The following was originally published on the Stellaris forums.
Hi all, and welcome to another glorious day in space! Today and tomorrow we’re going to do a two-parter. We’re going to look at two potentially habitable planets which orbit a nearby star, Gliese 581.
Gliese 581 is an M-class red dwarf, which makes it easy to spot planets around it and means that her Goldilocks Zone is extremely close. We are fairly sure that she has three planets (b, c and e) and may have a number of additional planets as well. Of these possible planets, Gliese 581d and g are potentially habitable. Sadly none of the confirmed planets are likely to be habitable.
First, Gliese 581d.
What we know
If she exists, Gliese 581d is between 5.6 and 13.8 times as heavy as Earth. Her orbital period and rotational speed mean that wherever you stand on her surface, the sun will rise and set once per “year”, and that “year” will be 67 Earth days long.
Two months of darkness sounds terrifying to me, and not just because I suffer from seasonal affective disorder. It might also cause extreme temperatures to exist, baking during the day and freezing during the night. However, if Gliese 581d has enough of an atmosphere (which it certainly has the gravity for) then that might help to spread the heat around and ensure that while nights are chilly, they aren’t freezing. The ocean currents might help too.
What we hypothesise
I said ocean currents because there is a very good chance that Gliese 581d is an ocean world. From what we can tell about the outer debris ring further out in the system, Gliese 581d had an enormous amount of ice around it. If this ice was around during planet formation then it’s not unreasonable that Gliese 581d would have had as much ice as rock accumulate, and when that melted then you would have a planet with immensely deep oceans.
(By the way, when astronomers say “ice” they don’t mean frozen water. They mean “anything that isn’t hydrogen, helium or a rock.” To astronomers, Neptune and Uranus are made of ice. They’re trying to popularise the term “ice giants” for gas giants made of gases which are neither hydrogen nor helium. While this term sounds impressively Norse, I don’t personally use it.)
This presupposes that Gliese 581d has enough of an atmosphere to develop a greenhouse effect, of course. If it doesn’t then the ice will be, well, ice.
(Artist’s impression. We do not have any photos of any exoplanets that are this detailed. I wish we did.)
What would it be like to live there?
What follows is supposition. I am not a biologist or an ethnologist, and we don’t even know that the planet exists.
Gliese 581d is heavy, meaning that on the surface most life would be small. However, her strong gravity would mean a dense atmosphere which would mean that flight would be much easier.
There would be no islands, only a continuous ocean. In our world, storms tend to form over water and exhaust themselves on land: without land the storms would be terrifying things. I don’t know anything about meteorology but if the 67-“day” day/night cycle didn’t disrupt them then it may well be that the hurricanes would be eternal.
However, there’s light and water and that means life. The surface layers of the ocean could be full of life, from plankton and amoeba to larger creatures that feed on them. Photosynthesising bacteria would find a ready home, as would anything larger which is able to float. Floating plant islands may form, drifting around the currents andaccumulating layers of symbiotic fungi and insects.
If there were sentient Gliese 581d-ites, they would probably live in small colonies, easily able to flee from the enormous storms and avoid drifting into dangerous areas. When two groups met they might trade, exchange population or make war, but they would eventually split apart again. If they were to travel the stars then they would take this mindset with them, distrusting complex societies and being uncomfortable unless they could move around as they wished. The ocean is endless, after all, and space is the same.
Sadly, if there are no islands then there is no mining which means no metals. If complex life evolved on Gliese 581d, or if she were colonised, itwould be hard for them to develop an industrial base. The technology and social organisation required for space flight would be very difficult.
The oceans, meanwhile, would be extremely deep. How deep? Look at a cutaway diagram of Earth and imagine that in place of the mantle, it has water. That’s how deep. We think Gliese 581d would not have tectonic activity, so there wouldn’t be earth-style seabed hot vents. The depths would be forbiddingly cold and dark. Anything that lives down there would have to be adjusted to the cold and the pressure, which on Earth often means “is very large.” Gliese 581d could have some truly enormous, ancient creatures lurking in her depths.