How drifting and evaporating pebbles shape giant planets

Abstract

Atmospheric abundances of exoplanets are thought to constrain the planet formation pathway, because different species evaporate at different temperatures and thus radii in the protoplanetary disc, leaving distinct signatures inside the accreted planetary atmosphere. In particular the planetary C/O ratio is thought to constrain the planet formation pathway, because of the condensation sequence of H 2 O, CO 2 , CH 4 , and CO, resulting in an increase of the gas phase C/O ratio with increasing distance to the host star. Here we use a disc evolution model including pebble growth, drift and evaporation coupled with a planet formation model that includes pebble and gas accretion as well as planet migration to compute the atmospheric compositions of giant planets. We compare our results to the recent observational constraints of the hot Jupiters WASP-77A b and τ Boötis b. WASP-77A b's atmosphere features sub-solar C/H, O/H, H 2 O/H with slightly super-solar C/O, while τ Boötis b's atmosphere features super-solar C/H, O/H and C/O with sub-solar H 2 O/H. Our simulations qualitatively reproduce these measurements and show that giants like WASP-77A b should start to form beyond the CO 2 evaporation front, while giants like τ Boötis b should originate from beyond the water ice line. Our model allows the formation of sub-and super-solar atmospheric compositions within the same framework. On the other hand, simulations without pebble evaporation, as used in classical models, can not reproduce the super-solar C/H and O/H ratios of τ Boötis b's atmosphere without the additional accretion of solids. Furthermore, we identify the α viscosity parameter of the disc as a key ingredient regarding planetary composition, because the viscosity drives the inward motion of volatile enriched vapor, responsible for the accretion of gaseous carbon and oxygen. Depending on the planet's migration history through the disc across different evaporation fronts, orderof-magnitude differences in atmospheric carbon and oxygen abundance should be expected. Our simulations additionally predict super-solar N/H for τ Boötis b and solar N/H for WASP-77A b. We conclude thus that pebble evaporation is a key ingredient to explain the variety of exoplanet atmospheres, because it can explain both, sub-and super-solar atmospheric abundances.

Breaking Degeneracies in Formation Histories by Measuring Refractory Content in Gas Giants

“…In locations affected by such processes, the abundances of these elements in the gas and solid phases can become much larger than solar values. The accretion of enriched gas and solids from these regions can be an important source of heavy element enrichment in planetary envelopes (e.g., Schneider & Bitsch 2021a;Bitsch et al 2022). We note that although refractory carbon is also expected to sublimate at the soot line, the resulting carbon-rich gas does not recondense when it diffuses beyond the soot line because of the irreversibility of refractory carbon sublimation.…”

Confirmation of Subsolar Metallicity for WASP-77Ab from JWST Thermal Emission Spectroscopy

“…The low metallicity for WASP-77Ab inferred by Line et al (2021) is surprising from an origins standpoint because gas giants are expected to accrete planetesimals that pollute their growing atmospheres during formation. Nevertheless, recent work suggests that subsolar atmospheric abundances for closein giant planets indicate formation beyond the CO 2 snow line (a > 5 au; Bitsch et al 2022). The results for WASP-77Ab are also surprising given the recent series of strong CO 2 detections in hot Jupiter atmospheres that are indicative of enhanced atmospheric metallicities (Alderson et al 2023;Bean et al 2023;JWST Transiting Exoplanet Community Early Release Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.…”

Nitrogen as a Tracer of Giant Planet Formation. II. Comprehensive Study of Nitrogen Photochemistry and Implications for Observing NH3 and HCN in Transmission and Emission Spectra

“…The composition of giant planet atmospheres offers valuable clues to the planet formation process. In recent years, a number of studies have suggested that atmospheric elemental ratios, such as the carbon-to-oxygen ratio (C/O), can diagnose a giant planet's formation location and accretion history within a disk (e.g., Öberg et al 2011;Ali-Dib et al 2014;Helling et al 2014;Madhusudhan et al 2014Madhusudhan et al , 2017Piso et al 2015Piso et al , 2016Thiabaud et al 2015;Cridland et al 2016Cridland et al , 2017Cridland et al , 2019Eistrup et al 2016Eistrup et al , 2018Eistrup et al , 2022Öberg & Bergin 2016;Booth et al 2017;Espinoza et al 2017;Booth & Ilee 2019;Öberg & Wordsworth 2019;Ohno & Ueda 2021;Schneider & Bitsch 2021a;Turrini et al 2021;Bitsch et al 2022;Mollière et al 2022;Pacetti et al 2022;Eistrup 2023). Recent studies also suggest that nitrogen provides valuable insights into planet formation processes (Piso et al 2016;Cridland et al 2020;Ohno & Ueda 2021;Turrini et al 2021;Notsu et al 2022).…”