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

2021

A&A

Upcoming studies of extrasolar gas giants will give precise insights into the composition of planetary atmospheres, with the ultimate goal of linking it to the formation history of the planet. Here, we investigate how drifting and evaporating pebbles that enrich the gas phase of the disk influence the chemical composition of growing and migrating gas giants. To achieve this goal, we perform semi-analytical 1D models of protoplanetary disks, including viscous evolution, pebble drift, and evaporation, 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. The gas phase of the protoplanetary disk is enriched due to the evaporation of inward drifting pebbles crossing evaporation lines, leading to the accretion of large amounts of volatiles into the planetary atmosphere. As a consequence, gas-accreting planets are enriched in volatiles (C, O, N) compared to refractories (e.g., Mg, Si, Fe) by up to a factor of 100, depending on the chemical species, its exact abundance and volatility, and the disk’s viscosity. A simplified model for the formation of Jupiter reveals that its nitrogen content can be explained by inward diffusing nitrogen-rich vapor, implying that Jupiter did not need to form close to the N2 evaporation front as indicated by previous simulations. However, our model predicts an excessively low oxygen abundance for Jupiter, implying either Jupiter’s migration across the water ice line (as in the grand tack scenario) or an additional accretion of solids into the atmosphere (which can also increase Jupiter’s carbon abundance, ultimately changing the planetary C/O ratio). The accretion of solids, on the other hand, will increase the refractory-to-volatile ratio in planetary atmospheres substantially. We thus conclude that the volatile-to-refractory ratio in planetary atmospheres can place a strong constraint on planet formation theories (in addition to elemental ratios), especially on the amount of solids accreted into atmospheres, making it an important target for future observations.

Section: Methodsmentioning

confidence: 99%

Section: Simple Growth Model For Jupiter and Saturnmentioning

confidence: 99%

How drifting and evaporating pebbles shape giant planets

2021

A&A

“…( 31) and (32) from Paardekooper (2014)). The dynamical torque can also help to significantly slow down inward migration of low mass planets, preventing large-scale migration of planets (Ndugu et al 2021).…”

Section: Migrationmentioning

confidence: 99%

“…In this work we focus thus on a disk model that includes the growth and drift of pebbles (Birnstiel et al 2012) as well as evaporation and condensation at evaporation fronts. We then model the growth of planets via the accretion of pebbles (Johansen & Lambrechts 2017) and gas (Ndugu et al 2021) inside these disks, while tracing the chemical composition of the accreted material to derive the atmospheric C/O ratio as well as the heavy element content. Our model approach is outlined in the cartoon shown in Fig.…”

Section: Introductionmentioning

confidence: 99%

How drifting and evaporating pebbles shape giant planets

2021

A&A

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.

“…Planets that start to accrete gas efficiently start to open gaps in the protoplanetary disk and then migrate on a much longer timescale that depends on the depth of the gap (Kanagawa et al 2018). More massive planets open deeper gaps and thus migrate more slowly, also influenced by the gas accretion rate itself Bergez-Casalou et al 2020;Ndugu et al 2021). The migration speed in the type-II regime, and thus also the distance over which planets migrate, also crucially depends on the disk's viscosity.…”

Section: Planet Migration and The Timing Of Planet Formationmentioning

confidence: 99%

Dry or water world? How the water contents of inner sub-Neptunes constrain giant planet formation and the location of the water ice line

Raymond

Buchhave

et al. 2021

A&A

In the pebble accretion scenario, the pebbles that form planets drift inward from the outer disk regions, carrying water ice with them. At the water ice line, the water ice on the inward drifting pebbles evaporates and is released into the gas phase, resulting in water-rich gas and dry pebbles that move into the inner disk regions. Large planetary cores can block the inward drifting pebbles by forming a pressure bump outside their orbit in the protoplanetary disk. Depending on the relative position of a growing planetary core relative to the water ice line, water-rich pebbles might be blocked outside or inside the water ice line. Pebbles blocked outside the water ice line do not evaporate and thus do not release their water vapor into the gas phase, resulting in a dry inner disk, while pebbles blocked inside the water ice line release their water vapor into the gas phase, resulting in water vapor diffusing into the inner disk. As a consequence, close-in sub-Neptunes that accrete some gas from the disk should be dry or wet, respectively, if outer gas giants are outside or inside the water ice line, assuming that giant planets form fast, as has been suggested for Jupiter in our Solar System. Alternatively, a sub-Neptune could form outside the water ice line, accreting a large amount of icy pebbles and then migrating inward as a very wet sub-Neptune. We suggest that the water content of inner sub-Neptunes in systems with giant planets that can efficiently block the inward drifting pebbles could constrain the formation conditions of these systems, thus making these sub-Neptunes exciting targets for detailed characterization (e.g., with JWST, ELT, or ARIEL). In addition, the search for giant planets in systems with already characterized sub-Neptunes can be used to constrain the formation conditions of giant planets as well.