TOI-1695 b: A Water World Orbiting an Early M Dwarf in the Planet Radius Valley

Abstract

Characterizing the bulk compositions of transiting exoplanets within the M dwarf radius valley offers a unique means to establish whether the radius valley emerges from an atmospheric mass loss process or is imprinted by planet formation itself. We present the confirmation of such a planet orbiting an early M dwarf ($T_{\mathrm{mag}}=11.0294\pm 0.0074,M_s=0.513\pm 0.012M_{\odot },R_s=0.515\pm 0.015R_{\odot },T_{\mathrm{eff}}=3690\pm 50K$): TOI-1695 b ($P=3.13$ days, $R_p=1.90_{-0.14}^{+0.16}{R}_{\oplus }$). TOI-1695 b's radius and orbital period situate the planet between model predictions from thermally-driven mass loss versus gas depleted formation, offering an important test case for radius valley emergence models around early M dwarfs. We confirm the planetary nature of TOI-1695 b based on five sectors of TESS data and a suite of follow-up observations including 49 precise radial velocity measurements taken with the HARPS-N spectrograph. We measure a planetary mass of $6.36\pm 1.00M_{\oplus }$, which reveals that TOI-1695 b is inconsistent with a purely terrestrial composition of iron and magnesium silicate, and instead is likely a water-rich planet. Our finding that TOI-1695 b is not terrestrial is inconsistent with the planetary system being sculpted by thermally driven mass loss. We present a statistical analysis of seven well-characterized planets within the M dwarf radius valley demonstrating that a thermally-driven mass loss scenario is unlikely to explain this population.

22 pages, 11 figures. Accepted in AJ