Black planet

Some years ago some small planets were detected around 55 Cancri. 55 Cancri was at the time thought to be a high-carbon star; moreover, innermost planet e was deemed high-radius for its mass. A flurry of papers came out asking if this planet was carbon; the books Exoplanets and Planet Factory evaluated these. Elizabeth Howell offered a summary of this science in 2017. Last I looked (today) they're wondering whether Lavaworld or Cyanideia.

But if 55 Cancri e isn't diamond planet, the models they were juggling back in 2012 are equally plausible for some other planet, perhaps hiding among other stars.

To constrain the process here is "Oxidation of the Interiors of Carbide Exoplanets". Authors: Harrison Allen-Sutter, Emily Garhart, Kurt Leinenweber and S-H "Dan" Shim of Arizona State; Vitali Prakapenka and Eran Greenberg of the University of Chicago. I found out when ScienceDaily relayed its 9 September press-release.

They think that a water ice planet infused with silicon-carbide will, once it accretes enough mass, crush the ice and carbide. (They allow for an iron core at the base of it all.) Released will be silicon-dioxide, one pure carbon molecule, and two hydrogens. The silica is of course just sand - glass, rather - and the carbon will crush into diamond that high pressure. They also allow for magnesium-carbide, but that doesn't change the base chemistry here. Hydrogen escapes, or maybe joins with looser carbon soot for methane.

As they point out, methane (like hydrogen) is a "reducing agent" - as the metallurgists put it. Any free oxygen here will burn. The point of this star-system is that it hasn't much oxygen, and our planet is binding what little it's got into glass. What happens here at the end is a carbon-iron alloy core, silica-diamond mantle binding up a lot of water. The initial crust would be silica (and, er, magnesia) but above that I'll call it different: black soot and graphite. Very thick methane atmosphere, with some proportion of carbon dioxide and monoxide which I'll get to.

I expect comets and hydrous meteors to deliver additional water. This water pools if this planet migrates to our habitable-zone... rather, slightly outside it, because methane is a greenhouse gas. Inward (or just later as the star ages and heats), the new water boils and joins this thick atmosphere; if our planet stays outward, it's a big static Antarctic glacier.

There are no tectonics. I further suspect this iron-carbon core is stiff and cannot spin like our purer iron core does. It's not as dense and the authors are looking at 2-8 Earth masses... so, the planet is big. That affects escape-velocity. The weak magnetic field is on the surface or beneath it.

If this unprotected planet is in the habitable-zone, the sun is breaking its methane back to soot and hydrogen like Venus. And the surface albedo of charcoal-world is super low. Any surface water evaporates if it doesn't just plain boil; that vapour is getting broken to atomics too. Which does free the oxygen... to un-reduce the hydrocarbons. There's no ozone. I don't know if the planet is keeping its free hydrogen but if so, it's clustering in its highest atmosphere and is mostly deuterium.

At the end, I expect a carbon-dioxide sheath over a black cinder planet. Some nitrogen. Few and wispy clouds, nightside. Trace water-vapour heavily deuteric. Hey, at least the acid is very weak; and it might be cooler. Oh, and copious craters in this black soil.