The planet Gliese 581d is back in the news. It's a planet 20 light-years away, orbiting the red dwarf star Gliese 581. What makes it interesting is that Gliese 581d circles its parent star just at the outer edge of what's called the "Goldilocks zone" -- the belt in which the planet would be warm enough to have liquid water and a climate in which life might arise.
Recently, the European researchers who first identified Gliese 581d as a potential lifebearing world have examined the possibility that it is tidally locked to its star -- turning one face perpetually to its sun the way the Moon keeps one side facing the Earth. This is likely because Gliese 581d orbits fairly close to its dim primary, about half the distance at which Mercury orbits the Sun. (You have to crowd in close to a faint star like a red dwarf.)
Tidally locked worlds, even in the habitable zone, are poor candidates for life because one side is perpetually dark and cold. One of Larry Niven's first short stories was set in "the coldest place" in the Solar System, which turns out to be the center of the dark side of Mercury. (This was back when it was thought that Mercury was tidally locked to the Sun. Astronomers found out otherwise during the period between when Niven sold the story and when it appeared in print. Always a gentleman, he offered to withdraw it, but his editor Fred Pohl ran the piece anyway with a note explaining what had happened.)
It's not hard to see that with one side of the planet perpetually roasting and the other size a subzero icecap, over time most of the planet's water and carbon dioxide would wind up as ice on the dark side, while the sunward side becomes a barren, sun-baked desert.
However, the researchers have done models of heat circulation on Gliese 581d (we really need a better name for it) and have determined that with a sufficiently dense atmosphere, the greenhouse effect would keep the dark side warm enough to prevent that. The magic point seems to be a pressure of about 10 bars, which is to say 10 times the atmospheric pressure at sea level on Earth.
What a weird and alien place that planet would be if we could visit it! It's a massive body, at least six times more massive than the Earth, which means its surface gravity would be crushing. It would likely have a deep planetwide ocean (likely covered by floating sea ice on the dark side), and above that an atmosphere thick enough to crush a person. But such a massive atmosphere would thin rapidly with altitude (look up "scale height" to see why), so a mere 300 meters above the surface one might find pressures bearable by humans.
Any life on such a world would likely be restricted to the oceans -- especially since it's quite possible a "super-Earth" like Gliese 581d would have no dry land anyway. In 2008 and 2009 groups in Ukraine and Australia beamed radio messages at Gliese 581. Even if there is life in the oceans of Gliese 581d (assuming there are even oceans), I don't see how aquatic beings would ever learn about electricity, radio waves, or computers.
Could we send a space probe there? The fastest space probe is the speedy New Horizons probe, en route to the NOT A PLANET ANYMORE Pluto. Its maximum velocity was about 16 kilometers per second. At that speed a vehicle would take about 400,000 years. So if our ancestors hadn't foolishly spent their budgets on stone handaxes instead of space exploration, we'd be getting our first images of Gliese 581d just about now.
But these new results have more significance than just whether or not one planet 20 light years away could have life. Red dwarf stars like Gliese 581 are the most common type of star in the Universe. They make up about three-fourths of all stars. If they can have life-bearing worlds, then that's where the majority of life beyond Earth is likely to be: on planets of red dwarfs (dwarves?). If Gliese 581d can have liquid water, then we shouldn't just confine the search for lifebearing worlds and intelligent beings to stars like the Sun.