I’ve come across two really interesting articles this morning on the habitability of planets, and the factors that affect that. Cloud cover is extremely important, and the more the better for keep a planet warm. Depending on the power of the star, it can add a great deal of room orbit in which the planet can travel yet. The atmospheric composition and pressure are also highly significant.
This is both in general, and in relation to one specific planet that has been capturing the minds of scientists recently – Gliese 581d. This is not Gliese 581g, which is the planet which might not exist at all. This instead we’re certain it exists, it’s the question of where it sits in the habitable zone that affects the science being undertaken.
At this point, I’ll step aside and let the articles take over.
A habitable zone can be defined in many ways, but for our immediate purposes, defining it with reference to liquid water on a planetary surface makes sense. Sure, we believe that life could exist beneath the surface on places like Europa, where surface water is out of the question, but the key issue is this: Are there atmospheric features that we could use to make the call on habitability? It’s an important issue because with our current and near-future technology, this is how we can plan to investigate life on planets around other stars. We can study exoplanetary atmospheres already and we’re getting better, but we can’t drill through exoplanetary ice.
GJ 581 has been a major player in the exoplanet story since the 2007 announcement from the Geneva team of the potential habitability of GJ 581 c. Gliese 581 itself is an M-class red dwarf, smaller and cooler than our Sun, and a variety of studies have suggested that a habitable climate might be possible even on a world that, like GJ 581 c, is tidally locked to a star like this one, presenting the same face to the star throughout its orbit. Subsequent work, though, makes a strong case that GJ 581 c is more like Venus than anything else, while GJ 581 d, further out, may just make it inside the habitable zone, depending on conditions in its atmosphere.