Kepler-138 is...weird

[ProtoJeb21]

A new study came out about th Kepler-138 system today. It’s a large red dwarf a little over half the size of the Sun with three confirmed planets: a hot Mars-like planet, a dense rocky Super-Earth, and a Gas Dwarf two thirds the mass of Earth. This new study, however, changes everything we thought we knew about the system. 

https://arxiv.org/abs/1805.12520

The sizes of the planets were determined to be significantly larger than previous estimates. This was due to a different method of sizing the star, which has proven difficult due to discrepancy between various analyses and modeling of Kepler-138. All three planets are much larger than originally studies determined, both in terms of radius and mass. Kepler-138b the “Hot Mars analogue”, is actually like a smaller version of TRAPPIST-1d that someone left in the oven. It’s 70% the size of Earth and 19% the mass, with a surprisingly low density that suggests a thick volatile envelope comprising a large portion of its mass. It is unclear what this volatile envelope is made of, but water seems more likely than hydrogen because Kepler-138b is too small and too hot for a stable hydrogen envelope. 

Kepler-138c and d are both almost 70% bigger than Earth, and like previous studies showed, they have radically different compositions. Kepler-138c is 67% larger than Earth and over five times more massive, with a high density suggesting a rock and iron dominated composition. However, its mean density is low enough for a thin volatile layer, which has a better chance of being hydrogen-based due to the planet’s higher gravity and cooler temperatures. It’s likely not a significant hydrogen layer, though, and the error range for Kepler-138c’s mass and radius still allows for an Earth-like composition. Kepler-138d, on the other hand, is about the same radius but barely more massive than Earth, with a low density most likely explained by a thick hydrogen atmosphere. Just like last time, it’s a Gas Dwarf, but when compared to Kepler-138c it makes little sense. In fact, the entire system is messed up, as all three planets likely formed much further out and migrated to their current positions, probably swapping orbits on the way. It’s something that has yet to be seen in any other system. 

[p1t1o]

I have never heard of a real "gas dwarf" before, fascinating.

I mean, except for when I asked this question over at XKCD:

http://forums.xkcd.com/viewtopic.php?f=18&t=124518&sid=ed573812f136e0561eb2e5b451068aa3