Revision 1

Accepted · first revision · record

Title: What produces the gap between super-Earths and sub-Neptunes?
Plain: Close-in small planets come in two sizes with a gap between them. Starlight boiling away thin hydrogen envelopes, heat from the planet's own core doing the same, or differences in how planets formed could each explain it.
Precise: For planets with $P < 100\,\mathrm{d}$ the occurrence rate drops by a factor $\ge 2$ at $R_p \sim 1.5-2.0\,R_{\mathrm{earth}}$, with the gap center decreasing with period roughly as R ~ P^-0.1. Discriminate between XUV photoevaporation (completed in $\sim 10^8\,\mathrm{yr}$), core-powered mass loss ($\sim 10^9\,\mathrm{yr}$), gas-poor late formation, and a water-world population, using the gap dependence on period, stellar mass, age and metallicity.
Settled by: Gap location and depth versus stellar mass, instellation and age in a large sample with precise radii and masses, compared with the distinct scalings predicted by each mechanism.
Kind: phenomenon
Answer type: identification
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: 2017