Much has been made of the announcement about the recently announced discovery of a potentially habitable planet around a nearby sun. The planet, dubbed Gliese 581g is estimated to be around 3 times the Earth’s mass, and about 1.5 x it’s diameter (depending on composition).
The excitment over this particular exoplanet – the 492nd discovered – is that it lies in the “goldilocks zone” where th etemperature is not too hot, not too cold. That is, its distance from the star is such that the surface temperature is likely to be such that liquid water could exist on the surface, something that is key to life as we know it.
However, the announcements touting “Scientists discover habitable planet” and “Earthlike planet discovered around nearby star” are grossly misleading.
It is true that Gliese 581g lies in the goldilocks zone. However, we know nothing of its composition, its amosphere, or its rotation. This latter is important, as the planet lies only 0.15 AU’s from its parent star. In contrast, Mercury 0.39 AU’s from our sun. 581g’s proximity to the star means the planet is likely tidally locked – in other words, one side always faces the star, the other away. One side would be a permanent, hot day, the other a permanent, frozen night. Even with an atmosphere to carry heat back and forth between the two sides, much of the water and other atmospheric gases could freeze out on the night side, leaving a thinner atmosphere, and permanent desert on the day side. The temperature along the terminator (the “twilight zone”) should be intermediate, allowing for liquid water, but not if all the water is frozen on the night side.
On the other hand, the star Gliese 581 is a red dwarf star (which is why the goldilocks zone i sso close), which is very stable and long lived, so if there were conditions suitable to life, it would have a good ling time to evolve.
Until we have the tools available to analyze the spectrum of the atmosphere, we really can’t know any more, as much as we want to speculate.
This doesn’t mean we shouldn’t get excited – far from it. The discovery of Gliese 581g tells us something very important. It tells us that there are two earth(ish) size planets orbiting in the goldilocks zone in a tiny volume of space (relatively speaking) – Gliese 581g and our own. the statistical implication is that goldilocks planets are likely common in the galaxy. And that means that even if 581g is a lifeless, half-frozen wasteland, there are more earthlike planets out there.