An ultra-short-period transiting super-Earth orbiting the M3 dwarf TOI-1685

Abstract

<jats:p>Dynamical histories of planetary systems, as well as the atmospheric evolution of highly irradiated planets, can be studied by characterizing the ultra-short-period planet population, which the TESS mission is particularly well suited to discover. Here, we report on the follow-up of a transit signal detected in the TESS sector 19 photometric time series of the M3.0 V star TOI-1685 (2MASS J04342248+4302148). We confirm the planetary nature of the transit signal, which has a period of <jats:italic>P</jats:italic><jats:sub>b</jats:sub> = 0.6691403<jats:sub>−0.0000021</jats:sub><jats:sup>+0.0000023</jats:sup> d, using precise radial velocity measurements taken with the CARMENES spectrograph. From the joint photometry and radial velocity analysis, we estimate the following parameters for TOI-1685 b: a mass of <jats:italic>M</jats:italic><jats:sub>b</jats:sub> = 3.78<jats:sub>−0.63</jats:sub><jats:sup>+0.63</jats:sup> <jats:italic>M</jats:italic><jats:sub>⊕</jats:sub>, a radius of <jats:italic>R</jats:italic><jats:sub>b</jats:sub> = 1.70<jats:sub>−0.07</jats:sub><jats:sup>+0.07</jats:sup> <jats:italic>R</jats:italic><jats:sub>⊕</jats:sub>, which together result in a bulk density of <jats:italic>ρ</jats:italic><jats:sub>b</jats:sub> = 4.21<jats:sub>−0.82</jats:sub><jats:sup>+0.95</jats:sup> g cm<jats:sup>−3</jats:sup>, and an equilibrium temperature of <jats:italic>T</jats:italic><jats:sub>eq</jats:sub> = 1069<jats:sub>−16</jats:sub><jats:sup>+16</jats:sup> K. TOI-1685 b is the least dense ultra-short-period planet around an M dwarf known to date. TOI-1685 b is also one of the hottest transiting super-Earth planets with accurate dynamical mass measurements, which makes it a particularly attractive target for thermal emission spectroscopy. Additionally, we report with moderate evidence an additional non-transiting planet candidate in the system, TOI-1685 [c], which has an orbital period of <jats:italic>P</jats:italic><jats:sub>c</jats:sub> = 9.02<jats:sub>−0.12</jats:sub><jats:sup>+0.10</jats:sup> d.</jats:p>