Searching for a dusty cometary belt around TRAPPIST-1 with ALMA

Abstract

Low mass stars might offer today the best opportunities to detect and characterise planetary systems, especially those harbouring close-in low mass temperate planets. Among those stars, Trappist-1 is exceptional since it has seven Earth-sized planets, of which three could sustain liquid water on their surfaces. Here we present new and deep ALMA observations of Trappist-1 to look for an exo-Kuiper belt which can provide clues about the formation and architecture of this system. Our observations at 0.88 mm did not detect dust emission, but can place an upper limit of 23µJy if the belt is unresolved or smaller than 4 au, and 0.15 mJy if resolved and 100 au in radius. These limits correspond to low dust masses of ∼ 10 −5 − 10 −2 M ⊕ , which are expected after 8 Gyr of collisional evolution unless the system was born with a > 20 M ⊕ belt of 100 km-sized planetesimals beyond 40 au. This 20 M ⊕ mass upper limit is comparable to the combined mass in Trappist-1 planets, thus it is possible that most of the available solid mass in these systems was used to form the known planets. A similar analysis of the ALMA data on Proxima Cen leads us to conclude that Proxima Cen cannot have been born with a belt of 100 km-sized planetesimals more massive than 10 M ⊕ between 20-40 au. Future characterisations of debris discs around low mass stars should focus on nearby and young systems which have not suffered significant collisional evolution beyond 10 au.

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

“…We note there are some ongoing efforts to characterize the outer part of the TRAPPIST-1 system (Marino et al. 2020 ) to better constrain the whole architecture of the system and possibly look for exocometary or exoasteroid belts.…”

Herschel Observations of Disks around Late-type Stars

“…Currently, we do know that circumstellar disks around young stars evolve and dissipate, but the physical mechanisms responsible and the timescale for disk dispersion are not yet observationally constrained. On the other hand, stellar age could be playing a role as, on average, most field late-type stars are older thus allowing for additional collisional evolution within the disk which would lead to a lower luminosity for the same initial disk mass (i.e., Marino et al 2020). Here, we present the results of a small Herschel imaging survey of young or nearby M dwarfs.…”

Risks for Life on Proxima b from Sterilizing Impacts

“…If a debris belt exists around Luyten's star (see Region C described in Pozuelos et al 2020), secular resonances with mini-Neptunes GJ 273d or GJ 273e could drive impacts on the potentially habitable planet GJ 273b. The existence of a giant planet (Boss et al 2017) and a debris belt (Marino et al 2020) exterior to the TRAPPIST-1 planets would affect the sterilization risk in that system.…”