Аннотация
The detection of Earth-size exoplanets around low-mass stars -- such as
Proxima Centauri b and the TRAPPIST-1 system -- provide an exceptional chance
to improve our understanding of the formation of planets around M stars and
brown dwarfs. We explore the formation of such planets with a population
synthesis code based on a planetesimal-driven model previously used to study
the formation of the Jovian satellites. Because the discs have low mass and the
stars are cool, the formation is an inefficient process that happens at low
periods, generating compact planetary systems. Planets can be trapped in
resonances and we follow the evolution of the planets after the gas has
dissipated and they undergo orbit crossings and possible mergers. We find that
planet formation in the planetesimal accretion scenario is only possible around
stars with masses $M_\star 0.07 M_sun$ and discs of $M_disc \ge
10^-2~M_sun $. Hence, in order to form planets ($M_p 0.1 M_øplus$)
around low-mass stars ($0.05 M_\star 0.25 M_sun$), relatively
massive discs are required. Our results show that one third of the synthetic
planetary systems have at least one planet with $M_p 1 M_øplus$, but we
are not able to form planets larger than $5 M_øplus$, showing that planets
such as GJ 3512b form with another, more efficient mechanism. We find that the
large majority of the planets formed have a large water content and most of our
synthetic planetary systems have 1, 2 or 3 planets, but planets with 4,5,6 and
7 planets are also common, confirming that compact planetary systems with many
planets should be a relatively common outcome of planet formation around small
stars. Our results provide information to guide current and future surveys and
aid in the interpretation of TESS and PLATO data.
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