As study continues for K2-18's planet, and for TOI-270's planet d (the third): models improve for the Neptune-mass planets we got at Sol h and Sol i. I'm halfway afraid to name Sol h.
The story here, which Edward D. Young, Sarah P. Marcum, Aaron Werlen, and Paula N. Wulff tell, is that the Solar System formed with a carbon-monoxide snow line somewhere further out in the Kuiper Belt than we now find Pluto, which is a nitrogen / water iceworld mostly devoid of CO. Also our seventh and eighth planets lack ammonia... like the extrasolars just noted. Compare 59 Virginis b (or B).
This points to a magma ocean making up most of both our "ice" giants. The magma has dissolved high-pressure volatiles. This magma is about half the radius: more for Neptune, less for Uranus. Over that, the atmosphere - mostly hydrogen and helium - extends for the rest. This convects (hydrogen is a decent conductor), to the surface which then radiates infrared which the JWST can spot.
The model may be tentative, because as you know we've only sent flybys to either, but ... so were the "ice giant" models.
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