How drifting and evaporating pebbles shape giant planets

Abstract

Recent observations of extrasolar gas giants suggest super-stellar C/O ratios in planetary atmospheres, while interior models of observed extrasolar giant planets additionally suggest high heavy element contents. Furthermore, recent observations of protoplanetary disks revealed super-solar C/H ratios, which are explained by inward drifting and evaporating pebbles enhancing the volatile content of the disk. We investigate in this work how the inward drift and evaporation of volatile-rich pebbles influences the atmospheric C/O ratio and heavy element content of giant planets growing by pebble and gas accretion. To achieve this goal, we perform semi-analytical 1D models of protoplanetary disks, including the treatment of viscous evolution and heating, pebble drift, and simple chemistry to simulate the growth of planets from planetary embryos to Jupiter-mass objects by the accretion of pebbles and gas while they migrate through the disk. Our simulations show that the composition of the planetary gas atmosphere is dominated by the accretion of vapor that originates from inward drifting evaporating pebbles at evaporation fronts. This process allows the giant planets to harbor large heavy element contents, in contrast to models that do not take pebble evaporation into account. In addition, our model reveals that giant planets originating farther away from the central star have a higher C/O ratio on average due to the evaporation of methane-rich pebbles in the outer disk. These planets can then also harbor super-solar C/O ratios, in line with exoplanet observations. However, planets formed in the outer disk harbor a smaller heavy element content due to a smaller vapor enrichment of the outer disk compared to the inner disk, where the very abundant water ice also evaporates. Our model predicts that giant planets with low or large atmospheric C/O should harbor a large or low total heavy element content. We further conclude that the inclusion of pebble evaporation at evaporation lines is a key ingredient for determining the heavy element content and composition of giant planets.

Confirmation of Water Absorption in the Thermal Emission Spectrum of the Hot Jupiter WASP-77Ab with HST/WFC3

“…For example, a key prediction of the core accretion theory of planet formation is that atmospheric metallicities should be inversely proportional to planet mass (Fortney et al 2013). Furthermore, the carbon-tooxygen ratio (C/O) provides information on the mechanisms through which hot Jupiters form and migrate to their current locations (Öberg et al 2011;Madhusudhan et al 2014;Mordasini et al 2016;Ali-Dib 2017;Espinoza et al 2017;Schneider & Bitsch 2021).…”

The Effect of Accretion Rate and Composition on the Structure of Ice-rich Super-Earths

“…It is important to note that different assumptions regarding the mechanism of formation and accretion (such as pebble accretion) lead to very different M  profiles (e.g., Lambrechts & Johansen 2012;Bitsch et al 2015;Ida et al 2016;Alibert et al 2018;Grishin et al 2020;Schneider & Bitsch 2021). These studies assume that the growth is rapid enough so that the core can attract a significant H-He envelope from the surrounding nebula.…”

Chemical Composition Of Bright Stars In The Northern Hemisphere: Star-Planet Connection

“…We refer to comprehensive theoretical studies concerning possible scenarios of planet formation and star-planet chemical composition relations, which try to incorporate and explain the observational results (e.g. Thiabaud et al 2015;Bitsch & Battistini 2020;Adibekyan et al 2021;Cowley et al 2021;Nibauer et al 2021;Schneider & Bitsch 2021;Schulze et al 2021, and references therein).…”