Last week we talked about TRAPPIST-1 and its seven planets. Turns out those planets have a whole lot of water (or at least they have very low densities, so they probably have a whole lot of water). And yes, it’s entirely possible that something could be swimming around in all that water. But the paper I cited last week wasn’t really about water or alien life. Not really.
I mean, the stuff about water was important, but it wasn’t the real point of the paper. The real point was that such water-rich planets could not have formed so close to their star. They must have formed farther away, somewhere beyond TRAPPIST-1’s frost line, so that they’d be able to accumulate large quantities of water (and/or other volatiles) in the form of ice. Then they migrated inward.
It would be sort of like if the ice-covered world of Pluto, or any of the large, icy moons of the Outer Solar System, were suddenly transplanted to the Inner Solar System. All that frozen nitrogen and frozen methane would sublimate, turning into a generously thick atmosphere. And all that frozen water would melt, turning into a deep, deep ocean—a global ocean so deep it would make Earth’s oceans look like puddles.
That’s what the TRAPPIST-1 planets are probably like: Pluto-like worlds that thawed.
The inward migration of the TRAPPIST-1 planets—sorry, I mean exoplanets—is sort of the opposite of what happened in our own Solar System. Our gas giants, according to the Nice model, started out closer to the Sun and then migrated away (except for Jupiter, which moved a little closer to the Sun).
That was the real point of that paper I cited last week. This is also the kind of thing that made TRAPPIST-1 so scientifically interesting in the first place: the alignment of those seven exoplanets makes it really easy for us to study orbital dynamics in a multi-planetary system, and to compare and contrast what we learn with what we know about our own Solar System.
The stuff about water and potential alien life… that was just a nice bonus.
Planet Pailly 
The thing I don’t understand though is that Earth formed within the frost line here. (Didn’t it?) Even if comet impacts brought the water to Earth (as I understand it, a problematic hypothesis), why couldn’t that happen in the Trappist-1 system?
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I think it’s a matter of scale. Earth is less than 1% water by mass. Some of the TRAPPIST-1 planets are (according to that paper) 50% water, maybe more. If all that water was delivered by comets (or asteroids, if that’s the alternative hypothesis you’re alluding to) there would have to have been enough comet and/or asteroid impacts to double the mass of a planet.
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Ah, ok, thanks. I didn’t catch that the TRAPPIST-1 planets were that high a percentage of water.
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The innermost planets (b and c) are supposed to be about 15% water, which is still a lot. The outer ones (g and h) are around 50%, and the ones in the middle are presumably somewhere in between, if I remember correctly. I think it was planet’s d and e that required the most guesswork. And of course, this is all according to that one paper. I found another paper I want to read that, based on the abstract, may tell me a slightly different story.
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