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, rp=1.738−0.076+0.087 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 (Rs=0.630±0.015 R⊙,
Ms=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 6.91−0.85+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 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=21.8−0.8+0.9 days, mpsini=13.0−5.3+3.8
M⊕) that cannot be firmly ruled out by our data