The following was originally published on the Stellaris forums.
Hello and welcome once again. The last two days have been pretty exhausting, dealing in enormous concepts like dark matter, cosmic voids and the universe itself. We’ve seen some truly amazing things like the Great Attractor, the Laniakea Supercluster and the Boötes Void. I think we should talk about something more human-scale for now.
Let’s talk about another exoplanet, 55 Cancri f. In 2015 it was named “Harriot” in a naming contest, after the astronomer Thomas Harriot.
Harriot is between 33 and 58 times the mass of Earth, which sounds enormous until you realise that she’s a gas giant and that only makes her one-seventh the mass of Jupiter. She’s orbiting an orange K-class star only slightly smaller than the Sun. This makes a change from the M-class red stars that most known exoplanets orbit.
Harriot is in the habitable zone of a habitable star. However, baby gas giants like her are still gas giants and therefore uninhabitable. But she could have moons.
We don’t know of any exoplanet moons yet – they’re too small to identify. But we can take a guess at what they might be like by looking at the moons in our own solar system. Earth has a big moon (1.2% of the mass of the primary.) If Harriot had a moon of the same relative mass, that moon would be between one-third and two-thirds of the mass of the Earth.
Let’s imagine the best possible case: a super-moon two-thirds of the mass of the Earth. Let us call this hypothetical moon Thomas.
Thomas would be very light, too light to hold onto a nitrogen-oxygen atmosphere in the long term. She’d need some way to refresh it. Fortunately, being this close to a gas giant (even a small gas giant) would result in intense tidal pressures which would do interesting things to her core. Amongst other things, this would give her volcanoes, which are an excellent way of pumping gas into her atmosphere. It could also give her a molten core, which would in turn generate a magnetic field, which would help protect her from solar activity from the nearby orange star.
We think that Harriot has water clouds, which would mean that if Thomas is made of the same matter as her primary (as seems likely) then she would have water, which means oceans.
I’ve done some quick calculations and there’s a stable orbit at 1.9 days from Harriot. This would be a long way out, near the Lagrangian points, which means that Thomas would be close to the Trojan asteroids that Harriot might gather. However, it would also help with sunlight. Every time Thomas goes behind Harriot, she would fall into shadow and no longer receive any light at all. Since her orbit is fast, it would help make these “eclipses” short and prevent them becoming too cold. She would need a thicker atmosphere than Earth in order to stop the night side from getting too cold, but fortunately she has volcanoes and this is a thing that volcanoes do.
What would it be like to live there?
To start with, you’d only weigh 87% as much. In the long term this would mean that people’s bodies would adjust to lose bone mass, but it would also help to overcome some of the difficulties associated with humanity’s upright posture. I get backaches a lot and this would be great for me. Birds might especially enjoy this, and could grow to great sizes.
Harriot sits at the outer edge of the habitable zone, and Thomas orbits a long way out; this means that she’ll frequently get cold. Depending on where on Thomas you live, there’ll be a time during their 260-day year when the days are hot and the nights are cold, and a time of year when the reverse is true. It’ll never get particularly warm, though.
A cycle of hot and cold is something that Earth’s tundra regions are used to. The brief “hot day” season would be an opportunity for plants to briefly flower and animals to reproduce, and then settle down to prepare for the colder times. Tundra regions are also known for possessing larger-than-usual animals such as caribou, since large body size is a good way of preserving body warmth. Thomas’s lower gravity would make this large size easier.
People who live in the tundra tend to practise transhumance. This is nothing to do with bioaugmentation, but is the practise of migrating between “winter” and “summer” pastures, often to the point of having separate towns in the two places. This sort of lifestyle naturally breeds a cyclic inevitability into the culture and the people’s mindset, and teaches them to treat territory as something you travel to and from, not something you dwell in. Transhumance-practitioners are normally also very fond of huge annual markets and parties when they return to meet one another and exchange their products.
When they went into space, the people of Thomas might take this mindset with them. They might build huge numbers of space stations and domed colonies, all of which are only crewed part of the time. There would be a season to expand, and then a season to return to their homes with their produce and trade it with one another. Their home moon might naturally become the site of an enormous annual market to which other species would begin to come. Trade, after all, enriches all parties, and birds love nothing more than to return to their nests with their shinies.
Back when I started this series, someone suggested doing an episode on bird people which live on a tundra world. On this hypothetical moon orbiting the baby gas giant of Harriot, we appear to have achieved that.