Revealing a population of water worlds among small exoplanets

Caroline Piaulet ( Université de Montréal )

Recent studies have shown that a few low-density planets intermediate in size between the Earth and Neptune may have bulk and atmospheric compositions rich in volatile ices such as water rather than hydrogen. While mass and radius are not enough to tell apart a hydrogen-rich from a volatile-rich atmosphere, small low-density planets are highly susceptible to atmospheric escape, which leaves the volatile-rich scenario as the most plausible composition. Here, we show that these atmospheric mass-loss considerations reveal a subpopulation of small planets that are strong volatile-rich "water world" candidates. We provide constraints on the degenerate solutions for their bulk volatile content assuming a four-layer core, mantle, hydrosphere, and hydrogen (or water) atmosphere, as well as mass-radius relations appropriate for planets with insolations of 2-1000 S_oplus. The compositions of planets in this sub-population range from ~ 10 to 20% water by mass for no hydrogen to 0.1% hydrogen by mass in the absence of water. We demonstrate using 1D hydrodynamic atmosphere mass-loss calculations that the inferred amounts of hydrogen on these planets would be lost in less than 1 Gyr. Finally, this hypothesis can be readily tested with JWST for a few prime candidates.