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

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.

Interpreting high spatial resolution line observations of planet-forming disks with gaps and rings: the case of HD 163296

“…Such information will be crucial to relate the forming planet population to the observed exoplanet population (see e.g. Lodato et al 2019;Ndugu et al 2019).…”

Probing the impact of varied migration and gas accretion rates for the formation of giant planets in the pebble accretion scenario

“…These models include type I (Paardekooper et al 2011) and type-II migration (Baruteau et al 2014), as well as gas accretion (Piso & Youdin 2014;Machida et al 2010). Ndugu et al (2018Ndugu et al ( , 2019) follow the disc model of Bitsch et al (2015a). used a modified pebble-accretion approach with relatively smaller pebble sizes than used in previous studies (Bitsch & Johansen 2017;Ndugu et al 2018;Chambers 2018;Brügger et al 2018;Ndugu et al 2019), showing how planetary growth can out perform planet migration.…”

“…Ndugu et al (2018Ndugu et al ( , 2019) follow the disc model of Bitsch et al (2015a). used a modified pebble-accretion approach with relatively smaller pebble sizes than used in previous studies (Bitsch & Johansen 2017;Ndugu et al 2018;Chambers 2018;Brügger et al 2018;Ndugu et al 2019), showing how planetary growth can out perform planet migration. The pebble-based planet population synthesis models compute the final mass and location of planets depending on the initial time when the planetary embryo was placed in the disc, the location at which they started to grow and the disc parameters.…”

Planetary core formation via multi-species pebble accretion

“…A planetary core may also grow into a full planet by accreting aerodynamically coupled bodies via gas drag, popularly known as pebble accretion (Johansen & Lacerda 2010;Ormel & Klahr 2010;Lambrechts & Johansen 2012;. Though the current framework of planet formation by core accretion of planetesimals or pebbles is the most successful, it cannot satisfactorily attribute the observed substructures at wider orbital locations to planets (Lodato et al 2019;Ndugu et al 2019;Nayakshin et al 2019).…”

“…However, previous works that studied core growth via pebble accretion (e.g., Guillot et al 2014;Morbidelli et al 2015;Bitsch et al 2015;Bitsch & Johansen 2017;Ndugu et al 2018;Brügger et al 2018;Ndugu et al 2019;Ndugu et al 2021) were based on two standard prescriptions. Firstly, the studies used a single spatiotemporal dominant particle size or Stokes number, which is assumed to carry most of the solid mass.…”