Where can a Trappist-1 planetary system be produced?

TL;DR

This study models the evolution of protoplanetary discs around M dwarfs to determine conditions under which a Trappist-1-like planetary system can form, emphasizing the importance of initial disc mass and dust resilience to photoevaporation.

Contribution

It provides the first detailed analysis of how external photoevaporation influences dust retention and planet formation potential in Trappist-1-like systems.

Findings

Dust quickly grows beyond photoevaporation entrainment size.

High initial disc mass (>30% of stellar mass) is needed for planet formation.

Even in low UV environments, significant dust loss occurs early.

Abstract

We study the evolution of protoplanetary discs that would have been precursors of a Trappist-1 like system under the action of accretion and external photoevaporation in different radiation environments. Dust grains swiftly grow above the critical size below which they are entrained in the photoevaporative wind, so although gas is continually depleted, dust is resilient to photoevaporation after only a short time. This means that the ratio of the mass in solids (dust plus planetary) to the mass in gas rises steadily over time. Dust is still stripped early on, and the initial disc mass required to produce the observed $4\,M_{\oplus}$ of Trappist-1 planets is high. For example, assuming a Fatuzzo & Adams (2008) distribution of UV fields, typical initial disc masses have to be $>30\,$per cent the stellar (which are still Toomre $Q$ stable) for the majority of similar mass M dwarfs to be viable hosts of the Trappist-1 planets. Even in the case of the lowest UV environments observed, there is a strong loss of dust due to photoevaporation at early times from the weakly bound outer regions of the disc. This minimum level of dust loss is a factor two higher than that which would be lost by accretion onto the star during 10 Myr of evolution. Consequently even in these least irradiated environments, discs that are viable Trappist-1 precursors need to be initially massive ($>10\,$per cent of the stellar mass).