The two-planet transiting system LHS 1140 has been extensively observed since
its discovery in 2017, notably with Spitzer, HST, TESS, and ESPRESSO, placing
strong constraints on the parameters of the M4.5 host star and its small
temperate exoplanets, LHS 1140 b and c. Here, we reanalyse the ESPRESSO
observations of LHS 1140 with the novel line-by-line framework designed to
fully exploit the radial velocity content of a stellar spectrum while being
resilient to outlier measurements. The improved radial velocities, combined
with updated stellar parameters, consolidate our knowledge on the mass of LHS
1140 b (5.60±0.19 M⊕) and LHS 1140 c (1.91±0.06 M⊕)
with unprecedented precision of 3%. Transits from Spitzer, HST, and TESS are
jointly analysed for the first time, allowing us to refine the planetary radii
of b (1.730±0.025 R⊕) and c (1.272±0.026 R⊕).
Stellar abundance measurements of refractory elements (Fe, Mg and Si) obtained
with NIRPS are used to constrain the internal structure of LHS 1140 b. This
planet is unlikely to be a rocky super-Earth as previously reported, but rather
a mini-Neptune with a ∼0.1% H/He envelope by mass or a water world with a
water-mass fraction between 9 and 19% depending on the atmospheric composition
and relative abundance of Fe and Mg. While the mini-Neptune case would not be
habitable, a water-abundant LHS 1140 b potentially has habitable surface
conditions according to 3D global climate models, suggesting liquid water at
the substellar point for atmospheres with relatively low CO2 concentration,
from Earth-like to a few bars.Comment: 31 pages, 18 figures, accepted for publication in ApJ