flesh/bone/air cavity ratios, and I expect this will hold more generally for all life-as-we-know-it: organisms will have approximately the same density as the solvent they're made of, be it water or something else), and so giant squid can grow impressively large without either an endoskeleton or exoskeleton.

Another barrier to arthropod size is the way they breathe, acquiring air by diffusion through the skin. This is dependant on having a large surface area, and so doesn't scale up well. However, this isn't strictly tied to being a vertebrate - spiders and scorpions have evolved book lungs. It's not even guaranteed that all life forms will actually need to breathe at all - somewhere in the universe there may be macroscopic life forms that derive all chemicals they need from solid food. Although gasses do have their advantages. Who knows...

So basically, life will find a way.

Jim Baerg said...

If you don't like a world that has one side permanently facing its sun, consider the possibility of an earth size world in a modestly elliptical orbit around a red dwarf. With enough eccentricity a Mercury type 3:2 tide locking would be more stable than 1:1 locking. This would make for some long & cold nights, but likely no colder than a Canadian winter.

It doesn't look like the Gleis 581 planets have the 3:2 locking though, the eccentricities of their orbits are zero with the error bars of the measurements.

If you are interested in easily terraformable planets for colonization rather than already habitable planets, a world orbiting relatively far from a red dwarf so that it is still tide locked & gets less heat than earth might be good. It should have all its volatiles frozen out on the dark side & a mirror in space could warm them enough to give the planet air & liquid water.

Milo said...

Life/climate on non-1:1 resonating planets hasn't been studied much, that I know of. (Exercise for the reader?)

Such a place would have a very slow diurnal cycle, and it would be tightly interlocked with the seasonal cycle, to the point that the two can't be told apart - both light/darkness and axial tilt (and, for that matter, perihelion summer/apohelion winter arising from an eccentric orbit) would join together to form a somewhat complex, but regular (over non-geologic time scales) seasonal pattern.

Everywhere on the planet would spend significant amounts of time in total darkness (even accounting for red dwarf planets' short year lengths, we're still talking weeks at a time). This doesn't kill life - we had forests at the poles back when they weren't iced over. But it would greatly encourage techniques such as migration or hibernation to cope with the dark periods.

Thucydides said...

Earth's moon is a pretty spectacular long shot, if the angle of the impactor had been a bit different, the end result might have been a rapidly spinning Earth that wobbled like a top (having entirely absorbed the impactor), or several moons, or a train of asteroids in our orbital space where the proto Earth and Impactor were.

I wonder if the conditions of Venus were set by a similar impact that was spectacularly "wrong", reversing the rotation of the planet and releasing so much heat that the planet outgassed a massive atmosphere which evolved to the heat trap we see today.

Convergent evolution seems to be a powerful force, sharks are not related to fish at all but also share the form factor of fish, dolphins, whales and Ichthyosaurs. Penguins and seals also share a fairly broad similarity to the form factors of marine life, despite their different origins and the fact they are amphibious to a certain degree. I have a fairly old book called "After Man" which looks at the principles of evolution in the context of a post human world; penguins are supposed to evolve to fill the niche of the extinct whale population, and of course eventually come to resemble whales, including analogues to Orcas and Baleen whales.

The physics of waterborne propulsion and flight would seem to demand certain principles be followed, so analogues of fish and birds *should* look familier in broad outline wherever we find them. How land life will evolve is probably harder to pin down, there are not as many constraints laid down by the laws of physics and the ecological niches can be exploited in multiple ways, it will be interesting to see what evolution can come up with.

Milo said...

Thucydides:

"sharks are not related to fish at all"

That depends on how you define "fish", I guess. Most people use "fish" to mean "vertebrates that are not tetrapods" (or maybe just gnathostomates rather than vertebrates), which is paraphyletic. The vast majority of "fish", containing most well-known species, belong to the clade of actinopterygians, which would be a good definition of "fish" for those who like monophyly.

However, the particular trait we are discussing here - a fish-like shape - was in fact ancestral to gnathostomates, and sharks, actinopterygians, and coelacanths all retain this shape from their common ancestor, rather than having evolved it independently. (Of course, they still each applied their own tweaks to the original design, some of which have converged.) By contrast, while ichthyosaurs and dolphins technically had a fishlike ancestor, they had long since lost that shape and had to re-evolve it independently.


"I have a fairly old book called "After Man" which looks at the principles of evolution in the context of a post human world; penguins are supposed to evolve to fill the niche of the extinct whale population, and of course eventually come to resemble whales, including analogues to Orcas and Baleen whales."

Reading that, I can't help but imagine a comically over-inflated balloon penguin.


"The physics of waterborne propulsion and flight would seem to demand certain principles be followed, so analogues of fish and birds *should* look familiar in broad outline wherever we find them."

Among vertebrate-analogues - yes. Bats, birds, and pterosaurs all took flight by adapting their forearms into wings which they flap (although they chose different preexisting arm/hand bones to base their wings on). However, all three started out as tetrapods. Insects evolved flight entirely differently, and while they still flap their wings rather than using propellor or jet propulsion or wackiness like that, those wings look very different, often come in fours rather than twos, and do not seem to have been modified from limbs.

Luke said...

Rick,

The picture is pretty, but the blues are too bright. With a lot less blue light from the sun, there will be less blue light scattered from the sky, which would probably be much darker overall, perhaps a very dark blue. The sea, which is also blue because of scattering of blue light, would also be darker, perhaps nearly black. Then, of course, the sunlight would be more yellow.

I took a copy and modified it, making the saturated blues darker and multiplying by a color temperature of 3600 K. The result is here
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienSky.jpg

Milo said...

At the same time, though, remember that even a "red dwarf" still has a color temperature comparable to an incandescent light bulb - although Luke might be right on the effects it has on air and water, for normal objects the human eye will still just see colors more or less normally.

Natives, of course, probably wouldn't be seeing the same colors as us at all. The number of color channels they see in and the peak light frequencies of those channels can be pretty much anything (even on Earth they vary quite a bit), but in particular they're more likely to make use of what we consider near infrared wavelengths, which a red dwarf emits a lot of.

Luke said...

Right, it is actually a bit whiter than a normal incandescent (which range from 2700 K to 3300 K), closer to the whiteness of a halogen (from 3300 K to 3600 K). Still, indoor incandescent lighting still looks yellower than outdoor lighting. If you view the image against a yellow background, you will get a better idea of how your eye would interpret it. For example, this lizard is lit by halogens
http://panoptesv.com/HBD/Pics/baskingspot1.JPG
and you can see how the camera recorded a very yellow colored picture - although the spots directly under the lamps are oversaturated and look white. Hmmm, maybe I'll have to oversaturate some of the glare in the picture ...

Turbo10k said...

Urgh. Don't know if I should read this first, the cities thread or catch up with the peace thread I haven't even looked at yet...
:(

Bookmarking.

Gridley said...

"If any worlds are 'for us' beyond the one we evolved on and any we may one day terraform."

Oh come on, that's just unbridled pessimism. There are how many stars in just this galaxy? How many galaxies?

Even if the odds are 100,000,000,000 to one against any given star having an Earth-like planet, there are still probably other Earth-like planets in our own galaxy.

Tony said...

Gridley:

"Even if the odds are 100,000,000,000 to one against any given star having an Earth-like planet, there are still probably other Earth-like planets in our own galaxy."

I personally wouldn't qualify things that much. We have yet to find in science any way in which we are particularly special. Taking that fact in hand, and taking judicial notice of the number of planetary systems already confirmed, I would be willing to bet that there is at least one Earth-like planet in size, gravity, atmosphere, and presence of carbon-based life, within 100 ly of our solar system.

Milo said...

If you're looking for worlds that actually have human-compatible biochemistry without terraforming, then the chances are less "100,000,000,000 to one against" and more "a googolplex to one against".

If you're looking for worlds with water-carbon-and-oxygen based life (i.e., fairly similar to us) but still no human-compatible biochemistry, then the odds are much better... but we can't settle there.

Fernando said...

Hi! Long time follower, first time poster from Argentina here.
I think something nobody considered here is how HUGE storms would be in a planet like Gliese 581g. A world with such mass probably will be composed of rock like Earth, and will attract so many comets that will be covered by a kilometers deep ocean (assuming that there are enough comets in the system, and enough of them crash on this world). No geological feature would be tall enough to reach the surface and modify the climate, and huge hurricanes will form if the water is warm. Without any land barriers, storms will keep growing and growing, covering the planet with clouds and moving great masses of air. Like the Great Red Spot in Jupiter, these storms could live years, even centuries. So the surface of Gliese 581g won't be the calm sea in the picture; it will be a titanic, biblical storm with no end.
That is, of course, assuming that the gravity of the planet will attract enough comets to form a worldwide ocean, and that temperature is warm to form hurricanes in the first place. Truth is, we don't know. However, I think that submarine life would not be very affected; in fact, they may benefit by storms moving warm water from the equator to the temperate areas. But storm-clouds constantly covering the skies would not be good for photosynthesis, and could even cause a runaway global warming effect.
Anyway, we will not know anything of this world until we actually send a interstellar probe there. But is fun to speculate. I'm already writing about a ecology in these "super-earths", centered around free floating coral like animals, that move with the currents and feed on the "aerial plankton" that grows in the storm-clouds, where they can feed directly from sunlight. These giant corals are home to thousands of species, like reefs on Earth...
I love this blog! Sorry if you could not understand this post; English is my second language, and I'm trying my best to practice it.

Milo said...

Fernando:

"A world with such mass probably will be composed of rock like Earth, and will attract so many comets that will be covered by a kilometers deep ocean (assuming that there are enough comets in the system, and enough of them crash on this world)."

Let's hope it isn't - that kind of ocean would be a major barrier to life. All the nutrients would sink to the bottom, which doesn't get light.

Fortunately, this is far from a foregone conclusion. We have very little idea on how a planet's gravity actually affects the number of comet strikes it gets, and that's aside from the many other things that also affect it. We haven't been able to get a statistical sample, after all. So I'm highly skeptical of any theory that claims to predict a heavy world's water content with such accuracy.


"No geological feature would be tall enough to reach the surface and modify the climate, and huge hurricanes will form if the water is warm."

And if it is a planet-wide ocean, than who cares? Hurricanes are only destructive to stuff on land, or maybe (to a lesser degree) near the surface of the ocean. Deep down, the water shelters you from the weather.


"So the surface of Gliese 581g won't be the calm sea in the picture; it will be a titanic, biblical storm with no end."

You could probably still find a calm spot somewhere. The Great Red Spot only covers a tiny portion of Jupiter (albeit one larger than Earth).


"That is, of course, assuming that the gravity of the planet will attract enough comets to form a worldwide ocean, and that temperature is warm to form hurricanes in the first place. Truth is, we don't know."

Temperature will vary across the planet, of course. Hot on one side, cold on the other, gradient in between.

Even if all else were equal, climate patterns on a tidally locked planet are going to be quite different from those on a rotating one. We don't have a good reference point in this regard.


"These giant corals are home to thousands of species, like reefs on Earth..."

Reefs are slow-growing environments that need somewhat stable conditions. I don't think they'd do well on storm-tossed currents.


"Sorry if you could not understand this post; English is my second language, and I'm trying my best to practice it."

Your English is fluent... really. I already forgot you came from Argentina by the time I got to the end of your post.

Tony said...

Fernando:

"That is, of course, assuming that the gravity of the planet will attract enough comets..."

If you're extrapolating from Jupiter's proven ability to attract multiple impacts in human time scales, think about this: According to Wikipedia, Jupiter has a mass equal to 317.8 Earth masses. On that scale:

mE = 0.00315 mJ
mH = 0.01259 mJ

Where:
m: mass
E: Earth
J: Jupiter
H: hypothetical superearth, m = 4 * mE

Which isn't conclusive of anything, but puts the relative scales in constext. A superearth is very much closer to the Earth in gravitational potential than it is to Jupiter.

Milo said...

For comparison, Neptune is only 17.15 Earth masses, and Uranus is only 14.54 Earth masses. Both already far below Jupiter's league. Any rocky world is almost certainly going to be lighter. (This also means that it's basically impossible for a rocky planet to have so much gravity that humans can't walk around on the surface.)

But maybe it's possible that your mass relative to other planets in the same solar system is more important than your absolute mass... In that case, Hot Neptune Gliese 581b will be the one getting all the glory (as far as we know).

Jim Baerg said...

Re - hurricanes on Gleise 581g

On earth corriolis effect is crucial for generating hurricanes. The revolution period & ( as tidal locking is almost certain) the rotation period of Gleise 581g is 36.6 earth days, so the coriolis effect there will be small. Hurricanes as such won't happen there.

According to the article linked by Rick in the OP the surface winds would be 0 to 40 knots. Presumably winds would be much steadier than on earth which would make wind power much more practical than on earth.

Tony said...

Milo:

"But maybe it's possible that your mass relative to other planets in the same solar system is more important than your absolute mass... In that case, Hot Neptune Gliese 581b will be the one getting all the glory (as far as we know)."

Which raises another point. Terrestrial moons of gas giants have been popular in fiction at least since the introduction of the "fourth moon" of Yavin to the moviegoing public. Yet knowing what we know about gas giants' ability to attract debris, such planets might not be all that healthy to live (or even evolve) on.

Milo said...

Tony:

"Yet knowing what we know about gas giants' ability to attract debris, such planets might not be all that healthy to live (or even evolve) on."

Titan shows that it can have an atmosphere and surface liquids that are neither too much nor too little. We haven't found life there, of course, but it's possible that that's because life really does need water after all, or because it just hasn't evolved to be macroscopically visible, or because of some factor we're not even aware we should be looking for yet.

Does Titan have an unacceptably high impact rate? I don't recall ever hearing of anything like that.

Anyway, I'm not so convinced if even a high impact rate would actually be lethal. Sure, you'll have more mass extinctions, but life on Earth has weathered many of those (most of them weren't caused by impact events), and life elsewhere will too. As long as there's enough of a gap between major impacts for ecosystems to re-evolve, and the impacts don't flood you in super-deep oceans, you should be fine.

UmbralRaptor said...

It also depends on how fast the impactors are coming in. Titan doesn't get whacked as hard as the Galilean moons, which afaik, helps it hang onto its atmosphere.

Citizen Joe said...

I wanted to bring up the Drake Equation regarding habitable planets. I believe there are some biases built in based on the assumption of a uniform distribution of stars. However, I'm pretty sure that the galactic habitable zone is fairly narrow. Too close to galactic core and the radiation will prevent life. Too far out and you don't have enough heavy elements for planets. So there is sort of a galactic Goldilocks Zone as well.

The stellar Goldilocks Zone assumes Earth like planets and atmospheres and orbital mechanics. Tidal locking can make ribbon worlds with narrow bands of habitability, even out beyond the ice line. An L1 lagrangian satellite could act as a solar shield for a planet too close to its star. This would also act as a shield against solar flares.

AdShea said...

@Milo why couldn't we settle on an earth-like planet that had oxygen-carbon-but-not-earth-compatible life? It would be an ecological atrocity, but I'm pretty sure we could burn off and import our own ecology far easier than terraforming something that doesn't have the right temperatures and atmosphere.

Tony said...

AdShea:

"@Milo why couldn't we settle on an earth-like planet that had oxygen-carbon-but-not-earth-compatible life? It would be an ecological atrocity, but I'm pretty sure we could burn off and import our own ecology far easier than terraforming something that doesn't have the right temperatures and atmosphere."

We'd probably have to import a complete, competitive biosphere, from nitrogen-fixing bacteria to apex predators. Just the handedness of sugars (that is, supposing right-handed sugars are not metabolically special in some way) would casue complete biosphere replacement in half the candiadtes, on average. That's a canonically, memetically hard job, and I'm not sure we'll ever know enough to do it successfully.

Thucydides said...

The question of resources in a red dwarf star system is interesting. Since the system is condensed out of a very small gas and dudt cloud, I would imagine that the system overall would be relatively resource poor as compared to ours.

On the other hand, since the star is so much smaller, there might not have been a strong t-tauri wind event at ignition, and the overall gas and dust could would not have been energized to any great extent, so volatile elements would be available much closer to the star (essentially a Kruiper belt where the asteroid belt is for us).

How this affects the planet itself is hard to say. If there is a "Jupiter" analogue outside the asteroid belt then the icy bodies will be projected into eliptical orbits which may intersect the rocky planet, seeding atmospheres and oceans. Since the sun is so close, these bodies might be drawn into the sun rather than the planet instead.

I would guess a relatively dry planet with shallow seas or maybe a shallow ocean similar to Oceanus Borealis on Mars a billion years ago. The hydrological cycle might include the flow of water from the hot pole to the cold pole, condensation into ice fields and a slow march of glaciers back towards the warm hemisphere. Ice might also be subducted (a large planet should have an active core and plate tectonics) and released centuries or millenia later in volcanic eruptions. How life will evolve under these circumstances is interesting to contemplate.

Milo said...

Thucydides:

"If there is a "Jupiter" analogue outside the asteroid belt"

Currently, the Gliese 581 system looks like this:
e. Pseudo-Mercury. The second smallest extrasolar planet detected so far, and the smallest one that orbits a main sequence star. Granted, it's still 1.7 Earth masses, so not exactly small.
b. Hot Neptune. The biggest planet in the system known so far. Looks incongruous for its position, compared to our solar system's neat "rocky planets inside, gas giants outside" ordering.
c. Pseudo-Venus. Just barely possibly habitable if conditions are just right, but is probably too hot.
g. Pseudo-Earth. The third smallest extrasolar planet discovered so far (second smallest around a main sequence star), and also right in the middle of the liquid water zone, so habitability is quite likely... with the entertaining caveat that it's tidally locked (as are most other planets here).
d. Pseudo-Mars. Just barely possibly habitable if conditions are just right, but is probably too cold. More likely than Gliese 581c, though.
f. The next heavier known planet in the system after Gliese 581b, making it look vaguely like Sol's gas giants, but we don't even know what it is. It could be either a remarkably huge rocky planet or a remarkably tiny gas giant.

The distance between 581d and 581f suggests an asteroid belt between them is actually quite plausible, but that's purely my own speculation.

New planets might still wait to be discovered! For one thing, you'd expect the system to have some planets lighter than the currently known "pseudo-Mercury", which at 1.7 Earth masses seems like a rather odd match for the title.


"I would guess a relatively dry planet with shallow seas or maybe a shallow ocean similar to Oceanus Borealis on Mars a billion years ago."

Okay, so one person tells me that Gliese 581g is likely to end up drenched kilometers deep, and another person tells me Gliese 581g should be expected to have very shallow seas. Let's just settle on "we don't know yet", okay?

Thucydides said...

But if we settle on "we don't know yet", what will we have to argue about? ;)

aaa said...

First time poster here too.

A planet in tidal locking with its star could be really interesting in terms of energy generation. Could we expect permanent winds between the cold side and the hot side? Or even better, we have a permanent hot source and a cold source; just imagine some planet-spanning thermal engine that produces lots and lots of work.

Anonymous said...

I still think that GL 581g is one of the most interesting planet discovered, so far. I think that we humans should concentrate on finding out as much as possible about this world. It would go a long ways toward us making more informed theories about "Earth-like" worlds and how native like could exist on it, or how we humans could live there.

Ferrell

Luke said...

aaa:

From what I have read, tide locked worlds do have strong permanent winds, but they are not necessarily what you would expect and depend a lot on the rotation/revolution period. For shorter period orbits you get complex banded wind patterns around the world, and in some cases you are supposed to get permanent cyclonic storms over the sub-stellar pole. These simulations have not so far taken into account the effects of terrain.

But yes, with strong constant winds you could install wind turbines for energy if you lived there, without worrying about the wind not blowing like happens here on earth from time to time.

For that matter, solar power will be easier because the sun is in the same position overhead all the time. Thus, there will be no need to track the sun and you can just turn your panels to directly face the sun at all times. Our current silicon photocells probably wouldn't work too well, though, because the lower energy photons would mostly be below the direct q=0 bandgap of silicon.

Milo said...

Thucydides:

"But if we settle on "we don't know yet", what will we have to argue about? ;)"

I'm more interested in the question of, assuming that this planet has a somewhat Earthlike ocean cover (say, oh... between 30% and 95% of the surface area), simply because to our knowledge this would be most beneficial for life, what would this life then look like?

Unfortunately this would really require looking at climate simulations that are beyond my skill to do accurately.

On the dark side, you are likely to have a chilly but liquid ocean covered in a sheet of ice. (Not guaranteed - liquid surface water here is possible with enough greenhousing - but you'd rather have your liquid water where you have light.) Underneath the ice, life would be possible, but due to the lack of photosynthesis it would be sparse - probably limited to some hydrothermal vents, and to sparse communities feeding on plankton drifting in from the lit side.

At the terminator, it's still going to be fairly chilly - you could greenhouse the planet to the point that the terminator is the optimum life zone, but chances are the locals will see the terminator the way we see our poles - livable, since it gets sunlight, but still pretty icy. This is the coldest place you're likely to see megafauna like polar bears. Hopefully you have a world ocean large enough for ocean currents to flow across the terminator in both directions, carrying nutrients out (so those sparse plankton-feeding communities don't starve) and carrying small amounts of water in (so your lit side doesn't dry out - mind you, you only need enough current to cancel out the water that gets transported by rain, which will be a pretty miniscule fraction of the ocean).

As you move closer to the inner pole, you're going to see gradually warming climates, leading to tundra-analogues, forest-analogues, etc.

Beyond a certain point it may (depending on the planet's overall temperature) start getting so hot that multicellular life can no longer cope and bioproductivity starts to go down again, since the permanently overhanging sun would heat things far more than even our tropical rainforests.

All these would deviate from their Earth analogues due to wildly differing weather patterns - different temperature gradients and nearly no rotation means different wind patterns, and evaporation patterns are also obviously different. Geography (continents, seas, altitude effects on temperature, rainshadows, sunshadows - you can have those on a planet where the sun never moves!) would further influence biomes significantly. Rainfall majorly affects what biome you end up getting, moreso than temperature (it just happens that temperature affects rainfall), and I can't tell you what rain patterns would look like on this planet - that's where it gets interesting.

Climates on such a planet would be very stable (though short-term weather probably still wouldn't), with no seasons to mess things up. Thus, seasonal behaviors like migration, hibernation, and leaf lossage would probably not be selected for on a tidally locked planet. (Some animals might still find an advantage in having a specific breeding season, but its calendar date would be more or less random. You might also see circuit nomads who move to new pastures while waiting for the old ones to regrow the food they harvested.) Shame - anything that does have a motive to migrate (like sentient traders?) would have an awesomely convenient navigation beacon in the sky.

Jim Baerg said...

Note on navigation on a tidelocked world:
On the light side it would be very easy to determine ones angle from the substellar point & hard to determine where along the circle at that angle one is, so 'latitude' would be the angle from the substellar point & there would be a 'longitude problem' at least as difficult as the one sailors on earth had a few centuries ago.

On the dark side observations of stars would give latitude in the ordinary earthly sense & longitude could be determined if one has an accurate clock.

Re: windpower on a tidelocked world. I wonder if there is some way a biological system could devise a sort of 'reverse muscle' so that a 'plant' could use wind induced motion to make organic chemicals from CO2 & H2O? That would make for an interesting ecology on the windy & not too cold parts of the dark side.

Milo said...

Merely inventing accurate clocks wouldn't solve the longitude problem here. On Earth, clocks work by measuring the location of the sun in the sky, and comparing it against your clock's time to determine what time zone you're in (since time zone = longitude, at least in the sense I'm using it here - time zones people actually use are rounded to the nearest hours and tweaked to obey borders, of course). On Zarmina, the sun doesn't move (much), so there's nothing to compare your clocks against.

Maybe you could measure the sun's location against the plane of the ecliptic, but I'm not sure how you'd go about determining that. Or, depending on where the magnetic poles are, you could use those as a secondary reference point.

Hey, I got it - if the planet has a moon, said moon would still orbit around the planet (in fact, the month would be the only astronomically-defined calendar unit - they have no days, and they're not likely to be able to tell the year length if their axial tilt and orbital eccentricity are so low they have no seasons), giving you something to measure your clocks against, and also giving you something to determine the plane of the ecliptic by. The moon would also provide some light so travellers on the dark side can still see, but it wouldn't be enough for photosynthesis. Also remember that you can have more than one moon (don't get led astray by Earth's example!), which would give you quite a bit to play with when finding a solution (and also fill out your starved calendar).

Navigation on the dark side is likely to not come up until quite late in the civilization's history. There's not much there you'd care to go to, except for scientific research vessels looking for extremophiles, and maybe if you need to take a roundabout route to circumnavigate an inconveniently placed continent (like our Europe/East Asia trade before the Suez Canal).

It just occurs to me that people on a tidally locked world would take a very long time to develop any sort of concept of astronomy. No dark skies to look at! They would be quite surprised when they travel to the dark side (safely snug in many layers of blankets) and find all those stars in the sky. On the other hand, this means that astronomic effects bright enough to be visible in daylight (supernovas, really bright planets like Venus supposedly is sometimes, maybe some comets or meteors, ...and of course, moons) would be quite spectacular to them, and could have significant mythology associated.



"Re: windpower on a tidelocked world. I wonder if there is some way a biological system could devise a sort of 'reverse muscle' so that a 'plant' could use wind induced motion to make organic chemicals from CO2 & H2O? That would make for an interesting ecology on the windy & not too cold parts of the dark side."

GENIUS!

I don't know how realistically plausible it is but the idea sounds awesome! AWESOME!

Milo said...

More thoughts on moons...

The outer edge at which moons can remain in orbit is given by the Hill sphere: distance_planet * cbrt(mass_planet / mass_sun / 3).

The inner edge at which moons can remain in orbit is given by the Roche limit: cbrt(1.5/pi * mass_planet / density_moon)

These are lenient assumptions. The practical outer limit for prograde satellites will be more like 1/2 or 1/3 of the Hill sphere, while real objects will break up before reaching the Roche limit.

Gliese 581 is 0.31 Solar masses, giving a mu of 41140856405.58 km^3/s^2.

Gliese 581g is 3.1 Earth masses, giving a mu of 1235660 km^3/s^2. It orbits at 0.14601 AU, or 21.843 Gm, or 1.214 lightminutes, or 72.86 lightseconds.

This gives a Hill sphere of 470775 km (1.57 ls). Compare with a Hill sphere of about 1500000 km (5 ls) for Earth.

And a Roche limit of - well, we'd need to decide on a density for the hypothetical moon. Let's go with 5.5 ton/m^3, about the upper limit in our solar system. Then we get a Roche limit of 11715 km (0.0391 ls), compared with 8034 km (0.0268 ls) for Earth.

The proximimity of the planet to the star, that caused tidal locking in the first place, definitely also results in a narrower band that a moon can stay stable in, but there's still quite some space.

Orbital period is 2*pi * sqrt(distance_moon^3 / mass_planet / G), which gives us an orbital period range of 2 Earth hours (in fact, the orbital period at the Roche limit depends only on the chosen moon density, not on the planet's mass!) to 21.13 Earth days, although a range of 5 Earth hours to 1 Earth week is more likely. Technically the month perceived by people on the planet should also be adjusted for the planet's rotation period (36.562 Earth days).

Luke said...

Milo:

With less blue light to scatter, the sky will be darker than that of earth. This might mean that people on the surface could see bright stars and planets even on the dayside. Those big, close planetary neighbors that this planet has might end up being as visible as Venus is in the evening skies of earth.

Jim Baerg said...

"Maybe you could measure the sun's location against the plane of the ecliptic, but I'm not sure how you'd go about determining that."

Some of the other planets in the Gleise581 system are both much bigger & much closer to Gleise 581g than Venus is to Earth, so maybe they will be commonly visible in daylight & so make the longitude problem trivial.

Anonymous said...

The development of hypothetical human colonies on a tidally locked oxygen-atmosphere planet could prove interesting as well. On the dayside, assuming the colonists retained similar waking and sleeping patterns to people on earth, windows would need heavy curtains or shutters to keep out the sunlight so people could sleep. Crops and livestock from Earth would probably need similar arrangements, with barns for the livestock and perhaps field-sized domes for the crops. Any settlements on the terminator or nearer nightside would have the opposite problem, and might employ streetlights capable of replicating the strength and spectrum of daylight during the waking period.
It occurs to me that dayside settlements could be true '24-hour cities', since without the disadvantages of having to work the 'night shift', such as lack of daylight and isolation from people with a regular working day, equal numbers of people could be active during each shift.

R.C.

Elukka said...

People can adapt. No idea how well crops would do, though.

My sleep rhythm gets messed up quite often and I can sleep fine during the day when I'm tired. I realize it varies on an individual basis but I doubt people would have much trouble if they lived there for an extended period of time.

Those up above the arctic circle seem to do well enough during the midnight sun.

Milo said...

R.C.

"Crops and livestock from Earth would probably need similar arrangements, with barns for the livestock and perhaps field-sized domes for the crops."

Field-sized domes? Too complicated. I don't think it's an issue anyway - plants should be able to adapt to permanent sunlight pretty easily. In fact, I wouldn't be surprised if most animals and people do as well - we have people living inside the polar circle on Earth, after all, and they have to deal with both season-long days and season-long nights. They might still have shutters, for comfort if not actual necessity, although the younger generation who grew up on the colony rather than settling from Earth may not see the need for them.

(See here.)

Crops or livestock will probably need to be adapted some way or another, though, be it to the light, gravity, weather, or whatever. But creating new breeds of our domestic animals and plants is something that we have done since the Bronze Age. Even without genetic engineering, artificial and natural selection will work pretty quickly to create breeds suited to the exotic conditions.


"Any settlements on the terminator or nearer nightside would have the opposite problem, and might employ streetlights capable of replicating the strength and spectrum of daylight during the waking period."

It would be easier to have the outside be permanently gloomy (current streetlights give enough light to see by), and only use intense lights indoors.

In fact, you shouldn't even need streetlights. Anywhere that gives enough light for photosynthesis easily gives enough light to see by. At most, light might occasionally drop to below human perception limits on particularly overcast days.

Another potential source of darkness: solar eclipses. However, between the increased difficulty of holding a large moon this close to the sun, and the increased size of the sun in the sky, it's unlikely the planet would have a moon large enough to fully block the sun.


"It occurs to me that dayside settlements could be true '24-hour cities', since without the disadvantages of having to work the 'night shift', such as lack of daylight and isolation from people with a regular working day, equal numbers of people could be active during each shift."

The biggest disadvantage of having 24-hour cities is that people in different shifts have difficulty socializing with each other. But this is a problem already faced by international communication between different time zones, like... us here. So it can be dealt with.

Speaking of which, a tidally locked planet would have no real time zones as such. The only cycles they have available to measure time by - months, years of other planets bright enough to be visible, this planet's year if there's some way to measure it, and arbitrary time units - are the same planet round. Much easier! Our messy fudging would look incredulous to them.

Over time they may even experiment with systems that don't involve 24-hours sleep cycles. There are already places on Earth where people take a nap at noon (to get out of the scorching sun). How averse would human biology be to taking frequent short naps? At the very least, there would probably be little stigma to irregular sleep schedules. Just go to sleep when you're tired and wake up when you're not.

For aliens that evolved there, there's no telling if their biology would even have a concept of "sleep" at all. It's very likely they wouldn't.

Milo said...

Actually, speaking of light needed for photosynthesis, would the sunlight have enough of the right kind of light to support photosynthesis in Earth plants, or would you need something adapted to photosynthesize with redder wavelengths?

That could be a serious barrier to colonization.

Luke said...

Milo:

Plants tend to do poorly when grown under incandescent lights (here, incandescents includes halogens, which are just incandescents with halogen gas to scavenge evaporated tungsten deposited on the glass and re-deposit it on the filament, allowing the filament to operate at higher temperatures). Since incandescents (and halogens especially) have a similar spectrum to red dwarf stars, I expect that any plants colonists grow on planets around these stars will need to be genetically modified to use different photopigment, or will use artificial lighting.

Another issue is for human health. Vitamin D (actually a hormone, not a vitamin) is necessary for proper calcium regulation. Without it, you get malformed and fragile bones and muscle tremors (calcium is also needed for muscle contraction). In addition, vitamin D is found to be increasingly important to a variety of other processes, such as tumor suppression, genetic repair, and radiation resistance (just off the top of my head). Vitamin D is formed when the precursor molecule - pro vitamin D - is exposed to a specific narrow band of wavelengths of UV-B spectrum light while it is concentrated in the skin. Since red dwarf suns put out essentially no UV-B spectrum light, there will be no natural synthesis of vitamin D for earth derived life on these planets. You may need either chemical manufacturing of vitamin D for use as a food supplement, or artificial lighting that produces low intensity UV-B light at the peak wavelength for production of vitamin D. If the latter is used for interior lighting (perhaps as mature LED lighting technology being developed today) it could deal with this issue (in fact, the people may actually end up being healthier than those of us on earth who spend a lot of time indoors). The food supplement method might be necessary for outdoor livestock, however.

Thucydides said...

By the time we get there we will have mastered many techniques to bring the climate etc. to our liking.

My particular favorite is to insert a platoon of mirrors into the solar photosphere to institute a solar laser. The energy can be used to power industry, space services, and be reflected off a platoon of solettas orbiting the planet to illuminate the dark side and make the cold pole habitable.

Applying gravitational torque to the planet to break the tidal lock is another possible way of making the planet more habitable.

You can imagine many more

VonMalcolm said...

After Man brings back memories: remember The Night Stalker: a mini T-Rex that had evolved from a bat and had long arms attached to its hind-quarters?

(I am basically bookmarking.)

Rick said...

Welcome to a few new commenters!

Belated thanks to Luke for the excellent color reprocessing reprocessing of the image!

Regarding 'convergence' (from well upthread) I would say that airplanes DO resemble birds, in having the same basic arrangement of wing and stabilizing tail. As an aerodynamics text I have notes, this would be an unlikely shape to come up with on general geometric principles, but airplane designers had the example of birds from the outset.

Side note: What is fascinating to me is how much wing flapping led people astray, so that they paid little attention to wing cross section. Yet - with benefit of hindsight - you can stand on any sea bluff and watch gulls gliding, using wing shape.


Back to the main discussion. When I was reading about planetary climate a few years ago they did not know enough about the thermal effects of clouds to model them. I don't know if that has changed. Apparently clouds can act as either a heat blanket or a cooling reflector, depending in circumstances including the type of cloud.

Cumulonimbus clouds are particularly bright and reflective, so a permanent storm over the hot zone, directly facing the star, like Earth's Intertropical Convergence Zone writ big, MIGHT cool it to a human suitable temperature. On the other hand, it might be 40 C and saturation humidity.

My bias is to expect lots of water on a super earth, but as noted upthread we haven't an actual clue, and can only argue about sheer speculation. That said, carry on!

Milo said...

Thucydides:

Wow! Let's improve the climate by moving a planet! A planet that's heavier than Earth!

For comparison:
rotational energy = 1/2 * moment * spin^2

Where the "moment of inertia", or "angular mass", is a little tricky to define, but Earth's is supposedly 8*10^37 kg*m^2, and so Gliese 581g would have one of around (very loosely) 5*10^38 kg*m^2. To impart on Gliese 581g one rotation per week, or some 10 microradians per second, you would need an energy of about 6 exatons of TNT (or a couple hundred megatons of antimatter), which for comparison could also be used to melt most of Earth's crust.

If you can throw around that kind of energy, you probably don't need habitable planets anymore except as a zoo.

Oh, and remember that adding some half-hearted rotation is worse than nothing. Now instead of half your planet being in freezing permanent night that permits no life, everywhere on your entire planet will at some point spend significant amounts of time in freezing permanent night that permits no life. Unless you can rotate fast enough that nights aren't too long, you're better off not bothering.

Planets cannot be moved, okay? It's only a little easier than creating new planets from scratch from available asteroids. Planets are terrain. You work with the terrain you have, or you hop on your FTL starship and find a different planet. (If you don't have an FTL starship, or maybe a Bussard ramjet and a lot of patience, just forget about interstellar colonization. Seriously.)

This is also completely unnecessary, since the point of our discussion is that a tidally locked planet could be pretty comfortable to live on without needing such absurd measures! Well, half of it is, and that's enough - it's not as though every square meter of Earth is a paradise, and Gliese 581g has more total surface area to spare.

Why the insistance on torturing innocent planets into Earth's image?

Milo said...

Rick:

"Regarding 'convergence' (from well upthread) I would say that airplanes DO resemble birds, in having the same basic arrangement of wing and stabilizing tail."

I disagree. Airplanes and birds may use tails for vaguely similar purposes, but they look nothing alike. Compare that with shark/ichthyosaur/dolphin dorsal fins, which, more than just converging to "some kind of vertical stabilizer", clearly all have very close to exactly the same shape and location on the body, despite not having an evolutionary relationship.

Luke said...

Rick:

Belated thanks to Luke for the excellent color reprocessing reprocessing of the image!

Unfortunately, I found out afterward that it is distributed under the creative commons noncommercial no derivative works license. My reprocessing is clearly a derivative work. I'll probably take it down soon, just to keep good karma.

Luke

Thucydides said...

Applying gravitational torque to a planet can be done, most easily by directing asteroids to do flypasts of the planet in question. The energy exchange will take a long time, or a mass chain of asteroids (or using asteroids to move a small moon on the correct trajectory).

Of course, there may be other means to do so, and by the time we are able to do things like go to distant exoplanets we might also be able to manipulate the Higgs field and spin the planet like a top once all the inertia is gone...certainly not mid future at all.

Being able to travel to distant stars implies the ability to manipulate huge amounts of energy (either brute force like STL starships or more subtle manipulations of space-time), so if anyone is so inclined, I doubt there would be very much difficulty in manipulating planets.

Milo said...

Thucydides:

"Applying gravitational torque to a planet can be done, most easily by directing asteroids to do flypasts of the planet in question."

And how are you moving those asteroids? Yeah, if you take it as given that you can move huge amounts of mass at will, then that lets you accomplish a lot. People tend to gloss over the matter of how difficult it is to move all that mass.

And gravitational torque is going to apply very small amounts of rotation for the amount of effort each asteroid flyby takes. It's the rough equivalent of trying to dig a tunnel through a mountain by blowing at it. Except now you have to ship in the "air" from a remote part of the solar system.


"Of course, there may be other means to do so, and by the time we are able to do things like go to distant exoplanets we might also be able to manipulate the Higgs field and spin the planet like a top once all the inertia is gone...certainly not mid future at all."

Well yeah. If you have magic that surpasses the currently known laws of physics, then who knows what you could do with it. Now, I'm not against magic, since I'm postulating FTL travel. But it's kind of pointless to speculate about it.

All things considered, I still say that I'm more interested in what life could look like on a tidally locked planet, not how to brute force it into becoming just another humdum planet little different from Earth (which is a massive amount of effort for a small increase in livability). I'm sure that, when we have the ability to travel the stars, it will be a long long time before we run out of naturally humdum planets in the Milky Way.

Yeah, maybe these future humans have the power to design planets with whatever characteristics they want, even create new planets from scratch if they put their minds to it - but in that case there's hardly any point speculating about the implications of any planet's natural characteristics. The answers can be anything you want them to be.

Luke said...

Okay, old copyright violating picture has been taken down. Here are some new ones:

Dry Falls, WA
http://panoptesv.com/Brushstrokes/Pics/SciFi/ThisWorld.JPG

How Dry Falls, WA would look if it was on Gleise 581g
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld.JPG

Mattawa, WA
http://panoptesv.com/Brushstrokes/Pics/SciFi/ThisWorld2.jpg

Mattawa on Gleise 581g
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld2.jpg

Olympic peninusla
http://panoptesv.com/Brushstrokes/Pics/SciFi/ThisWorld3.jpg

Olympic peninsula on Gleise 581g
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld3.jpg

Somewhere between Lompoc, CA and San Jose, CA
http://panoptesv.com/Brushstrokes/Pics/SciFi/ThisWorld4.jpg

And on Gleise 581g
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld4.jpg

Nick P. said...

Luke:

Wow, so a lot of those cheesy low-budget Sci-Fi movies and TV shows I've seen in the past that like to make liberal use of bizarre color filters might actually be on to something.

Life imitates art again it seems.

VonMalcolm said...

Re-bookmarking (I must have failed to check the follow-up comment box when posting last time.)

Milo said...

Reposting Luke's post with proper links:

Dry Falls, WA
http://panoptesv.com/Brushstrokes/Pics/SciFi/ThisWorld.JPG

How Dry Falls, WA would look if it was on Gliese 581g
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld.JPG

Mattawa, WA
http://panoptesv.com/Brushstrokes/Pics/SciFi/ThisWorld2.jpg

Mattawa on Gliese 581g
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld2.jpg

Olympic peninusla
http://panoptesv.com/Brushstrokes/Pics/SciFi/ThisWorld3.jpg

Olympic peninsula on Gliese 581g
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld3.jpg

Somewhere between Lompoc, CA and San Jose, CA
http://panoptesv.com/Brushstrokes/Pics/SciFi/ThisWorld4.jpg

And on Gliese 581g
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld4.jpg

Please remember this blog can do those, okay?

Milo said...

And Luke, I still say you're overdoing it. Our eyes readily adapt to different color temperatures - a green leaf, viewed indoors (in incandescent lighting that is even redder than Gliese 581g), still looks green. Everything may have a subtly "warmer" tone to it, but we would still recognize the colors as more or less the same and would probably only notice the difference when the scenes are put right next to each other. (I tried looking at your Earth Mattawa picture with my monitor's color temperature set to 9300K and 6500K - both much bluer than either Sol or Gliese 581, of course, but the only values my monitor permits. The latter results in the yellow plants looking slightly more prominent and the green ones slightly darker, while the former results in the yellow ones looking more faded and the green ones being more in the foreground. Anything that is yellow, green, gray, blue, etc. remains yellow, green, gray, blue, etc.) Your leaves look more rust-brown than green! (And don't say you changed them because the plants actually do have a different leaf color - reddish leaves would be less good at photosynthesis on Gliese 581g than on Earth.)

I can accept the idea that the sky and the water might look different, since the method by which those receive their color is quite different from the method by which most objects receive their color, and so may get tinted to a different degree than other objects, so our eyes won't adjust right (I haven't done the math, though). However, hands off everything else. It should look nearly the same as on Earth, with at worst some subtle tweaking that is hard to conciously notice.

(Sky color, at least, will also depend on how thick the atmosphere is...)

This site explains a little about color perception:
"Our perception is also very adaptive to the color of the light the subject is viewed under. For instance, consider scenes illuminated by different light sources, such as florescent, tungsten, and daylight. Most fluorescent lights are quite greenish, tungsten is very red, and daylight, normal. But our eye / brain compensates, and all of these scenes look perfectly normal and natural. Recorded on film, these scenes will have the color bias of the light source, which, if considered objectively, we would have to say is correct. But viewed in a photograph, which is subjective, we would consider the lighting in such scenes as wrong." (My bold.)

Your edited photographs perhaps show what an Earth-made camera would record on Gliese 581g, but not what our eyes would actually perceive.

Luke said...

Milo:

I did change the colors of the vegetation. I'm not sure what color they would be, but the wouldn't be green.

And here's another one for you
http://panoptesv.com/Brushstrokes/Pics/SciFi/AlienWorld5.jpg

Luke said...

Milo:

Changed the hue a bit, they're not quite as orange now. To me it looks more like a scene lit by incandescents.

Milo said...

Another thought re: effect of scattering on astronomy: would you be able to see stars in Earth daylight, if you looked through a filter that blocked bluer wavelengths of light (while passing through redder ones), with no other visual assistance?

Jim Baerg said...

Re: Changing the orbit or rotation or a planet.
As Milo notes the energy requirements to that would be enormous.
However, I think there would be some marginal cases where a relatively modest effort could produce a large effect if it was desired.

If the orbit of the planet is near circular then one face permanently facing the star is the stable situation, but if the eccentricity of the orbit is greater than some amount a 2:3 revolution:rotation ratio like Mercury
is more stable. If the orbital eccentricity is in a certain range a relatively modest nudge could shift the planet between 1:1 & 2:3 spin locking.

Would 1:1 or 2:3 spin locking make for a more pleasant place to live? If a planet had Gleise 581g's orbital period but a 2:3 spin locking most points on the planet would get 36 earth days of light & 36 of dark. As someone who likes skiing I wouldn't mind the 36 days of cold, but the dark would be inconvenient. I think a colony on such a planet could put up a mirror in roughly 24 hour orbit. The mirror should reflect enough sunlight to allow human color vision but need not be enough for photosynthesis. Something similar would be wanted for a terraformed Venus or any other planet with similarly slow rotation.

Milo said...

Doesn't tidal locking take millions of years, though?

I was under the impression that, going purely by orbital mechanics, all planets will gradually drift towards becoming tidally locked. However, depending on the particular situation, this might take anywhere from "a few million years after the planet forms" (i.e. instantaneously) to "longer than the projected lifetime of the parent star" (i.e. never).

While you could in some situations, theoretically, change the system's "steady state" with a minor nudge, it would take impractically long amounts of time to actually converge into that "steady state".

Luke said...

Milo:

Another thought re: effect of scattering on astronomy: would you be able to see stars in Earth daylight, if you looked through a filter that blocked bluer wavelengths of light (while passing through redder ones), with no other visual assistance?

I have wondered the same thing. I know the SR 71 used a star tracker that worked in daytime for navigation, but the SR 71 also flew above most of the atmosphere.

Milo said...

Another consequence of different light wavelengths: less light for marine organisms. Water strongly absorbs red light, so below certain depths only blue light will make it through (and even that is eventually absorbed).

Thus a redder sun means that, given equal amounts of light at the surface, the deep seas will more quickly turn black. This means that the life zone where photosynthesis can take place will be shallower. (On Earth, the zone is estimated as reaching down to about 200 meters - fortuitiously matching the depth of continental shelves. I wonder how marine life coped in eras when the shelves weren't so accomodating.) Or to put it more bluntly, life in the seas is (at least in one way) harder on Gliese 581g than on Earth. Given that life started in the seas, this is a little inconvenient.


So speaking of which, even though it has little to do with Gliese 581g, I'm driven to ask: does anyone know what the spectra of other solvents like ammonia, methane, ethane would look like? (What color would they appear to be at the surface? What wavelengths would be most visible deeper down? How quickly would light levels fall off with depth?)

Also, has anyone estimated the optimal zones for planets with liquid surface ammonia/etc., and determined how small the planet's sun would have to be for such a planet to be tidally locked?

Rick said...

More wonderful images and discussion. The location 'somewhere between Lompoc, CA and San Jose, CA' looks very familiar - I'm pretty sure that spot is about 4 km north of where I live.

Love the visible planets in the sky!

The relation between what a photo records and what the eye perceives is way above my pay grade. But we notice day by day subtle changes in the cast of light, and I think we would notice that - a golden hue like some late-afternoon light.

Of course in time anyone living there would regard it as normal and cease to 'notice' it.

I've never seen any discussion of suitable zones for other planets, the spectra of other fluids that might form 'seas,' etc. Interesting questions, though!

Jim Baerg said...

How stable is the orientation of a tide locked planet?

IINM the moon is slightly off spherical & the long axis points toward & away from the earth. Since there has been negligible tectonic activity on the moon in the last billion years that long axis hasn't changed.

However, an earth size or larger world like Gleise 581g will have tectonic activity that would shift the mass distribution so that the most stable orientation of the planet then differs from the orientation it has. Then there would be a sudden (by geologic standards) shift in the planet & life would have to deal with changed climate zones after possibly eons of stability.

Non-native intelligent life colonizing a tide locked world might decide that a different orientation would put more land in a pleasant climate zone. Perhaps mirrors to selectively melt a carefully chosen part of the darkside icecap could produce a desired shift in orientation.

Milo said...

Jim Baerg:

"IINM the moon is slightly off spherical & the long axis points toward & away from the earth. Since there has been negligible tectonic activity on the moon in the last billion years that long axis hasn't changed."

I haven't checked, but that sounds like the result of tidal effects. Just as the moon causes tides on the earth, the earth would cause tides on the moon.

The thing is that the earth rotates so fast that the moon's tides on the earth keep pulling in a different direction, so only liquids (water) can keep up with the ever-changing tides. By contrast, the moon always has the same side pointed toward the earth, so even solid rock would gradually get stretched into its tidal equilibrium shape.


"Then there would be a sudden (by geologic standards) shift in the planet & life would have to deal with changed climate zones after possibly eons of stability."

I would be more worried about climate zones shifting due to tectonic drift (which affects not only what latitude you're on, but also changes sea currents, raises mountains that cause rainshadows and sunshadows, etc.), so most of the planet remains in the same orientation but now your continent is in a different place relative to most of the planet. Life copes by getting up and moving, or by evolving to fit new conditions. Even plants can "move" on the timescale we're discussing here - as the continent drifts poleward, only seeds that the wind carries terminatorward of their parent will tend to flourish, and so the forests will gradually shift terminatorward. As long as there's still land in that direction to settle.

In fact, that could be a serious problem for evolution on tidally locked planets. On Earth, there have been eras where most of the land was in one hemisphere, with the other one being almost entirely ocean. On a tidally locked planet, if most of the land happens to have ended up on the dark side, where does land-based life evacuate? Or if you have less than 50% sea cover, than what if most of the sea gets shifted onto the dark side?

So how much tectonic activity should we expect anyway? Would the planet's slow rotation reduce tectonics? Would tidal heating be a factor, like it's commonly cited for gas giant moons?

There are other sources of instability, like volcanism/meteor impacts changing greenhouse levels, new life forms evolving or migrating in from another region and forcing ecosystems to adapt to their influence...


"Perhaps mirrors to selectively melt a carefully chosen part of the darkside icecap could produce a desired shift in orientation."

I don't know if tectonic effects can actually change a planet's orientation by a meaningful amount (I thought that was largely the result of orbital perturbration from other objects in the solar system?), but you would definitely need to do more than melt a few meters of water on the surface of the crust.

Jim Baerg said...

"On Earth, there have been eras where most of the land was in one hemisphere, with the other one being almost entirely ocean."

Like the present era. The Pacific ocean takes up most of one hemisphere.

"I don't know if tectonic effects can actually change a planet's orientation by a meaningful amount (I thought that was largely the result of orbital perturbration from other objects in the solar system?), but you would definitely need to do more than melt a few meters of water on the surface of the crust."

The Antarctic ice cap is a few km thick. I don't see any reason for the darkside ice cap on a tide locked planet to be much thinner.

Stephan Berlin said...

Properties of the Kepler mission

To my understanding, Kepler detects exoplanets by registering recurring "dips" in the stars observed brightness.

A certain number of dips is needed for detection: you can figure that would be at least two for a first estimate, at least three for a verified match.

Kepler has been active only since May 2009. The first detected planets were published in January of 2010. Those were all planets with an orbit extremely close to their star, naturally, as only they could make three or more passes in eight or less months.

Gliese 581g, again, is a planet with a close orbit, but not so close as the planets from the first batch, as more time was available to detect it.

So, as Keplers mission time proceeds, more planets with an orbit similar to Earth will be detected; as Milo wrote above, that is not because those are more common; but it's also not precisely about sensitivity of equipment! It's about the time needed for the detection of planets with certain orbits, dictated by the properties of the Kepler mission. We just need some more patience.

On the downside, planets with an orbit more far from their star will have a lower chance of passing between their star and Kepler, thereby decreasing the chance of detection. This would explain what I perceive to be a "slow down" in published detections over the year 2010.

KraKon said...

Working backwards:

"Those were all planets with an orbit extremely close to their star, naturally, as only they could make three or more passes in eight or less months."

You mean there will never be a chance for us to detect, say, an exo-Neptune. Despite it being a big and mighty gas giant, it's cold and only passes betwwen us and its star once every 164 years !

"I don't know if tectonic effects can actually change a planet's orientation by a meaningful amount"

I think that depends on the mass ratios of the two bodies (say Pluto/Charon), and the viscosity of the planet's composition. A hot planet with a large liquid core could absorb most of the tectonic effect through compression of fluids. A cold rocky planet would bear the brunt of the gravitational tensions and variation.

"In fact, that could be a serious problem for evolution on tidally locked planets. On Earth, there have been eras where most of the land was in one hemisphere, with the other one being almost entirely ocean."

Reminds of how much water Earth actually has...

"On a tidally locked planet, if most of the land happens to have ended up on the dark side, where does land-based life evacuate? Or if you have less than 50% sea cover, than what if most of the sea gets shifted onto the dark side?"

If we have a tidally locked planet with lots of water and convinient heat distribution through a thick atmosphere, I see the worst case scenario where half the planet is ice, the terminator a sea of meltwater, then anything above quickly transitions from saturation-point humidity to arid desert. In other words, the only land to set foot on is well beyond the water you need.
Flash idea:
Life develops in the marshes just above the terminator, where water levels are kept low because of the heat, yet humidity is at saturation point thanks to giant rainforest clouds above it. The evolutionary pattern would be entirely amphibian for a long period of time, protecting themselves from the direct sunlight under a few cm of water. Life would tardively develop from there into fish-like forms that exploit the vast expanses of colder water towards the terminator, or landforms that can support the heat for a little while or burrow to stay underground...

And if we have enormours winds, wouldn't that translate into a massive sandy desert on the hot pole, blowing sand all over the place?
Flash idea:
Underwater life forms use nutrients carried by sand blown from the hot pole that land on water and dissolve for the benefit of aquatic plants. Still doesn't solve the sunlight problem through...

"So how much tectonic activity should we expect anyway? Would the planet's slow rotation reduce tectonics? Would tidal heating be a factor, like it's commonly cited for gas giant moons?

"There are other sources of instability, like volcanism/meteor impacts changing greenhouse levels, new life forms evolving or migrating in from another region and forcing ecosystems to adapt to their influence..."

Probably something we are wholly unable to predict.

On planet-wide storms.

As the planet doesn't rotate, there won't be any atmospheric cells powered by the planet's rotation. This seriously hinders storm formation as these cells force hot wind up a cold front...

Maybe a planet wide cloud cell, going from one pole to another, powered only by convection? It'll die out at the poles though...

Milo said...

KraKon:

"If we have a tidally locked planet with lots of water and convinient heat distribution through a thick atmosphere, I see the worst case scenario where half the planet is ice, the terminator a sea of meltwater, then anything above quickly transitions from saturation-point humidity to arid desert. In other words, the only land to set foot on is well beyond the water you need."

It's not clear to me what you're postulating here. How much land/water?

To me, the worst-case scenario is that the light side is either 100% land or 100% water, or at least has so little land and water that it limits biome opportunities on there.

Let's take the 100% water case. The problem isn't that land animals could never evolve, since the fish could just wait a few eons for continental drift to bring in a continent before they try to crawl onto land. (Remember, we're talking about red dwarf planets. They have time to spare.) The problem is that land animals would have trouble evolving very far, since when in some later era the continents all drift back to the dark side, everything on land has practically nowhere to go, and so goes extinct. Some "land animals" could survive by going back into the ocean, like dolphins, but next time land appears they'll have to reinvent legs practically from scratch. And land plants will be gone too, don't forget that. On Earth, changing conditions from continental drift renders some organisms extinct, but enough survive to preserve major evolutionary innovations and repopulate the land.

So you won't get advanced land life. You could get tetrapods, but you won't get tetrapods that evolve into amniotes that evolve into synapsids that evolve into mammals that evolve into primates that evolve into humans.

Conversely, if you have more land than water, then most sea life will periodically go extinct. This is even worse, since life starts in the oceans - although hopefully microbial mats can adapt to land life early on. It's probably only later when you get complicated multicellular life that these extinctions really put a crimp in your evolution. Basically, picture today's oceans if all fish had gone extinct at the K-T event, and cetaceans ended up evolving to take all their niches.

Milo said...

KraKon:

"Life develops in the marshes just above the terminator, where water levels are kept low because of the heat, yet humidity is at saturation point thanks to giant rainforest clouds above it. The evolutionary pattern would be entirely amphibian for a long period of time, protecting themselves from the direct sunlight under a few cm of water."

...You need to protect yourself from direct sunlight, when you're at the terminator and have heavy cloud cover?


"And if we have enormours winds, wouldn't that translate into a massive sandy desert on the hot pole, blowing sand all over the place?"

Actually, I think the enormous winds are... umm... overblown. Many articles claim climate simulations that gave more Earthlike windspeeds, at least at the lower troposphere, which is what really matters. You could still have some storms, but so do we.

I admit bias, since this is how I like it. Earthlike wind strengths to facilitate life, non-Earthlike wind directions and patterns so that life is exotic.


"Flash idea:
Underwater life forms use nutrients carried by sand blown from the hot pole that land on water and dissolve for the benefit of aquatic plants. Still doesn't solve the sunlight problem through..."


Well, remember my suggestion that what little life exists on the dark side would depend on plankton and runoff from the light side. These could be aeroplankton, too, which could provide a way to supply nutrients on top of the ice sheets covering the dark side's ocean.


"Probably something we are wholly unable to predict."

We can't fully predict the course evolution will take, of course. We can't even confidently say anything about our own planet's evolution that isn't backed by fossils. But we can still try to recognize the broad strokes.

Luke said...

Milo:

To me, the worst-case scenario is that the light side is either 100% land or 100% water, or at least has so little land and water that it limits biome opportunities on there.

I'm going to do some wild speculating here - without modeling the dynamics, I don't know if it will work. But basically, consider the tidal forces operating on the planet. They tend to pull in out into an ellipsoidal shape, with the long axis pointing at the star. This orientation is stable - perturbations that knock the long axis away from parallel to the radius vector from star to planet results in a restoring force that brings the long axis back into the preferred orientation. So, suppose you have a large continent somewhere on our planet. This introduces a dyssymmetry in the distribution of mass. Tidal forces will tend to exert a torque on this extra continental mass sticking away from the planet's core, to either pull it toward the sub-stellar point or the anti-stellar point. If the torque is strong enough (and this is the point I am unsure about) you will end up with the largest continental land masses sitting on either one of these two extremes. This could leave us with a permanent warm side land mass for life to evolve on.

This is not enough to publish in a peer reviewed journal, but could suffice for fiction.

Thucydides said...

I would guess that situation would apply whenever the land masses get together and form or reform a supercontinent like Pangaea. At other times, continental drift would have very little effect on the planet's orientation.

The big danger to any native life on Gliese 581g may be precession of the axis, which will shift the hot and cold poles and move the habitable zones as well. Mars has had some pretty dramatic shifts in the past, which may explain the change from "warm/wet" to the current "cold/dry" climate. How this will work on a tidally locked planet is unclear to me; the gravitational forces holding the planet in a tidal lock will be pretty strong, but perhaps the actions of the other planets in the solar system might have soe effect?

Milo said...

Thucydides:

"I would guess that situation would apply whenever the land masses get together and form or reform a supercontinent like Pangaea. At other times, continental drift would have very little effect on the planet's orientation."

I think that if anything, it is more likely that tidal pull would influence continental drift, not the other way around.

Like Luke says, this is really something someone would need to simulate with some rather accurate computer models to be sure... We haven't even worked out how much normal continental drift you'd get in a very slowly rotating planet.

However... I just realized something. While the planet will be more elongated along the axis pointing toward/away from the sun, so will the seas. (Remember, these are tides we're talking about.) Since I'd expect them both to get pulled by the same amount (over really long time scales where rock's rigidity can be overcome), this means that the land elevation above or below sea level should not be influenced by whether you're at the poles or the terminator. So much for that.


"Mars has had some pretty dramatic shifts in the past, which may explain the change from "warm/wet" to the current "cold/dry" climate."

I think this is more a result of Mars losing its tectonic activity, and consequently, it's atmosphere. With its atmospheric pressure dropped below the triple point of water, it will forevermore remain dry, regardless of temperature. Unless you can put back the atmosphere.

Tectonic activity influences your atmosphere due to effects like a magnetic field (which protects against solar wind stripping your atmosphere) and volcanic outgassing (adds new air to replace losses), but I'm afraid I can't tell you the details (how important each is, what affects each's rates, etc.).

Precession of the axis would only cause local changes as the north and south poles move. There would still be north and south poles, and so climates would move with them.


"The big danger to any native life on Gliese 581g may be precession of the axis, which will shift the hot and cold poles and move the habitable zones as well. [...] How this will work on a tidally locked planet is unclear to me; the gravitational forces holding the planet in a tidal lock will be pretty strong, but perhaps the actions of the other planets in the solar system might have soe effect?"

If you can tilt the axis enough so it's not perpendicular to the ecliptic, then this will introduce seasonal effects to the planet. Within the polar circle, you would experience actual day and night cycles, with day during summer and night during winter (independantly of which side of the planet you're on - although that would still affect temperature). Below the polar circles, light and darkness would be based only on which side of the planet you're on, like on a proper tidally locked planet (independantly of whether it's summer or winter - although that would still affect temperature).

If you shift the axis relative to the planet's geography, but the tidal lock is stronger than your precession and restabilizes it so this new axis is still perpendicular to the ecliptic, then all you've done is shift the geography a bit relative to the poles (which return to their normal place). Although the underlying mechanics are different, effectively this is just continental drift.

Milo said...

This is not a problem, though. Constant climate change is a fact of life. Just look at Earth's evolutionary history. Life adapts - as long as there's something to adapt to. The danger is if there's nowhere left to evacuate to - if the planet shifts into a position where life would have trouble achieving high biodiversity even if that position were stable. (Good for biodiversity: a patchwork of land and sea in different temperature zones, supporting multiple biomes. Good for biodiversity: coastlines, which result in rainier land and sunnier sea. Bad for biodiversity: deep oceans with no coastline in sight. Bad for biodiversity: all water getting locked up on the frozen side.)

Of course, Earth has a non-life-supporting era too, back during Snowball Earth. Fortunately, such extremes are rare enough that we had time to go from invertebrates -> fish -> tetrapods -> amniotes -> mammals -> primates -> humans (and many other branches) before another similarly extreme condition arose. However, all-land-in-one-hemisphere is a condition that arises somewhat more often.

Stephan Berlin said...

I actually didn't think it through, but it's true: to detect an exoplanet with a Neptune like orbit, Kepler would need to observe its star for at least 328 years.

Furthermore, since Neptun's size is only the 200.000 part of its orbit, that would be the approximate chance that it would pass between its star and our position, making it detectable by Kepler in the first place, assuming that the alignment of orbits between star systems is evenly distributed.

Luke said...

Milo:

However... I just realized something. While the planet will be more elongated along the axis pointing toward/away from the sun, so will the seas.

Quite. This was one thing that always bugged me about GDW's otherwise excellent Aurore sourcebook (for ther 2300 AD game). Aurore was a tide-locked planet, with huge sub-stellar mountains due to the tides. In general, the surface of the world will follow the gravitational equipotentials (or orbital-gravitational-centrifugal equipotentials, for adding in additional inertial forces). It is easy to show that the gradient is always perpendicular to the equipotential, and the force is the gradient of the potential (with a minus sign), so if the surface deviated from the equipotential surface, there would be a gravitational force pushing it sideways until it settles into the equipotential surface. So, on a perfectly fluid world, even though the sub-stellar point will tend to bulge toward the sun, it will never seem like it is uphill.

this means that the land elevation above or below sea level should not be influenced by whether you're at the poles or the terminator. So much for that.

It works the other way around, I think. At least according to my proposed mechanism, any large concentration of mass which would naturally occur anyway (such as a continent) would cause a torque that shifts the planet's axis so that the mass is at the sub-stellar or anti-stellar region of the planet. This is opposed to mechanisms that have the tides shape a planet by deforming their hydrostatic equilibrium shape into something that rises above the sea at the sub- or anti-stellar regions (the tides will raise the land into a hump, but they will also, as you noted, raise the seas as well).

I am not sure how much my proposed mechanism can shift things around. After all, you don't find earth's equator shifting so that large land masses lie along it, and this would involve a very similar mechanism.

Milo said...

Luke:

"It is easy to show that the gradient is always perpendicular to the equipotential, and the force is the gradient of the potential (with a minus sign), so if the surface deviated from the equipotential surface, there would be a gravitational force pushing it sideways until it settles into the equipotential surface."

An intuitive way to see this is: Stuff rolls downhill. Therefore, in order for stuff to not feel like rolling anywhere, there has to not be a hill.


"I am not sure how much my proposed mechanism can shift things around."

I recommend remembering that if gravitational-centrifugal-tidal effects held perfect sway, then all planets would be perfectly smooth with no mountains anywhere. (I.e., the surface would follow those equipotentials you mentioned.) Overall, I think a planet (several thousands of kilometers of rock and iron) can flatten a crust (several tens of kilometers of rock and water) faster than the crust can meaningfully tilt the planet. The geologic time scales we're discussing here have not allowed crusts to be completely flattened, so I doubt a continent will have very significant effects on a planet's axial tilt before tectonic drift causes the continent to up and move somewhere else again.

Rick said...

Welcome to a couple of new commenters!

Our current detection methods, especially radial velocity and transit, are biased toward close-in planets, both in producing stronger signatures and producing them on a faster time scale.

Currently, distant planets are detectable mainly when young, because they are hot enough to be detectable directly in the IR, and because they can distort circumstellar dust rings in a detectable way.

Milo said...

Well, planets that young aren't going to have life now, but presumably there are young planets out there right now that are going to develop life someday...

KraKon said...

Continental drift?

I think tectonic activity and hot spots extruding lava are much more important factors in continental drift than whatever tidal influence the sun has on the planet...Is Gliese a tectonically dead planet?

Ocean planet:

I see problems with that. The only way to have an ocean deep into the hot side of the planet is for it to rain constantly to replace losses by evaporation. To rain constantly, you need big constant clouds. As pointed out to me before, big clouds constantly overhead means that sunlight doesn't reach you, but it alse means massive runaway global heating. Maybe climatic cycles where clouds slowly gain ground on the hot side, until they cover the whole planet, trigger Venus like conditions underneath, leading to the dissipation of the clouds (no more evaporation on the hot side as sunlight doesn't go through) and the regression all the way to an equilibrium point...

"The geologic time scales we're discussing here have not allowed crusts to be completely flattened,"

They can only be flattened long after the planet becomes a cold, hard rock, and then a few billion years. Erosion is a million times faster!

Milo said...

KraKon:

"Is Gliese a tectonically dead planet?"

We don't know! We haven't seen it!

I expect heavier planets to stay tectonically active longer, which counts in Zarmina's favor. However, I don't know how spin rate affects things. There's also the possibility of tidal heating to consider.


"The only way to have an ocean deep into the hot side of the planet is for it to rain constantly to replace losses by evaporation."

Umm... no. I don't know if you were aware of this, but water has a tendency to flow. As long as the ocean is high enough to flow over obstacles, its elevation will even out planet-wide, regardless of rain levels.

Elukka said...

On the note of detecting planets - how far away are we from snapping photos of nearby terrestrial exoplanets? There are already several direct pictures of gas giants... They're larger than Jupiter, some of them just on the limit between planet and brown dwarf, but somehow we do have pictures of exoplanets 129 light years away.

Sean said...

I think a premise behind the proposed Terrestrial Planet Finder was that it would be able to detect light reflected from planets using a telescope at least 100x more precise than Hubble.

Jim Baerg said...

I know I was the one who brought up the idea of tectonics producing a bump that would shift the stable orientation of a tide locked planet, but on further thought I realize that Isostasy would probably prevent that from happening. However since it has been taking millenia for isostatic rebound to undo the depression produced by the ice sheet over Hudson Bay, my suggestion that melting part of the darkside icecap might have such an effect still stands.

Re: continental drift putting all the continents on the dark side. That might not be all that bad.
Consider putting the present day earth in tide lock with the subsolar point in the middle of the Pacific Ocean. All the continents would get glaciers a few km thick on them which would lower sea level by a km or 2. This would expose a fair bit of land in the shallower parts of the Pacific.

Milo said...

Jim Baerg:

"However since it has been taking millenia for isostatic rebound to undo the depression produced by the ice sheet over Hudson Bay, my suggestion that melting part of the darkside icecap might have such an effect still stands."

Yes, and spinning a 19 zettaton planet using the tug of a thin section of its crust would take less than a millenium?


"Consider putting the present day earth in tide lock with the subsolar point in the middle of the Pacific Ocean. All the continents would get glaciers a few km thick on them which would lower sea level by a km or 2. This would expose a fair bit of land in the shallower parts of the Pacific."

Hey! I hadn't thought of that! Good observation!

Problem solved, then :) Yay!

Luke said...

Jim Baerg:

Consider putting the present day earth in tide lock with the subsolar point in the middle of the Pacific Ocean. All the continents would get glaciers a few km thick on them which would lower sea level by a km or 2. This would expose a fair bit of land in the shallower parts of the Pacific.

Papers I've read on heat transport by atmospheres of tide-locked planets indicate that this mechanism will be kind of iffy. Mainly because heat transport is so efficient that with an earth-like atmosphere the temperature on the dark side will be just about freezing. A thicker atmosphere puts the temperature above freezing. My understanding is that this is different from earth because earth's Hadley cells insulate different parts of the atmosphere from each other, preventing efficient heat transport. Of course, these simulations were fairly crude, and even our most sophisticated climate models (famously) are not always accurate at reproducing the climate.

Link:
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-45MFXSB-37&_user=2741876&_coverDate=10%2F31%2F1997&_alid=1499890508&_rdoc=1&_fmt=high&_orig=search&_origin=search&_zone=rslt_list_item&_cdi=6821&_sort=r&_st=13&_docanchor=&view=c&_ct=1&_acct=C000058656&_version=1&_urlVersion=0&_userid=2741876&md5=a08f438ea7240802a603dbd923d3b61d&searchtype=a

Summary:
http://www.treitel.org/Richard/rass/tidelock01.txt

This neglects heat transfer by the oceans, which would be likely to increase the dark side temperature further if oceanic currents go from the day side to the dark side (which might not be the case if all the land is on one side and all the ocean is on the other).

Anonymous said...

i think the new planet sounds a little like a piece of space junk flied into the orbit of this star.but it can be possible to have a new planet in that orbit.i would like to go, but it would take hundreds of years to get there.cant live that long!!!

Milo said...

Here's some new research on tide-locked climates (that's an article, the actual paper is here, Figure 1 and Figure 5 are the most readable bits).

It seems that when cooling rates are fast, temperature follows the behavior you'd expect - hot on the pole directly under the sun, growing colder in all directions outward. Prevailing winds seem to largely go from cold to hot, but not quite in a straight line and they look rather chaotic - although the cold-to-hot trend is still pretty consistent across the entire planet (as opposed to Earth, where one third of the planet has poleward rather than equatorward winds). It's a little hard to be entirely sure how straight the winds are because the Mollweide projection they're using, while very good for non-tidally-locked planets where climates tend to be distributed by latitude, also isn't exactly the most intuitive for a planet dominated by inner and outer poles rather than north and south poles. Still, to a first approximation, "winds blow from the dark side to the light side" will do. (Presumably these are surface winds and the air makes its way back to the cold side in higher-altitude winds that aren't shown?) On a real planet, geography and stuff will influence things anyway.

When they tweaked the model for slower cooling rates where atmospheric transfer plays more of a role, though, they got a weird phone-shaped distribution, with the hottest parts being surprisingly close to the terminator, and the next hottest parts lying on a chevron between these. The entire arrangement is skewed to the western side of the lit hemisphere. Prevailing winds are deflected around this phone, appearing to circle counterclockwise around the northern warm spot and clockwise around the southern warm spot. Far from the chevron figure, the winds are plain clean westward ones.

Unfortunately, they don't state what influences these cooling rates, what reasonable ranges we can expect, or how they might justify the vast difference in cooling rate they had to put into the model to produce this chevron.

Milo said...

Interestingly, these models don't support the common claim that the substellar point would be so hot it turns into a "desert". The major factor forming Earth's deserts is not heat, but rather the prevailing winds blowing all clouds away from the horse latitudes.

In the simple tidally locked model, winds blow toward the hot pole, meaning that as long as temperatures remain below boiling, it'll be more like a rainforest than a desert.

In the chevron model, the entire pattern is actually offset from the pole, but it still looks like the rainiest region would be the warm chevron. There is no substellar desert, although the rainforest regions are far removed from the inner pole by a distance of 15% the planet's diameter (60% of the distance to the terminator!), lying in directions 45 degrees from the equator in each direction.

Anonymous said...

ttc street car route car photo rally car snow cover car steering works auto de mediu