Planet formation around M dwarfs via disc instability

Abstract

Context. Around 30 per cent of the observed exoplanets that orbit M dwarf stars are gas giants that are more massive than Jupiter. These planets are prime candidates for formation by disc instability. Aims. We want to determine the conditions for disc fragmentation around M dwarfs and the properties of the planets that are formed by disc instability. Methods. We performed hydrodynamic simulations of M dwarf protostellar discs in order to determine the minimum disc mass required for gravitational fragmentation to occur. Different stellar masses, disc radii, and metallicities were considered. The mass of each protostellar disc was steadily increased until the disc fragmented and a protoplanet was formed. Results. We find that a disc-to-star mass ratio between ~0.3 and ~0.6 is required for fragmentation to happen. The minimum mass at which a disc fragment increases with the stellar mass and the disc size. Metallicity does not significantly affect the minimum disc fragmentation mass but high metallicity may suppress fragmentation. Protoplanets form quickly (within a few thousand years) at distances around ~50 AU from the host star, and they are initially very hot; their centres have temperatures similar to the ones expected at the accretion shocks around planets formed by core accretion (up to 12 000 K). The final properties of these planets (e.g. mass and orbital radius) are determined through long-term disc-planet or planet–planet interactions. Conclusions. Disc instability is a plausible way to form gas giant planets around M dwarfs provided that discs have at least 30% the mass of their host stars during the initial stages of their formation. Future observations of massive M dwarf discs or planets around very young M dwarfs are required to establish the importance of disc instability for planet formation around low-mass stars.

A bright inner disk and structures in the transition disk around the very low-mass star CIDA 1

“…These two observational results are consistent with the predictions of coreaccretion models for the formation of planets, in which giant planet formation is relatively inefficient in disks with low mass around very low-mass stars (e.g., Payne & Lodato 2007;Liu et al 2020). Alternatively, giant planets around M-dwarfs may form when the disk is gravitationally unstable (Mercer & Stamatellos 2020).…”

“…However, core-accretion models predict that it is very challenging to create the required pressure bump with an embedded planet under the physical conditions of CIDA 1, because a large fraction of the disk mass (5-45%, where higher values correspond to a disk with high viscosity) is needed to form such a massive planet in the first place. One alternative is that the initial disk mass is high enough such that the disk is gravitationally unstable and capable of forming massive planets (Mercer & Stamatellos 2020). In the case of CIDA 1, a Saturnmass planet is the minimum mass required to open a gap (with low viscosity) and to trap millimeter-sized particles, but the predicted integrated spectral indices for this scenario are too high compared to observations (Pinilla et al 2017).…”

The Terrestrial Planet Formation around M Dwarfs: In-situ, Inward Migration or Reversed Migration

“…We conclude that reversed migration may better match the observations, while the formation of planets massive than 3 M⊕ around low-mass stars is not yet clear (Zawadzki et al 2021). However, recent observations show the occurrence of giant planets around M dwarfs (Mercer & Stamatellos 2020), which will lead to forthcoming investigation of the terrestrial planet formation in the presence of gas-giants and the final architecture of planetary systems around M dwarfs.…”

Boosting the public engagement with astronomy through arts

“…Later on this was explained with the limitations of the planet formation in the protoplanetary disks -it is easy to understand this intuitively: the lower the stellar mass, the lower the mass of the protoplanetary disk, the lower the mass of the planets that can form inside them. The effect is boosted by an additional bias that the lower mass planets are easier to discover around lower mass stars, than around higher mass stars [40,44].…”