TOI-1235 b: a keystone super-Earth for testing radius valley emergence models around early M dwarfs

Abstract

Small planets on close-in orbits tend to exhibit envelope mass fractions of either effectively zero or up to a few percent depending on their size and orbital period. Models of thermally-driven atmospheric mass loss and of terrestrial planet formation in a gas-poor environment make distinct predictions regarding the location of this rocky/non-rocky transition in period-radius space. Here we present the confirmation of TOI-1235 b (\(P=3.44\) days, \(r_p=1.738^{+0.087}_{-0.076}\) R\(_{\oplus}\)), a planet whose size and period are intermediate between the competing model predictions, thus making the system an important test case for emergence models of the rocky/non-rocky transition around early M dwarfs (\(R_s=0.630\pm 0.015\) R\(_{\odot}\), \(M_s=0.640\pm 0.016\) M\(_{\odot}\)). We confirm the TESS planet discovery using reconnaissance spectroscopy, ground-based photometry, high-resolution imaging, and a set of 38 precise radial-velocities from HARPS-N and HIRES. We measure a planet mass of \(6.91^{+0.75}_{-0.85}\) M\(_{\oplus}\) which implies an iron core mass fraction of \(20^{+15}_{-12}\)% in the absence of a gaseous envelope. The bulk composition of TOI-1235 b is therefore consistent with being Earth-like and we constrain a H/He envelope mass fraction to be \(<0.5\)% at 90% confidence. Our results are consistent with model predictions from thermally-driven atmospheric mass loss but not with gas-poor formation, which suggests that the former class of processes remain efficient at sculpting close-in planets around early M dwarfs. Our RV analysis also reveals a strong periodicity close to the first harmonic of the photometrically-determined stellar rotation period that we treat as stellar activity, despite other lines of evidence favoring a planetary origin (\(P=21.8^{+0.9}_{-0.8}\) days, \(m_p\sin{i}=13.0^{+3.8}_{-5.3}\) M\(_{\oplus}\)) that cannot be firmly ruled out by our data.