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We finally found a roughly terrestial exoplanet in a habitable zone! It's in a 36 day orbit around a red dwarf about 20 light years away and has a mass of about 3 Earths. Which is genuinely very exciting, and shows just how far our planet-hunting techniques have come. But it's important to remember, particularly with all the hype this is generating, that we don't know much more than what I listed above.
If you want the real details, I recommend the original paper: The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581. It's a good read, with some interesting material in there. I particularly recommend reading through the process of how they deduced the existence of the new planet. It looks pretty solid, but it's anything but direct. Basically, they're looking at the radial velocity of the star. They have a couple hundred measurements of this over an 11 year period. They run this through some kind of Fourier Transform, and then start looking for evidence of periodic perturbations caused by orbiting planets. There is quite a bit of noise in this, dealt with on a largely ad hoc basis. This shouldn't be surprising when you're trying to measure the velocity of a star 20 light years away to a precision of a couple meters per second, but I point this out mostly to emphasize how little we know about Gliese 581g.
So, could it be habitable? Luckily, I've been reading a lot about planetary science recently for a potential project mine. Thus I feel confident in being able to say... maybe? It depends a lot on how the planet originally formed -- did it get enough volatiles (hydrogen, oxygen)? Did it get more carbon than can be sequestered out of the atmosphere? Or not enough -- the habitable zone is only habitable assuming quite a bit of greenhouse effect. Did its core retain enough heat, needed to spin for a magnetic field and drive plate tectonics for longterm greenhouse balance? We simply can't answer these questions -- we can't even put probabilities on them, since we just don't have enough data or good enough models yet. There is a lot to be worried about, though. The system is low in metals (meaning elements heavier than helium in this context), which doesn't bode well for a hot core. Without a hot core, you can't have a magnetic field protecting the atmosphere from being sputtered (yes, that is the technical term) away. It also means no volcanism, which means the carbon being pulled out of the atmosphere through weathering processes can't be replenished. This combination might be what doomed Mars. The planet is close enough to the star to be almost certainly tidally locked, with one side baking and the other side frozen. Some newer atmospheric models show ways you could still get a habitable planet in that configuration, but it's not a good sign.
That said, to find a terrestrial planet in the habitable zone this quickly after getting serious about planet hunting is very promising. If you think finding habitable planets is a good thing. It's exciting, but I'm increasingly unsure it's a good thing. The more common life is in the universe, the more ominous Fermi's Paradox becomes -- the more it looks like technological civilizations inevitably blow themselves up. But that's a subject for another post.
If you want the real details, I recommend the original paper: The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581. It's a good read, with some interesting material in there. I particularly recommend reading through the process of how they deduced the existence of the new planet. It looks pretty solid, but it's anything but direct. Basically, they're looking at the radial velocity of the star. They have a couple hundred measurements of this over an 11 year period. They run this through some kind of Fourier Transform, and then start looking for evidence of periodic perturbations caused by orbiting planets. There is quite a bit of noise in this, dealt with on a largely ad hoc basis. This shouldn't be surprising when you're trying to measure the velocity of a star 20 light years away to a precision of a couple meters per second, but I point this out mostly to emphasize how little we know about Gliese 581g.
So, could it be habitable? Luckily, I've been reading a lot about planetary science recently for a potential project mine. Thus I feel confident in being able to say... maybe? It depends a lot on how the planet originally formed -- did it get enough volatiles (hydrogen, oxygen)? Did it get more carbon than can be sequestered out of the atmosphere? Or not enough -- the habitable zone is only habitable assuming quite a bit of greenhouse effect. Did its core retain enough heat, needed to spin for a magnetic field and drive plate tectonics for longterm greenhouse balance? We simply can't answer these questions -- we can't even put probabilities on them, since we just don't have enough data or good enough models yet. There is a lot to be worried about, though. The system is low in metals (meaning elements heavier than helium in this context), which doesn't bode well for a hot core. Without a hot core, you can't have a magnetic field protecting the atmosphere from being sputtered (yes, that is the technical term) away. It also means no volcanism, which means the carbon being pulled out of the atmosphere through weathering processes can't be replenished. This combination might be what doomed Mars. The planet is close enough to the star to be almost certainly tidally locked, with one side baking and the other side frozen. Some newer atmospheric models show ways you could still get a habitable planet in that configuration, but it's not a good sign.
That said, to find a terrestrial planet in the habitable zone this quickly after getting serious about planet hunting is very promising. If you think finding habitable planets is a good thing. It's exciting, but I'm increasingly unsure it's a good thing. The more common life is in the universe, the more ominous Fermi's Paradox becomes -- the more it looks like technological civilizations inevitably blow themselves up. But that's a subject for another post.
Re: unclear on the whole good / bad thing...
An alternative scenario that I find reasonably plausible is that there is a threshold ahead of us that a "typical" civilization (if it doesn't destroy itself) will steady state against. Maybe there are long-lived solar-system sized civilizations out there but interstellar transport of live organics is just not worth it (in the sense of having a negative expectation of marginal civilizational flourishing, given its huge energy cost). Maybe they quickly passed through a brief period of high-power broadcast radio before settling on more efficient and secure local networking and directed point-to-point interplanetary networking. If there were such a civilization in the Gliese system, could we tell? Frankly, if we were in the Gliese system, could we tell? Has our civilization ever put out enough nondirectional radio power to be seen against the sun?
Until we find evidence of the ruins of civilizations slightly more advanced than us, the absence of evidence of civilizations much more advanced than us is quite adequately explained either by the hypothesis that the dominant small terms in the Drake Equation are behind us or that advanced civilizations tend to enter a steady state in which they do not produce the kind of evidence in question.