Wednesday, April 25, 2007

Worlds Beyond II - "Super-Earth"

Scarcely had the electrons dried on my last post about extrasolar planets than some pretty dramatic news hit the wires: discovery of the most Earthlike extrasolar planet yet, orbiting the star Gleise 581, about 20 light years away. The star is a dim red dwarf - though quite close as stars go you would need a telescope to see it - and it was already known to have two other planets.

Earthlike is a relative term - I wouldn't recommend buying real estate there yet, even if we had a way to get there. The planet is at least eight times Earth's mass, thus probably about twice its size - possibly rather more, if it is largely made of less dense stuff than rock, as big planets usually are. All we know about it is its distance from the parent star, 0.073 AU* (about 10 million km), and its minimum mass - the mass may be greater, for reasons I'll go into if someone wants me to.

* An AU, "astronomical unit," is the average distance from the Earth to the Sun - about 150 million kilometers, or 93 million miles for those of us in countries where they still use medieval units. Get used to it, because I will be using AUs a lot here.
We also know how bright the star is - not very (about 1/200 as bright as the Sun) - from which we can determine how much sunlight, or Gleise 581-light, it gets. Even this little bit of information, however, is enough to take a fair first guess at the planet's temperature, which comes to about 0-40 degrees C (32-104 F) - a temperature that permits liquid water, the most basic stuff of life. It could be colder if it is covered with ice that reflects away most of the light that otherwise would warm it, or much hotter if it has a dense greenhouse atmosphere like Venus. Still, it is at the right distance to have liquid water, which is a very good start. Conceivably - pure speculation here - it could have a lot of liquid water, perhaps a hydrosphere several thousand kilometers deep, making Earth's oceans look like a thin muddy film by comparison.

This discovery isn't just cool - it is awesome.


Nyrath said...

More to the point, the fact that this system is a mere 20 light-years away drastically increases the estimate of the number of human-habitable planets in our neck of the woods.

Good times!

Bernita said...

Definitely exciting!

Rick said...

Nyrath - It certainly hints that there are indeed planets in stars' habitable zones, besides the one we're sitting on.

I suspect that the biggest contraint on human habitability may be not right-size-right-place planets, but their biological history - Earth has only been human habitable for roughly a tenth of its history.

Still it makes the spine tingle.

Bernita - that was quick!

EA Monroe said...

Hi Rick. I saw over at Bernita's that you had started a blog so I stopped in for a visit. On recently, there were photos of a "square" nebula -- or something like that. It was strange!

Rick said...

Hi, ea!

Was it this:

(I've long forgotten how to hand-code links!)

Apparently the squareness is a perspective effect, and it really looks more like two martini glasses joined stem to stem - but a remarkable and wonderful sight in any case.

Sam said...

How cool.
Would a larger planet mean more mass and considerably more gravity? Less light, hmmm. Tricky for most plants, but a hosta heaven, lol.
Let's see: 'For sale - half a million acres in uncharted territory. Bring your flashlights and anti-gravity devices...'

Rick said...

Sam - yes, surface gravity on this planet might be about twice as much as on Earth (2 g, another bit of geekspeak I'll be using regularly). No one can be sure yet, because we don't know how big it actually is or what it is made of - only an estimate of how massive it is.

My personal guess is that as much as half of it might be water, an "ocean" several thousand kilometers deep. That would make it larger than a rocky planet of the same mass, since water is less dense than rock, but the surface gravity would be a bit more comfortable.

It actually gets about as much total light from Gleise 581 as we get from the sun - the star is a lot dimmer, but the planet is a lot closer to it. A large proportion of the light, however, would be in the infrared - pretty useless to plants on Earth, I suppose, but any local life presumably evolved for those conditions.

To borrow a line from Arthur C. Clarke, if it does have an ocean thousands of miles deep ... think of the fishing!