# Are the observed gaps in protoplanetary discs caused by growing planets?

## TL;DR

This study investigates whether the gaps observed in protoplanetary discs are caused by growing planets, using population synthesis models to compare theoretical predictions with observations, and finds discrepancies suggesting alternative explanations or missing physics.

## Contribution

The paper introduces a comprehensive planet population synthesis model including pebble and gas accretion, migration, and compares the results with observed disc gaps and exoplanet populations.

## Key findings

- High pebble mass needed for gap formation, only possible in metal-rich stars.
- Formed planets predominantly become gas giants, inconsistent with observed super-Earths.
- Discrepancies suggest gaps may not always be caused by planets or missing physics in models.

## Abstract

Recent detailed observations of protoplanetary discs revealed a lot of sub-structures which are mostly ring-like. One interpretation is that these rings are caused by growing planets. These potential planets are not yet opening very deep gaps in their discs. These planets instead form small gaps in the discs to generate small pressure bumps exterior to their orbits that stop the inflow of the largest dust particles. In the pebble accretion paradigm, this planetary mass corresponds to the pebble isolation mass, where pebble accretion stops and efficient gas accretion starts. We perform planet population synthesis via pebble and gas accretion including type-I and type-II migration. In the first stage of our simulations, we investigate the conditions necessary for planets to reach the pebble isolation mass and compare their position to the observed gaps. We find that in order to match the gap structures 2000 M E in pebbles is needed, which would be only available for the most metal rich stars. We then follow the evolution of these planets for a few My to compare the resulting population with the observed exoplanet populations. Planet formation in discs with these large amounts of pebbles result in mostly forming gas giants and only very little super-Earths, contradicting observations. This leads to the conclusions that either (i) the observed discs are exceptions, (ii) not all gaps in observed discs are caused by planets or (iii) that we miss some important ingredients in planet formation related to gas accretion and/or planet migration.