The making of a superEarth

The extended Kepler transit-detection mission, "K2", looked at the ecliptic. Two clusters along the way are Praesepe (alias Beehive) and Hyades, in Cancer and Taurus respectively; as clusters, they haven't moved much since the stars in them formed. Those stars, so planets, are considered about 600-800 My old.

JPL a month ago lumped together fifteen transits in the GKM range and posted some findings. The theory is the drain of a subNeptune into a superEarth over time, as tested at HD 63433. In large part due to Kepler's earlier sweep we know our universe - for older stars - has clusters of each, with a gap between 1.5 Earth radii and 2 Earths; two Earths may as well be the four Earths of Neptune-proper. A period of high heat would burn away the ices and gasses as make up a Neptunelike, leaving mostly the silicate/water core. T Tauri, the star-in-making, bursts high heat; the initial formation of a planet would have its gas burned away. Some of it, at least.

JPL were testing if a long term effect existed as well. Praesepe and Hyades stars aren't T Tauri. After 600-800 My, most are still on the main-sequence, heating up but gradually. (Heck, most stars are K or M, on that track for longer than our sun will last.) So: any patterns?

Turns out: yes, patterns. If Praesepe and Hyades stars - younger stars like HD 63433 - have planets detectable by transit at all, they are of subNeptunes (or hot Jupiters). Like: these planets are all subNeptune. For stars outside those clusters, with older (3-9 Gy) ages: only a quartersixth are subNeptune, the others being 1.5 Earth or less. This means planets are losing atmosphere over time - it is "saecular". It further suggests that wherever we see a 1.8 Earth it is becoming a 1.5 Earth; hence their rarity.

I confess surprise that so few 1 Earths were found in these clusters; I do not think our own planet (nor Venus) to be Neptunelike at formation. The Hadean rocks formed at 27 bar, which is high; but less than Venus' 93 bar.

For some scorched planets, maybe atmo-loss can continue to happen from the solar wind after 800 Mya. But those planets would have to be like 51 Pegasi's. Too large; not in scope.

I think JPL, in the wide K2 field, were mainly seeing old K stars with longer time between transits; so getting, like, Venus irradiance. Venus hasn't lost its atmo and it is a sub-Earth. The transiting older-planets' loss of atmosphere must then be happening from a noncatastrophic cause, therefore (for a start) nonstellar. That cause must be internal to the planet.

One cause might be a change from a nongreenhouse atmo to a greenhouse. But how often does that happen? Methane, carbon dioxide, water, CFCs, and ozone are all greenhouse. The preference of the research, for the source of the near-constant heat, is the core. We can consider nuclear fission, crashes by Theia-planetoids, and differentiation. All these are more likely in the planet's first years, which we've already exited. The JPL is left with: it's just overall slow to cool, so larger and more massive than Earth's.

When do less-massive planets stop boiling off their own air? Above a certain mass/density, the hot core is overridden by the mass, which explains how Neptune (for one) exists anyhow. As for the 2 Earth mass range, I suspect: about when they reach Venus' equilibrium, or that of a steamy waterworld or hycean. Once the world-ocean (at least) stops boiling, the planet might get habitable - but maybe never on land. On our own Earth, our very bones evolved to keep marrow-production protected from lateSilurian Earth's residual radiation. That's at 4.1-2 Gy age.

I bring all this up now because just a few days ago another team presented some October speculations on some seventeen under-insolated earthlikes, mostly detected by transit but also including (far side of?) Prox b. These were treated as if they were icy-crust waterworlds, like Europa; their properties reconstructed according to that density. They ruled out Kepler 11 b, 18 b, 60 bcd, and 414 b. At least Gliese/GJ 514, and I'd add Keplers 441 and 442 and 1229, as under 3 Gy, should have all their planets ruled out, also. Prox b meanwhile looks to be like Io. But of the others several, like Kepler 296 f, sit in that 1.5-2 Earth gap. Should they?

That team proposes we look for geysers on some of them. If geysers spew or do not spew, then these planets can be re-constrained. I wonder if Boulder's CUTE 'scope will check in on some of these, or if it is just for Hot Jupiters.