A New Mass and Radius Determination of the Ultra-Short Period Planet K2-106b and the Fluffy Planet K2-106c

Ultra-short period planets have orbital periods of less than one day. Sincetheir masses and radii can be determined to a higher precision than long-periodplanets, they are the preferred targets to determine the density of planetswhich constrains their composition. The K2-106 system is particularlyinteresting because it contains two planets of nearly identical masses. One isa high density USP, the other is a low-density planet that has an orbitalperiod of 13 days. Combining the Gaia DR3 results with new ESPRESSO data allowsus to determine the masses and radii of the two planets more precisely thanbefore. We find that the USP K2-106b has a density consistent with anEarth-like composition, and K2-106c is a low-density planet that presumably hasan extended atmosphere. We measure a radius of Rp=1.676-0.037+0.037 REarth, amass of Mp=7.80-0.70+0.71 MEarth and a density of rho=9.09-0.98+0.98 gcm-3 forK2-106b. For K2-106c, we derive Rp=2.84-0.08+0.10 REarth, Mp=7.3-2.4+2.5MEarth, and a density of rho= 1.72-0.58+0.66 gcm-3. We finally discuss thepossible structures of the two planets with respect to other low-mass planets.