The terrestrial planet formation around M dwarfs: insitu, inward migration, or reversed migration

Abstract

Terrestrial planets are commonly observed to orbit M dwarfs with close-in trajectories. In this work, we extensively perform N-body simulations of planetesimal accretion with three models of in-situ, inward migration and reversed migration to explore terrestrial formation in tightly compact systems of M dwarfs. In the simulations, the solid disks are assumed to be 0.01% of the masses of host stars and spread from 0.01 to 0.5 AU with the surface density profile scaling with r−k according to the observations. Our results show that in-situ scenario may produce $7.77^{+3.23}_{-3.77}$ terrestrial planets with an average mass of $1.23^{+4.01}_{-0.93} \ M_{\oplus }$ around M dwarfs. The number of planets tends to increase as the disk slope is steeper or with a larger stellar mass. Moreover, we show that $2.55^{+1.45}_{-1.55}$ planets with mass of $3.76^{+8.77}_{-3.46} \ M_{\oplus }$ are formed in the systems via inward migration, while $2.85^{+1.15}_{-0.85}$ planets with $3.01^{+13.77}_{-2.71} \ M_{\oplus }$ are yielded under reversed migration. Migration scenarios can also deliver plentiful water from the exterior of ice line to the interior due to more efficient accretion. The simulation outcomes of reversed migration model produce the best matching with observations, being suggestive of a likely mechanism for planetary formation around M dwarfs.

Assessing the Transiting Exoplanet Survey Satellite’s Yield of Rocky Planets Around Nearby M Dwarfs

“…Our Gaussian model also seems to agree (within a factor of a few) with the inward and reversed migration models for terrestrial planetary formation from Pan et al (2022), which found similar planet occurrence rates around M dwarfs. Their reversed migration model found a peak at intermediate radii, and subsequent decrease in occurrence, though their calculated occurrence with stellar radius relation is flatter than what we observe and peaks around 0.2 M e , which corresponds to radii significantly below the observed peak in the planet distribution.…”

“…Adopting the values of σ A from lightkurve and our power-law fit allows for a similarly good fit with a Gaussian that peaks at a similar (or slightly higher) stellar radius (≈0.42 R ⊕ ) and an amplitude at its peak around 6 planets per star as opposed to 3 planets per star. This does not necessarily contradict the estimation of λ ≈ 2.5 planets per star from Dressing & Charbonneau (2015) for similar reasons as the Gaussian model with σ A = 0, but does contradict, e.g., Pan et al (2022), which, with theoretical formation and migration models, did not find such a large-amplitude peak in planetary occurrence at intermediate radii, or indeed such a high planetary occurrence around any systems unless all of the planets were formed in situ.…”

“…Meanwhile, tiny host stars simply lacked the necessary material to form large numbers of planets. Pan et al (2022) also found a dependency between stellar mass and planet occurrence, resulting in a decrease in planet occurrence around the smallest of stars (M * < 0.2 M e ) in most formation scenarios.…”

Eccentricity Distribution beyond the Snow Line and Implications for Planetary Habitability

“…Additionally, planet formation simulations conducted by Raymond et al (2007) found that it may be difficult for M dwarf accretion disks to form terrestrial planets larger than ∼0.3 M ⊕ , potentially decreasing the probability of M dwarfs participating in habitable planet formation. Alternatively, M dwarf planets that form beyond the snow line and migrate inward may retain substantial water content on the crust and in the mantle (Ogihara & Ida 2009;Unterborn et al 2018;Pan et al 2022). Furthermore, ice-rich planetary embryos may migrate interior to the snow line that would otherwise have been blocked by the presence of a giant planet (Izidoro et al 2015;Bitsch & Savvidou 2021).…”

A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models

“…In the core-accretion model of planet formation, planets grow from initial over densities (known as cores) in a protoplanetary disk, which accrete dust and gas from the surrounding disk. Models in which initial cores grow through the accretion of ~1-km-sized solid bodies (called planetesimals) ( 8 – 10 , 39 ), or by accreting pebble-sized material ( 40 , 41 ), predict that very low-mass stars are only capable of forming compact systems of rocky planets on short-period orbits. The maximum mass of planets formed through core accretion in simulations of the planetesimal-driven scenario around low-mass stars is about 5M⊕ ( 9 ), and 3M⊕ in the pebble accretion–driven scenario ( 40 , 41 ).…”