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

Ultra-short period planets (USPs) have orbital periods of less than 1 d. Since their masses and radii can be determined to a higher precision than long-period planets, they are the preferred targets to determine the density of planets which constrains their composition. The K2-106 system is particularly interesting because it contains two planets of nearly identical masses. One is a high-density USP, the other is a low-density planet that has an orbital period of 13 d. Combining the Gaia DR3 results with new ESPRESSO data allows us to determine the masses and radii of the two planets more precisely than before. We find that the USP K2-106 b has a density consistent with an Earth-like composition, and K2-106 c is a low-density planet that presumably has an extended atmosphere. We measure a radius of R-p=1.676(-0.037)(+0.037)R(circle plus), a mass of M-p=7.80(-0.70)(+0.71)M(circle plus), and a density of rho = 9.08(-0.98)(+0.98) gcm(-3) for K2-106 b. For K2-106 c, we derive R-p=2.84(-0.08)(+0.10)R(circle plus) , M-p = 7.3(-2.4)(+2.5)M(circle plus,) and a density of rho = 1.72(-0.58)(+0.66)gcm(-3) We finally discuss the possible structures of the two planets with respect to other low-mass planets.