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

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⊕), 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±0.015 R⊙, M_s=0.640±0.016 M⊙). 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⊕ 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⊕) that cannot be firmly ruled out by our data.