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/nonrocky transition in period-radius space. Here we present the confirmation of TOI-1235 b (P = 3.44 days, r_p = 1.738_-0.076^+0.087 R⊕), a planet whose size and period are intermediatebetween the competing model predictions, thus making the system animportant test case for emergence models of the rocky/nonrockytransition 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 aset of 38 precise radial velocities (RVs) from HARPS-N and HIRES. We measure a planet mass of 6.91_-0.85^+0.75 M⊕, which implies an iron core mass fraction of 20_-12⁺¹⁵% in the absence of a gaseous envelope. The bulk composition of TOI-1235 b is therefore consistent with being Earth-like, and we constrain an 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, suggesting that the former class of processes remains 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_-0.8^+0.8 days, m_p sin i = 13.0_-5.3^+3.8 M⊕) that cannot be firmly ruled out by our data.