Carbon ocean world

Carbon-dioxide can exist as a liquid at high pressure. Venus is a planet with high pressure CO₂. But it is hot there: high pressure CO₂ is a greenhouse gas. Once a world has already gone Venus, a megastructure might shade the place to cool the high-pressure atmosphere into a true ocean. How might this happen elsewhere, naturally?

I suspect that the natural starting volatiles in planet-building are ammonia, water, hydrogen, and methane. These are all hydrogen compounds. How do we lose the hydrogen: by raising the temperature, and boiling it off. Hydrogen gas boils first; then, ammonia and water. Radiation further breaks water vapour into hydrogen and oxygen ions: hydrogen escapes, oxygen burns the methane to water and CO₂, repeat for the water. Ammonia likewise but the leftover nitrogen is just... nitrogen. That is I think the process which did for Venus.

It occurs to me lately, however, that planets can migrate outwards as well as inwards.

Propose a planet maybe in the 1.3-flux range. A smaller planet, around the same time, is beneath it, in a mutual Kirkwood resonance 2:1. Both become hydrogen-poor. Add maybe a miniNeptune at the 0.25-flux range, like Ceres. This exists to soak up water-rich asteroids and deny them to the soot field.

The inner planet gets pulled into eccentricity. It uppercuts the protoVenus. The conjoined ball of molten rock shifts to a higher orbit also more eccentric.

[INTERJECT 6/26: Why not instead a Hot Jupiter getting sucked into the star, like Io raises tides on Jupiter? The problem here is that such a planet is assuredly a migrate. Anything outside that orbit (which there probably isn't within 4 AU) started water-rich.]

Now the planet cools. Again: low hydrogen. CO, CO₂, nitrogen, nitrous oxide is left.

I don't think this atmo is very thick. But luckily we still have some sooty rocks. Since our newborn dustball is in a new orbit, all those planetoids should find a lot to talk about.