Thursday, September 30, 2010

Descoberto um planeta habitável (II)?

Possible earthlike planet found in the Goldilocks zone of a nearby star!, por Phil Plait:

But the amazing thing is that the planet’s distance from the star puts it in the Goldilocks Zone: the region where liquid water could exist on its surface! (...)

If you’re too close to a star, it’s too hot to support liquid water. If you’re too far, it freezes. This defines a rough region from the star — the Goldilocks Zone, for obvious reasons — where liquid water can exist on the surface of a planet. This depends on the star, of course, but also on other factors like the planet’s atmosphere; Venus could have liquid water, but its super-thick atmosphere produces a runaway greenhouse effect which has heated it to 460° C (900° F). If Mars had a thick atmosphere, it might support liquid water! So the planet itself matters here too.

Gliese 581g, as the new planet is called, is in the zone where the temperature is just right. And with a mass of just three times that of the Earth, it’s unlikely to be a gas giant.

However, this does not mean the planet is habitable, or even very Earthlike. It may not even have any water on it at all. For now, we can’t know these things, so beware of any media breathlessly talking about life on this planet, or how we could live there.

There are some things we can speculate on with some solid footing. The orbital period of 37 days puts it pretty close to the star – since the star is a red dwarf, it’s cooler than the Sun, so being closer doesn’t necessarily mean you overheat. But it does mean the star exerts strong tides on the planet, which have the effect of slowing the planet’s rotation until it equals the orbital period. This has almost certainly happened to this planet, so in other words, one day on this planet = one year, and the planet always shows the same face to its star like the Moon does to the Earth.

That makes things a bit dicier for habitability. The side facing the star may get very hot, while the dark side gets very cold. If the planet has an atmosphere that gets mitigated somewhat (the hot air on the day side will flow over to the night side and vice versa, smoothing out the highs and lows in temperature), and may make the planet more clement. However, we have no clue if this planet has an atmosphere at all.

I also want to note that the mass found (3x Earth) is the minimum mass of the planet! It may be more massive, though it’s unlikely to be much more. The Doppler method doesn’t give an exact mass, only a lower limit. That’s frustrating, but that’s the way the math works out.
[Via Razib Khan]

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