TRAPPIST-1's strange coincidence

On topic of Laplacian systems, Eric Agol, ''et al.'' refined the TRAPPIST-1 system a bit. Here's the release.

These planets are much cooler than the other Laplacians noted so far, excepting Gliese 876 whose giants we cannot observe so directly. In fact the planets can be mapped to the Mercury-Mars line of insolation. Unlike GL 876's, TRAPPIST-1's worlds are rocky. Not, however, as rocky as is Earth. Or perhaps more rocky, more like Mars; we'll get to that.

The choices are: no core, a Mars-like small core, large core waterworld.

Since all these planets are about as dense as each other, for their structures the authors incline toward WWG1WGA. That was clearly not the case for the Laplacians in TOI-178 and, likely, not for GJ 876's either. But as the authors point out, it is too(?) coïncidental that different planets all line up to the same density, a coïncidence not seen at TOI-178.

"No core" here means the Elkins-Tanton / Seager Rustworld: the planets formed by iron-oxide and... not much else. The authors say, it remains to be seen whether a geochemical model can be constructed which results in high oxidation of iron throughout the processes of planet formation and evolution. All I can say to that is... I'll say.

I am inclined to have most of these as waterworlds, myself; especially the outermost. Micro-Neptunes. Although the inmost b and c worlds should be hotter than Venus [TOO MUCH SO 3/27/23 b is Mercurial 6/21 as is c]. d, with Earth's insolation, is only Mars sized and indeed Mars-dense. Coïncidences do happen: why shouldn't d be a warmer Mars with the others as water-heavy? The larger planets keep their (hot) water which [bc]d couldn't.

As the authors also point out, the composition of the star is unconstrained. Looking at Mg/Fe and Fe/Si ratios, especially (remember Planet Factory?). They're hoping someone books time on the JWS Telescope. They'd also like more d and h transits in hope of finding a planet i beyond them all. My prediction, for what it be worth, is that a planet i does exist, and might even be found but if so, it won't be found by transit.