I’m getting kind of sick of seeing the headline: “THIS PLANET IS HABITABLE,” when there is no way to know that without much more precise and sensitive measurements. What astronomers are actually saying is they’ve found a planet that sits in the “Habitable Zone” around its star. “Habitable Zone” is just a way of saying, “If all the conditions were exactly right, this is the distance a planet would need to be from the star it orbits to have a temperature at the surface amenable to liquid water.”
Fact is, this planet is tidally locked to its star. One side broils while the other freezes. If it has an atmosphere (very likely it does, being so much bigger than earth), then the atmosphere probably circulates very strangely, making areas near the terminator experience milder temperatures, but probably some extreme weather as well. Worse, because Gliese 581 is so small and relatively cool (compared to our own sun), planet g must be much closer to it than earth is to the sun–as evidenced by its very short year. The problem with such proximity to the star is the danger of flares. Small stars don’t put out nearly as much energy as stars the size of the sun, so a flare, even a small one, would temporarily raise the amount of energy the planet receives by a lot. So much so that any life would probably get cooked.
I’m certain someone could point out other reasons planet g is probably not habitable, but then, what does “habitable” really mean? For the sake of argument, we’ll say here it means Humans could live on the surface without much–if any–environmental protections. How likely is this?
Well, first the planet needs life. The reason is simple: Oxygen is highly corrosive and would soon bond to other atoms or molecules unless there is some large-scale process (like photosynthesis) constantly dumping oxygen into the atmosphere. Even then, it is very possible that the amount of oxygen, other toxins in the air, or simply too much or too little pressure would make the atmosphere unbreathable. Add to this the simple point that a planet at least three times the mass of earth (if we assume similar average density) would have a surface gravity probably something like 1.5 to 1.7 times that of earth—meaning if you weighed 200 pounds on earth, you’d weigh 300 pounds on planet g. Not impossible to live with, but dang uncomfortable. And for the love of Pete, don’t trip and fall!
So even if planet g had all kinds of things going right, like a temperate climate zone, no stellar flares, oxygen-producing bacteria—even with all of these things, the air might be too thin, or poisonous, or there could be any number of other reasons the planet would not actually be habitable. At least not as-is. But terraforming is a subject for another day.
So… Is it possible that planet g has life? I’d actually bet on it. On Earth, life has proven quite tenacious. We are constantly being surprised by the places we find life, even if it is only bacteria or lichen. For cripe’s sake, we know of some organisms that can survive in the high-radiation near-vacuum of space and in the boiling hot volcanic vents at the bottom of the ocean. So regardless of other conditions, and because of the way carbon likes to make complex molecules, if there’s liquid water on Gliese 581g, I would bet on there being some kind of life.
Nice place for a vacation, though?
No. It’s probably more like Hell than Club Med.