D-26: The Twins, Part Two

Hi everyone, and welcome back to the second part of our two-parter! Today we’re going to talk about the other possible habitable planet orbiting Gliese 581: Gliese 581g, also known as Zarmina after Zarmina Vogt, the partner of its discoverer.

(Image courtesy European Southern Observatory. Shout outs to all my peeps at ESO.)

As with Gliese 581d, Zarmina may or may not exist. The HARPS survery didn’t find it, which is disappointing because HARPS is the gold standard for exoplanet searching, but it hasn’t been ruled out either.

Zarmina is extremely exciting not just because it’s potentially habitable but also because it’s small. You may have noticed so far that all the planets I’ve discussed have been large planets orbiting small stars. This is because the closer the two are in mass, the easier they are to detect. Zarmina, however, is probably only a little over twice the mass of Earth, meaning that the surface gravity is probably only 30% or so above Earth normal. That’s enough that you’d feel it, but not enough to stop humans living there. In other words, the possible discovery of Zarmina shows that we can spot much smaller planets, and that they may be much more common than we thought.

The bad news is that she’s tidally locked. The good news is that she’s cold enough that this may not be so much of a factor.

A planet is said to be tidally locked when it always has the same side facing its star. We think that most planets around red dwarfs will be tidally locked unless they have moons. This will normally mean that one side bakes and the other side freezes. However, if there’s an atmosphere around it that can spread the heat around then it may be habitable. In this case this is likely, and is helped by the fact that Gleise 581 isn’t a very hot star. In fact, it’s likely that under most atmospheres Zarmina won’t have a temperature high enough to have liquid surface water.

If Zarmina has at least 20% carbon dioxide in her atmosphere, however, she’ll be warm enough that the part of the planet directly under the most sunshine will be like Earth’s tropics. This will be surrounded in turn by rings of other climates, getting steadily colder until at last it’s just ice floes and bare rock.

Oh yes, ice. Gliese 581 has a lot of ice around it, after all, so it’s likely that Zarmina has oceans. Unlike her sibling, she’s probably got more rock in the mix and so she’s unlikely to be entirely ocean – there could be islands and continents.

Steven Vogt, the discoverer and the partner of Zarmina’s human namesake, is one of the most inspiring optimists working in astronomy today. I really admire the man and in his honour, I’m going to write the rest of this piece with the most optimistic assumptions possible.

(Artist’s impression courtesy Vogt.)

What would it be like to live there?

Zarmina’s habitable patch is idyllic for humans, even if it might be small. (We don’t know how big; at a rough guess it’d probably be a continent-sized area.) It’s always daylight and always warm, which means there are no seasons. Crops would grow quickly. Further out the land becomes more and more marginal, more suitable for nomads or scattered villages than for civilisations.

This is the natural environment for a unified government. Power centralises easily when the most desirable land is concentrated: look at the Nile Valley. It might not be a dictatorship, but Zarmina is unlikely to have the same patchwork of squabbling nations that Earth has. If you disagree with the government then you’re either bigger than them in which case you’re now the new government, or else you’re smaller than them in which case you can run away and live in the cold places.

However, like all empires with marginal hinterlands, barbarian invasions from these cold places might be a recurring issue.

In real life, icy places are fantastic places to find meteorites. We know that the Gliese 581 system is far more full of debris than Earth’s is, and so there are likely to be proportionally more meteors. These would land all over the planet, but since most of it is frozen over, most meteorites would fall onto the ice and snow where they would be very visible and easy to recover.

Meteorites are a fantastic source of metals, including rare metals we don’t often get on Earth. Some of these rare metals are very useful for space flight.

This means that the people of Zarmina would be very accustomed to a particular mode of resource collection: “Gather an expedition and send it out into the barren places to bring back valuables. Defend yourselves against the barbarians and outcasts who wander the fringes. So sayeth the King.” If they go into space, they may well keep the same mindset. Other planets would just be resource-gathering operations for them rather than potentially colonisable new homes. Whether the government is a monarch or a republic, it’s likely to be centralised and rule over space just as it does over Zarmina itself.

So does Zarmina actually exist?

In the opinion of the eternally optimistic Steven Vogt, yes it does and there is definitely life there. Out of respect to him and his team, I shall not state my own more pessimistic opinion.


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