A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776. (arXiv:2009.08338v1 [astro-ph.EP])

A planetary system with two transiting mini-Neptunes near the radius valley transition around the bright M dwarf TOI-776. (arXiv:2009.08338v1 [astro-ph.EP])
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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=8.5mag, M=0.54+-0.03Msun) detected
during Sector 10 observations of the Transiting Exoplanet Survey Satellite
(TESS). Combining them 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-776b, a period of 8.25d, a radius of 1.83+-0.11Re,
and a mass of 4.66+-0.97Me; and for the outer planet, TOI-776c, a period of
15.66d, a radius of 2.06+-0.13Re, and a mass of 6.1+-1.5Me. The Doppler data
shows one additional signal, with a period of 35d, 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-776b 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 0.54Msun. Due to the
brightness of the star, the TOI-776 system is also an excellent target for the
JWST, 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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