A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776. (arXiv:2009.08338v2 [astro-ph.EP] UPDATED)

A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776. (arXiv:2009.08338v2 [astro-ph.EP] UPDATED)
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We report the discovery and characterization of two transiting planets around
the bright M1 V star LP 961-53 (TOI-776, J = 8.5 mag, M = 0.54+-0.03 Msun)
detected during Sector 10 observations of the Transiting Exoplanet Survey
Satellite (TESS). Combining the TESS photometry with HARPS radial velocities,
as well as ground-based follow-up transit observations from MEarth and LCOGT
telescopes, we measured for the inner planet, TOI-776 b, a period of 8.25 d, a
radius of 1.85+-0.13 Re, and a mass of 4.0+-0.9 Me; and for the outer planet,
TOI-776 c, a period of 15.66 d, a radius of 2.02+-0.14 Re, and a mass of
5.3+-1.8 Me. The Doppler data shows one additional signal, with a period of 34
d, associated with the rotational period of the star. The analysis of fifteen
years of ground-based photometric monitoring data and the inspection of
different spectral line indicators confirm this assumption. The bulk densities
of TOI-776 b and c allow for a wide range of possible interior and atmospheric
compositions. However, both planets have retained a significant atmosphere,
with slightly different envelope mass fractions. Thanks to their location near
the radius gap for M dwarfs, we can start to explore the mechanism(s)
responsible for the radius valley emergence around low-mass stars as compared
to solar-like stars. While a larger sample of well-characterized planets in
this parameter space is still needed to draw firm conclusions, we tentatively
estimate that the stellar mass below which thermally-driven mass loss is no
longer the main formation pathway for sculpting the radius valley is between
0.63 and 0.54 Msun. Due to the brightness of the star, the TOI-776 system is
also an excellent target for the James Webb Space Telescope, providing a
remarkable laboratory to break the degeneracy in planetary interior models and
to test formation and evolution theories of small planets around low-mass
stars.

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