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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...
You're looking for the simplest solution at a time when the available information is only getting better and more complex. It's likely to take a *lot* more work to rule out life on mars, and only slightly less work to confirm it.
Fermi's paradox only applies if you assume that if we'd been visited, you and I would know about it. I'm not talking government conspiracies, here, I'm talking about being so close to the the problem we can't even see ourselves in the middle.
The deeper we look into space, the more complex stuff we see. How much of that might have some sort of artificial origin that we don't understand? It's hard enough for us to tell if a terrestrial rock has been shaped into a tool, never mind what others might have shaped stars into.
Time was, we classified civilizations into categories based on the sheer volume of energy they consumed. More energy=more advanced civilizations. Except now that energy is more expensive than we had guessed, it might not be the best measure of advancement after all. Should we now start looking for more efficient ways of communication than radio?
I'm going to sound like a new age fruitcake if I advance any theories about it, but that's not my point. I just think it's premature to go making connections between the slime we find or don't find, and our own uncertain future. There's *plenty* to be worried about that we have a pretty good understanding- without also having to look to the stars to find ominous portents of danger.
I really liked Neuromancer's ending as it applied to this: Until something clever could emerge on earth, there's no other way to recognize the clear signal of other clever things.
It'd be like expecting an ant colony to appreciate the intelligence of a bee hive. There's no reason to expect them to interact at all, never mind establish diplomatic relations with each other.