The third planet TESS witnessed to transit the 1.94%-luminosity L 231-32 system 72 light years away, so TOI-270 d, is now modeled: metal-rich, miscible-envelope sub-Neptune
. "Metal" here is in the stellametric sense, lithium on up. We get this from San Antonio: Christopher R. Glein, Xinting Yu (余馨婷), Cindy N. Luu.
Planet d here is about 4.2 Earth masses. From the cloud-(or haze-)deck: 2.133 R🜨. I don't find a mutual planetary resonance as Trappist-1; the TOIs-270 are however non-eccentric such that tidal locking is certain. I assume d at least migrated rather than coalescing in situ.
The excitement in d is in its atmosphere above said deck, detected at the 400 K layer. The haze temperature below that should be 900-1100 K, heavy in carbon-dioxide and methane. I wonder if from the outside it looks dark green.
Detected gas does not include much hydrogen - so, not "Hycean". Also lacking: carbon-monoxide (so no water-mediated "runaway") - and nitrogen. Below the clouds, temperature will of course exceed 1000 K, actually in the 4000s K. On that surface should be low oxygen (and CO) "fugacity" pressure. Fugacity would determine the relative size of its metallic core, the geochemistry of its mantle and crust, the composition of its atmosphere, and the forces responsible for mountain building
.
Glein, Luu, and 余馨婷 think an ocean - of silicate lava.
Although this planet and, probably, system is no object of colonisation; the trio hold it out as a "Rosetta Stone" for miniNeptunes like it. Of which many exist.
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