This came up from Montreal (and ScienceDirect) yesterday: a presumed waterworld (or two). We're already aware of Quebecois water-dense worlds elsewhere so I'm more interested in how they found this one.
Kepler-138's system was found almost a decade ago by a transit-detecting telescope. The star has another double-digit parallax so 218 lightyears away. Transits numbered three here... directly.
What happened recently was the discovery of a Callisto, as it were; a fourth world as didn't match the other, inner three. 138e does not transit. So it is constrained only in its minimum mass... and an estimate on inclination. This outer planet did however force a recalculation of the inner three. Mainly of c and d; b is solidly a (hot) Marslike.
K-138c and 'd are still too hot where TOI-1452b is likely liquid. But all are now slated as low-density. For 138c and 138d they figure: steam, maybe supercritical below the steam, hot ice below that.
Carbon-dioxide and carbon-monoxide would also be low-density so Zim remains unsure how they figured these two Keplers as steamworlds as opposed (say) to carbonworlds. Same for TOI-1452 now I think on't.
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