Trappist-1's planets are in mean-motion resonance. Since we're still looking at this system, most of its planets remain unconstrained - except that they are not very dense. Alejandra Ross et al. have a study on similar MMR systems, not Trappist-1 itself.
The dataset numbers 24 systems. Kepler-36 and Kepler-105, specifically -36b and -105c, are standouts. Not only are these less dense than the others; they cannot even be made of stone. The Ross team implies that most MMR systems had formed in-situ.
Two other outliers: WASP-47 e, and good ol' 55 Cancri e. These, the study proposes, formed where they are like the other twenty. They were once Neptunians; but, in the heat and the tides, have boiled off most their atmo.
Those two Kepler planets - therefore their systems - must be migrates from out in the coal and ice regions, with (relatively) less stone and iron. Now they have supercritical "oceans". The paper wants to label the two, Icy Core Worlds. Various models are possible; suffice this blog that K-36 b is 1-5% water and K 105 c is a whopping 11-25%.
On the other side of density, Kepler-107 c is marked a superMercury. It happens this is the only one in the 'set. Elsewhere Trappist-1 - say - doesn't have any.
BACKDATE 9/13 Kyplanet.
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