Asantrea Planetary System

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by BrunoHirschkoff

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So uhm. I've been feeling for a while that the Asantrea setting is missing some important bits - namely timekeeping and a calendar system. But to develop a believable calendar, there's a bit of basic info you have to resolve. Like... what is the planet's orbital period, clearly. But to understand that, you have to know how far it is from its host star(s), based on their luminosity and magnitude. And the mass of the stars relative to the planet. And therefore the orbital velocity. And while we're there, how about we have a go at resolving the 3-body problem with math? Oh yes. The Kepler model.

This has been one of the nerdiest, and most enjoyable, rabbitholes I've gone down in a minute.

So.

Asantrea has binary suns, that's well established. Kesh and Aror orbit each other every 5 days at a distance of 0.062AU. Fast and tight. Kesh is a K1V type star; a little smaller and yellower and a lot dimmer than Sol. Aror is an M2V red dwarf. Between them they produce just under half of Sol's radiation, so by the inverse square law, I need to put Asantrea about two-thirds the distance from Kesh and Aror as Earth is from Sol, to receive a comparable amount of solar radiation.

And guess what, it works. Asantrea's year is 233 days long.

While I was there, I also did some math to work out the relative tidal forces of those two moons, and made some adjustments to make them more realistic. Seilyr, the verdant moon, is now larger but much further away, within about 150,000km of the outer limit of remaining in orbit with Asantrea. Saliel is now much closer, and very much smaller; a fraction of the size of Luna. It, too, hurtles around with an orbital period of 5.5 days, in near-perfect 4:1 resonance with Seilyr's 22.1 day period.

So then I also did some reading about liquid water on small celestial bodies - Seilyr's surface gravity is about the same as our moon's, would water even stick around? Turns out yes - it just needs a dense atmosphere, for which of course it needs a magnetosphere, for which of course it needs a hot, volcanic core. You can see how these rabbitholes just keep going.

Seilyr's magnetosphere is powered by the orbital resonance it has with Asantrea and Saliel, similar to Jupiter's moon Io, but far less pronounced. And because both bodies now have independent magnetospheres, they ***interact*** as they orbit. So Seilyr's atmosphere is a riot of nitrogen auroras and great volcanic plumes, all being chased across its surface by the substellar storm band, which crawls along with a tidal bulge below and monsoonal rains around it as the moon rotates slowly on its axis in a 3:2 resonance with its orbit of Asantrea.

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