Exomantles

Zimmerman (yesterday) points to the Keck observatory. Which has been pointing to polluted white-dwarf stars within 650 lightyears. These 23 dead stars have heavy elements that cannot have come from the star alone when it was main-sequence hydrogen, so must have dragged in rock from its erstwhile planets.

That would be mantle rock, the crust (presumably) being a thin rind like here on Earth so undetectable. [UPDATE 2/2/24: should be of interest to students of Theia within us.]

One of the twenty-three dwarfs, WD1145 + 017, had Olivine Websterite, a match for Earth. Which ex-star had made the news six years ago because it is still consuming its planet . . . [UPDTE 1/16/22 as some do.]

The other 22 planets' mantles do not look like ours. Nor like each others', nor like Mars or Venus. Looking around, the authors Keith D. Putirka and Siyi Xu seem to have been poasting about this since March, and Siyi Xu has a full dissertation before that (pdf); but we're only hearing about this now.

Which leads us to extrapolate to the other systems with exoplanets within 650 LY. We're one or two in 23-24 for this spiral-arm. But maybe not:

As noted, these 23 stars are dead and their planets wholly destroyed. But: in principle the clock can be rewound, to see what the star used to look like. None will be in the red-orange range; those li'l guys are all, like, still with us, like our own dear G 1-M_☉ Sun. (See it yourself: T = M^-2.5, where T is in E+10 years.) Honestly G stars are pushing it because if they're that old, they got formed when the heavy elements were scarce - too scarce I think for these planets. On the other side we all know when you get too hot, 8+ M_☉, the star goes neutron at the end. So I'd look to A and F (UPDATE 12/2 or at least something that started A or F). Like WD1145 + 017 used to be.

I am unsurprised that alien suns, in the searing A and F range, warm alien planets. To what level does a planet that lasts 550 million years even have time to differentiate a core from a mantle? (And forget about life. The K class is supposed to be the (auburn-)goldilocks class, slightly smaller than our Sun.)

Do keep in mind also that some of these dwarfs likely consumed Hot Neptunes or even Hot Jupiters. Hot Neptune simmers to itself for 550 Mya; star goes red giant (and how!), blasting its outer layers. Then star retracts. No wonder quartz makes up so much of so many of these ex-planets.

But on the off chance that a normal planet can exist with these sort of mantle, and survive four billion years - here's where we speculate. Some mantles might produce thicker crusts. Others might trap more water than our mantle has.

I'd like to know about metallicity though. Do certain compositions of A-F star produce certain types of planet? We had hype about 55 Cancri's brood, once upon a time; last I looked, we still had trouble constraining 55 e.