D-21: The Cradle

The following was originally published on the Stellaris forums.


Good morning, everyone! A big thank you to Murmeldjuret for filling in over the weekend, and I hope everyone enjoyed their updates as much as I did.

Today we’re going to talk about a very, very interesting star system. We’ve discussed several binary systems before, and briefly mentioned one three-star system (Gliese 667.) But what about a four-star system?

Meet HD 98800. If you dislike numbers, this is also called TV Crateris or TV Crt.

(Image courtesy Keck. Apologies for the extremely technical image but I couldn’t find a good visible-light one.)

HD98800 is composed of two pairs of sunlike stars (probably K class, which means orange stars slightly smaller than the Sun). Within each pair, the stars are very close together, close enough to fit within the orbit of Mercury. The two pairs orbit one another at a distance of about 50 AU, which isn’t close on a planetary scale but is very close when you’re dealing with stars. You can think of it as a binary system made up of binary systems; in fact that’s probably the best way of thinking about it.

I know you’re thinking it so say it with me: “Sup dawg, I heard you like orbiting so I put a binary system in your binary system so you can orbit while you orbit.”

50 AU is a distance that’s easily crossable. (For reference, Neptune is 30 AU away.) A species which evolved on a planet orbiting one pair of stars could send a probe to the other with 20th century technology, and could probably send crewed exploration vessels with 21st century technology.

So are there planets there?

Maybe one, but there’ll probably be more one day.

The stars in HD98800 are very, very young. They’re so young, in fact, that hydrogen fusion in their cores hasn’t properly begun. This means that while they’re slightly less massive than the Sun they’re also a little bloated. Give them a few billion years and they’ll be svelte K-class main sequence stars. They’ll also have planets, most likely.

HD98800B (that is, one of the pairs) is surrounded by an enormous ring of debris, mostly rock and gas and ice. We think that our Solar System formed just like this: a ring of debris around a young star. Gradually the debris will form into planets, and if the debris has enough ice in it and the planet is within the Goldilocks zone then maybe it’ll have an atmosphere and oceans.

Even more excitingly, we’ve spotted a gap in the debris ring between 1.5 and 2 AU out. This may seem innocuous, but gaps of this sort are usually made by planets clearing out their orbits. (We’ve seen Saturn’s moons do the same thing in her rings, which is why they have gaps.) The gap is definitely not within the Goldilocks zone right now, given the immature state of the stars, and probably won’t be even when they’re properly on the main sequence, but where there’s one planet there might be more.

HD98800A (the other pair) doesn’t have nearly as dramatic a debris ring (see image above), but it may still have some. Even if it doesn’t, multi-star systems are notorious for having planets migrate from one orbit to another, so it’s not impossible that some of B’s planets may end up orbiting A.

In some tens of billions of years, HD98800 could have multiple habitable planets, possibly around both pairs of stars.

(Artist’s impression. Image courtesy NASA / JPL-Caltech.)

What would it be like to live there?

On Earth, seafaring civilisations tend to arisearound estuaries, bays or clumps of islands. If you’re accustomed to taking a boat across the bay then it’s a natural step to proceed a little further, and a little further again the next day. Thepresence of nearby coasts to travel to provides a stepping stone to help society develop the necessary concepts for exploration and then for colonisation.

A lot of people, including me, think that the Moon played the same role for humans on Earth: it’s soclose and so perfect that it made us realise that something is up there and that up is a direction we can go in. Sadly, the stars are not so near.

The stars are near, however, if you live in HD98800. You could get there in only years, even without any sort of FTL. This might give the localsthe conceptual framework to develop a civilisation based around space travel, which will in turn help them travel a little further, and a little furtheragain the next day.

HD98800 is the cradle of stars, and the cradle ofplanets. One day it may be the cradle of empires.


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