Gliese (pronounced “GLEE-za” which means Israel will be embargoing it any day now) is a bit of an odd duck. It orbits a red dwarf, which means that plant life that uses photosynthesis isn't likely to fare well there. Not enough UV. Earth plants wouldn't do well at all. The star (Gliese 581) is very dim and very small, and Gliese is only about a half the distance from the star that Mercury is from our sun. As a result, the “year” is only 37 days.
Gliese 581g is the sixth planet found orbiting that particular star, and the first that isn't a gas giant.
Like our moon, Gliese is tidally locked, keeping one face turned to its primary, so a “day” on Gliese is...well, essentially forever. The sun does not move in the sky. If it's directly over head, that's where it stays. If it's on the horizon, you live in perpetual twilight. The far side is vampire heaven.
If it has an atmosphere something like our own (which is very unlikely) then surface temperatures would vary wildly. It might be constantly over 37 C. on the sunny side, and -100 C on the dark side. (The moon, which has effectively no atmosphere at all, varies by nearly three hundred degrees Celsius during its 28-day “day”.) In that case, the weather conveyor belts would kick up some fantastic winds.
But the atmosphere might be carbon dioxide, or methane, in which case the entire planet would be hotter than an oven as a result of the greenhouse effect. Venus is like that, and so maintains a temperature of a searing 800C at the surface, night and day. Makes Fresno look sorta decent by comparison.
Or it might have no atmosphere at all, in which case conditions would be about like on the moon. Given that it's roughly 3.5 million times further away than the moon is, it's doesn't seem worth getting excited about, does it?
There's also the question of size. It's two to three times the mass of the earth, and about 1.2 to 1.5 times greater diameter, so the surface gravity will probably be anywhere from 150% to 200% of earth's. If you weigh 100kg here, you will weigh between 150 and 200 kg there. It wouldn't be suitable for humans without genetic modifications. It is, however, safe to assume it may be less dense than earth, which is the densest planet in the solar system, and not just because it's infested with Republicans.
Back to the atmosphere, if any. Earth has oxygen in her atmosphere amounting to 20% of our air. Nitrogen makes up 78%. Now the thing about oxygen is that it's one of the most reactive gases there is, and will combine with dozens of other elements given the chance. The example most people are most familiar with is iron—oxygen combines with it and water to produce hydrated ferric oxides, commonly known as “rust”. There's a lot of it on Mars (which is why it's the red planet) and this suggests strongly that while there isn't any oxygen in the Martian atmosphere now, there was at one time.
Because it likes to combine with other minerals, oxygen usually doesn't last long in atmospheres unless it is constantly replenished. On Earth, that's done through plant life, which takes CO2 and converts it to oxygen. When the first oxygen producing life forms appeared on earth, roughly a billion and a half years ago, the planet didn't have any oxygen to speak of. If there were existing lifeforms, they would have found the oxygen toxic, and either diedoff or been forced into anaerobic (oxygen-free) environments, such as are found around sub-oceanic volcanic vents or deep underground. (We are coming to realize that anaerobic life may make up a greater mass than the lifeforms we're familiar with here on Earth's surface, which brings up the possibility that oxygen-exuders weren't the first life forms on the planet.) But without a source of renewal (and plant life is the only source we know of), a planet tends to lose whatever oxygen it has, usually in fairly quick order.
Plants as we know them wouldn't grow on Gliese, so if there is an atmosphere, it's unlikely to have oxygen.
On the other hand, water is a pretty good likelihood. That's based on nothing other then the fact that the stuff is EVERYWHERE. We've even found trace amounts on the Moon, which surprised the scientists. (Robert Heinlein, the SF writer who was ridiculed for postulating underground lunar ice, would be pleased). There's at least enough ice in Mars to fill the Great Lakes, and I wouldn't be surprised if there wasn't a lot more than that. The gas giants in our solar system all have at least traces of water in their atmospheres, so chances that Gliese has some are pretty good, actually, just playing the percentages.
So even though it's deemed a “Goldilocks planet”, Gliese is unlikely to be able to support human life. No high-spectrum light means no plants means no oxygen means Los Angeles only with the possibility of water.
But that doesn't mean it doesn't have life. When we talk about “Goldilocks planets”, we're talking about places that support us, and the stuff we eat and drink.
Life, however, has other ideas about what “Goldilocks” conditions might be. On the surface of earth, we find life everywhere from the frigid, arid ground of the Dry Valleys in Antarctica (places where it is frequently -40 in the summer, and which may have seen no precipitation in thousands of years) to the scorching dirt in the world's deserts to the scalding lips of undersea volcanic vents. In the north there are “ice worms”, who live in cave ice with no visible means of support.
Go deep into caves or into mines and you'll find life that has never experienced sunshine or felt oxygen. Some breathe sulphur dioxide and excrete concentrated sulphuric acid. Some life forms may not have to breathe at all, but simply draw energy needed for life from the heat found deep underground.
Here on earth, life not only exists where humans wouldn't want to live, it exists where humans couldn't possibly live. Indeed, we surface dwellers and the inhabitants of the oceans may be a minority of the mass of life on and in this planet.
Not only could we not live in each others' domains, but we're so alien we couldn't even eat each other. Which doesn't make us harmless to one another: oxygen is a poisonous and corrosive gas to anaerobes, and if anaerobic organisms get inside the human body, the results can be horrific, in part because there is no way the body can resist them. It simply hasn't evolved that way because there has been no evolutionary pressure to do so. Fortunately, oxygen-breathers and anaerobes don't mix in the same social circles.
Those of you old enough might remember the three days' quarantine the Apollo astronauts had to endure upon their return from the Moon. Most people thought it was silly, because “everyone knew” the Moon was a lifeless hunk of rock, so it was pretty unlikely that the astronauts could catch any bugs up there. Besides, weren't the astronauts in space suits and protected?
There's a picture of Eugene Cernan in the capsule on the return trip, and his face is lavishly smeared with moon dust that he got on himself climbing out of his suit. So much for protection.
And while we found no indication of life on the Moon, we no longer consider the possibility of life there to be impossible. Remote, certainly, but impossible, no.
In fact, if the Moon was lifeless before the first earth object hit it in 1962, it may not be now. Who's to say a tiny but very tough and resilient hitchhiker didn't end up on the Moon, and found a niche?
After all, the most likely explanation for how life came to earth, with its methane atmosphere, is equally preposterous.
So even though Gliese 581g might not strike you as an ideal vacation rental, don't assume it's uninhabited. It might not be.