# Pebbles versus Planetesimals: The case of Trappist-1

## TL;DR

This study compares pebble and planetesimal accretion scenarios in forming planetary systems like Trappist-1, analyzing their similarities, differences, and implications for planetary water content and formation pathways.

## Contribution

It provides a comprehensive simulation-based comparison of pebble versus planetesimal accretion, highlighting their effects on planetary system characteristics and water content.

## Key findings

- Both scenarios can produce systems similar to Trappist-1 in mass and orbital configuration.
- Planets formed from pebbles tend to be drier than those from planetesimals when full recycling is considered.
- Water content differences depend on the treatment of ablation and recycling processes.

## Abstract

We present a study on the formation of planetary systems around low mass stars similar to Trappist-1, through the accretion of either planetesimals or pebbles. The aim is to determine if the currently observed systems around low mass stars favour one scenario over the other. We ran numerous N-body simulations, coupled to a thermally evolving viscous disc model, including prescriptions for planet migration and photoevaporation. We examine the differences between the pebble and planetesimal accretion scenarios, but also look at the influences of disc mass, planetesimal size, and the percentage of solids locked up within pebbles. When comparing the resulting planetary systems to Trappist-1, we find that a wide range of initial conditions for both accretion scenarios can form planetary systems similar to Trappist-1, in terms of planet mass, periods, and resonant configurations. Typically these planets formed exterior to the water iceline and migrated in resonant convoys to close to the central star. When comparing the planetary systems formed from pebbles to those formed from planetesimals, we find a large number of similarities, including average planet masses, eccentricities, inclinations and period ratios. One major difference was that of the water content of the planets. When including the effects of ablation and full recycling of the planets envelope with the disc, planets formed from pebbles were extremely dry, whilst those formed from planetesimals were extremely wet. If the water content is not fully recycled and instead falls to the planets core, or if ablation of the water is neglected, then the planets formed from pebbles are extremely wet, similar to those formed from planetesimals. Should the water content of the Trappist-1 planets be determined accurately, this could point to a preferred formation pathway for planetary systems, or to specific physics that may be at play.