Saturday, April 17, 2010

Worlds in Disorder

Crescent Neptune and Triton
Via Sky & Telescope, news that planetary systems Out There are even more of a chaotic mess than we thought before. Several newly discovered transiting extrasolar planets turn out to be orbiting retrograde with respect to the rotation of their parent stars.

This, to put it mildly, was not expected. Planets discovered by transiting, passing in front of their stars, tend to be 'hot Jupiters,' orbiting rather close in, since such tight orbits are more likely to have the needed edge-on positioning.

If, as generally assumed, planets form from a protoplanetary disk of gas and dusk, they should all be born prograde, and the subsequent encounters that might leave a planet in a retrograde orbit would be expected to produce large orbits, not small tight ones.

'The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,' said Andrew Cameron, who reported the findings to the Royal Astronomical Society. (How cool is that?)

An earlier Sky & Telescope article, which I missed at the time, gave hints of these findings, but also made a rather curious statement, from MIT exoplanet specialist Joshua Winn: 'Now we've gotten a glimpse of the weird, wild systems we've been hoping to find all along. Theorists will have a lot of fun trying to get the planets in these orbits.'

The observational study of exoplanets is only 17 years old, but this is a discipline that has already forgotten its origins. Its entire history has been one big joke on the theorists, with punch lines that continue to unroll. When haven't observed extrasolar planetary systems been weird and wild?

It's somewhat forgotten now, but the first discovered extrasolar planets do not orbit a sunlike star but a pulsar, a post-supernova remnant that was about the last place anyone expected to find planets. The first planet of a sunlike star, 51 Pegasi b, discovered in 1995, is a hot Jupiter, something so unexpected at the time that there was an argument over whether it was a 'planet' at all, or belonged to some other class of substellar object. (No one knew how soon planetary status even in the Solar System would become a matter of controversy.)

From a science fiction perspective, if anyone before 1995 had set a story in a planetary system typical of those we've discovered since (currently about 48 multiplanet systems are known), it would have been dismissed as implausible. I would have dismissed it as implausible. Jupiter sized planets in sub-Mercury orbits? Get real.

Big oops. At least for now it is our own system whose orderly grand-plan structure seems to be the exception. This itself may be a mere selection effect - if we were observing a twin of the Solar System, we would only just be able to detect Jupiter, and not yet any of the others. The extrasolar systems we now observe may turn out to be in the minority, the ones that happen to be easiest to find.

The obvious lesson here is that the universe is full of surprises. The only slightly less obvious lesson is that it is hard to project from a sample size of one. We will be surprised again, and repeatedly. Meanwhile, though, we have a lot more freedom of invention when it comes to planetary systems than we thought not so long ago.


The image, via Astronomy Picture of the Day, is of a crescent Neptune with Triton. (View the image carefully!) Chaos creeps even into our own orderly system; Triton is the only large moon in a retrograde orbit, and is probably a former Kuiper Belt object, captured during a Solar System past that is now thought to have been fairly turbulent itself.


Related posts: Extrasolar planets don't just orbit backwards. They collide, and take death plunges into their parent stars.


I should add (and just did, on edit) to treat the comments as a handy open thread for any pertinent - or even impertinent - observations.

22 comments:

Anonymous said...

Wow. Weird worlds, bizzare star systems...so an Earth-sized moon of a Jupiter-sized planet orbiting in a star's 'Goldylocks' zone could exist...the most extreme orbits for a habital world...'life-as-we-DON'T-know-it' living on Pulsar planets...THE UNIVERSE HAS GIVEN US LEAVE TO USE OUR IMAGINATIONS TO THE FULLIST!!!!

Sorry, got carried away...

Ferrell

VonMalcolm said...

I wonder if Sol has swallowed a Hot Jupiter or other planet in its earlier years: Does the presence and location of Mercury rule against this possibility?

Would a Hot Jupiter effect the Habitable Zone of its particular star in any way? If it does Sci-Fi writers have to take notice and adjust their worlds accordingly.

Citizen Joe said...

The stuff I read about this predicts that these erratic orbits would preclude the existence of smaller worlds in that area since the gravity would have either crashed them or hurled them into space.

But yes, gas giants in habitable zones. Most moons end up being tidally locked and thus always face the gas giant. That means long hot days and long bitter cold nights. For example, Europa has a 3.5 day orbit so Europan days are about 3.5 Earth days long. You could try for the twilight zones, but I think they will shift around as the host gas giant orbits its star.

Cityside said...

It wasn't exactly a hot jupiter in the zone, but one 2300AD sourcebook detailed a tidally-locked garden world that was a superjovian satellite

http://rpg.drivethrustuff.com/product_info.php?products_id=25&it=1

Thucydides said...

Our own solar system provides plenty of examples of weirdness (once we know what to look for!)

A Mars sized protoplanet crashing into the Earth to create the Moon; another possible giant impact to reverse the rotation of Venus; Uranus axis of rotation tipped at almost right angles to its orbital path; Neptune being much closer than theory suggests it "should" be (probably through gravitational drag as it plowed through the debris of the early solar system, ejecting material into the Kuiper belt or beyond...

Some theories suggest hot Jupiters are the result of gravitational drag as the new planets moved through very dense clouds of protoplanetary material, which suggests there will be massive asteroid belts in the outer solar systems of these star systems (the remnants of materials ejected by the hot Jupiter as it spiraled inwards). Exo solar civilizations will be more likely to be based on asteroids and free flying colony structures in deep space than Heinlein's sodbuster planetary colonists (which means actual habitable planets will be the most valuable real estate in the galaxy).

One thing which I've only seen touched on once (Building Harlequin's Moon, by Larry Niven and Brenda Cooper) is the interaction between the magnetosphere of the hot Jupiter and the local sun. Imagine the flux tube between Io and Jupiter magnified by several orders of magnitude, and there is a colossal energy source for a very high energy civilization. Of course the solar storms will be pretty impressive as well.

Living on a tidally locked moon of a Jovian planet (or a planet orbiting a red dwarf star) will require lots of planning, but like a magic show, it can be done with mirrors: a fleet of solettas orbiting the moon, planet or in carefully calculated free orbits can ensure proper illumination at all times. The colonists can also just dig into the crust and use fusion power to illuminate the site with huge "lanterns"...

Anonymous said...

So these worlds orbit quite close to their suns, and are therefore hot? And we can see these worlds because, from our point of view they transit in front of their stars? Their orbital characteristics mess up our previous theories of planetary development?

Members of the Royal Astronomical Society, I submit that the name for this class of objects is obvious...

Ian_M

Rick said...

The current theory - worth what you paid - is that hot Jupiters migrated inward very early in their history, presumably scattering any inner protoplanets along the way.

Whether this process might also give the migrating planet an impressive moon system is, I'd guess, an open question. The Solar System has no really massive moons, but then it has no really massive planets either.

A few years ago I wrote this planetary climate sim to explore the effects of high axial tilt, elliptical orbits, and long rotation periods. (Warning: It is both crude and user-hostile.)

A giant moon in a week long tide-locked orbit would have a spectacular diurnal cycle, and not even mad dogs or Englishmen had better go out in the noonday sun for more than a few seconds.

But as inland regions heat up, coasts will get a good afternoon sea breeze - perhaps approaching hurricane force.

A general thing I learned - to the degree that my simple model is valid - is the importance of oceans as thermal ballast. For example, by my model Arrakis would not be habitable. Lower latitudes would get broiling hot in summer, high latitudes would be Antarctica cold, and the transition zone would be too narrow and shifting to be of use.

I guess I'm not eligible for the Royal Astronomical Society (even apart from being 'Murrican), since the more appropriate name for a hot Jupiter is not a bit obvious to me. Though something about horns, pitchfork, sulphur, and brimstone comes to mind.

Anonymous said...

I can't be the only one who looked at this article and thought 'Hot Tranny Mess'. Right?

Right?

Ian_M

Thucydides said...

Lots of discussion on planetary formation, stars, space etc.

http://crowlspace.com/

Jean Remy said...

I think one of the issues with finding all those weird counter-model star systems and no normal Sol-model ordered (for a certain measure of ordered) system, is that the oddballs as far far easier to find. Sol-type stars are relatively dim, rather bland yellowish dwarven stars with unremarkable masses. We're more likely to get good visible data from larger stars.

I think Asimov once said in the Foundation trilogy that it was a "remarkably large gas giant, larger than most ever found in the entire Galaxy"... turns out THAT was wildly wrong. Jupiter is sort of unremarkable as far as gas giants go. I believe the critical mass of a Hot Jupiter is something like 11 Jupiter masses?

So we find superduper massive 11-Jupiter-mass orbiting massive, several-Sol-masses stars... and we find it odd that they do NOT behave the way our solar systems do?

We live in a Universe pretty much governed by the Rule of Weird. I love it when computer models completely fail to predict something, because then there's a mad scramble to figure out what went wrong

AND SCIENCE HAPPENS!

Stevo Darkly said...

An excellent blog that would also be of interest to readers of this one is:

http://www.centauri-dreams.org

It's a very interesting and pretty level-headed look at technological and astronomical news that relates to the possibility of, someday, interstellar travel.

Major subject areas include:

- Is manned interstellar travel possible? What tech advances or disoveries in physics might enable this?

- If interstellar travel is possible, where should we go? If interstellar communication is possible, where should we look or listen?

- What are we likely to find? What's it like Out There?

- If we find other intelligent species Out There, will we be able to communicate with them?

- I've noticed that the commenters here and at Centauri Dreams often discuss the same topics and themes -- especially of late.

There's lots of discussion there about "hot jupiters" too.

These two blogs could probably benefit from some cross-pollination.

Oh, the CrowlSpace blog that Thucydides mentioned above is a good one too, that also discusses subjects of interest to this crowd. Although it's not updated as often as the Centauri Dreams blog (which usually has a new post every day).

Cityside said...

"Hot Tranny Mess"

Google that at your own risk.

Rick said...

I think I'll avoid googling it at all.

Thanks for the links! More ways to spend time!

Anonymous said...

"Hot Tranny Mess"
EEEEWWWW!!!! and I didn't even have to Google it...

Ferrell

Citizen Joe said...

Ian: I'm less concerned that you would think "Hot tranny mess" than your assumption that everyone else would think that.

Rick said...

Well, actually only that someone else would come up with it.

Probably we all need to get out more.

Most of the stars searched by the various explanet search teams are solar type, and most of their planets are also (to us) oddballs. But there are definitely selection effects at work - basically we are finding the sorts of planets our instruments and techniques are good at finding.

Stevo Darkly said...

"But there are definitely selection effects at work - basically we are finding the sorts of planets our instruments and techniques are good at finding."

Very true. At this point in time, assuming hot Jupiters are "common" would be like assuming that lost car keys tend to collect beneath street lamps.

Rick said...

Yes. And that they tend to be on large or sparkly key rings.

But in the next few years we will start to learn a lot more. If a fair number of sunlike stars have 'normal Jupiters' in near circular orbits a few AU out, the radial velocity search times will have observed them through a full orbit. Or will have failed to observe many such planets. Plus other search methods are becoming more active.

We'll know a lot more - and in some ways be more constrained - in the next few years. That makes inventing planetary systems a tricky business.

My impression, based on far too little reading, is that SF has not yet much exploited what we've learned about extrasolar planets.

jollyreaper said...

Just to be contrarian -- when what we're observing flies so much in the face of theory, perhaps the observations are incorrect?

Stevo Darkly said...

In other words, "When the data conflict with the theory, abandon the data"? :) :) :)

Actually ... I am not an expert, but my impression is that the observations are pretty convincing/reliable. I have not seen anybody argue otherwise.

Citizen Joe said...

The raw data doesn't say anything, it is the interpretation that gets us these weird worlds.

Rick said...

Interesting question! The observations are indirect and subtle.

I'd guess that the problem is with the theories, constructed on a narrow foundation, the Solar System being the only planetary system we can observe closely. Samples of one are always tricky to theorize from!