[Paper Review] Interpreting the atmospheric composition of exoplanets: sensitivity to planet formation assumptions

P. Mollière, Tamara Molyarova+14 more|arXiv (Cornell University)|Apr 28, 2022

Stellar, planetary, and galactic studies226 references154 citations

TL;DR

This study develops a methodology to invert exoplanet atmospheric compositions into constraints on planet formation, focusing on HR 8799e. By testing different formation model assumptions—such as disk chemical evolution and pebble accretion—it finds that chemical evolution reduces the need for radial migration, while pebble accretion can reproduce observed abundances but often underpredicts atmospheric metallicity, highlighting model-dependent uncertainties in formation inference.

ABSTRACT

Constraining planet formation based on the atmospheric composition of exoplanets is a fundamental goal of the exoplanet community. Existing studies commonly try to constrain atmospheric abundances, or to analyze what abundance patterns a given description of planet formation predicts. However, there is also a pressing need to develop methodologies that investigate how to transform atmospheric compositions into planetary formation inferences. In this study we summarize the complexities and uncertainties of state-of-the-art planet formation models and how they influence planetary atmospheric compositions. We introduce a methodology that explores the effect of different formation model assumptions when interpreting atmospheric compositions. We apply this framework to the directly imaged planet HR 8799e. Based on its atmospheric composition, this planet may have migrated significantly during its formation. We show that including the chemical evolution of the protoplanetary disk leads to a reduced need for migration. Moreover, we find that pebble accretion can reproduce the planet's composition, but some of our tested setups lead to too low atmospheric metallicities, even when considering that evaporating pebbles may enrich the disk gas. We conclude that the definitive inversion from atmospheric abundances to planet formation for a given planet may be challenging, but a qualitative understanding of the effects of different formation models is possible, opening up pathways for new investigations.

Motivation & Objective

• To address the critical challenge of inferring planet formation histories from atmospheric compositions, which remains difficult due to model uncertainties.
• To investigate how different formation model assumptions—such as disk chemical evolution and pebble accretion—affect the interpretation of observed atmospheric abundances.
• To develop a systematic methodology for assessing the consequences of formation model choices when interpreting exoplanet atmospheres.
• To test the feasibility of inverting atmospheric compositions into formation constraints using HR 8799e as a case study.
• To identify key obstacles and model dependencies in the atmospheric-to-formation inference process, especially for directly imaged planets.

Proposed method

• Applies a nested sampling method to constrain formation parameters based on atmospheric composition, enabling probabilistic inference under different model assumptions.
• Integrates state-of-the-art planet formation models with chemical equilibrium calculations to predict atmospheric abundances under varying formation scenarios.
• Tests the impact of including chemical evolution in protoplanetary disks, which alters elemental abundances over time due to condensation and evaporation processes.
• Evaluates pebble accretion models with varying initial compositions and accretion rates, accounting for potential gas-phase enrichment from evaporating pebbles.
• Uses atmospheric retrieval techniques to compare predicted compositions with observed data, particularly for HR 8799e.
• Employs equilibrium chemistry models (e.g., Mollière et al. 2017) to compute molecular and atomic species abundances across temperature and pressure regimes relevant to exoplanet atmospheres.

Experimental results

Research questions

• RQ1 How do different planet formation model assumptions—such as disk chemical evolution and pebble accretion—affect the predicted atmospheric composition of exoplanets?
• RQ2 To what extent can the observed atmospheric composition of HR 8799e be reproduced without invoking significant radial migration?
• RQ3 Can pebble accretion models reproduce the observed atmospheric metallicity and C/O ratio of HR 8799e, and what are the limitations?
• RQ4 How do uncertainties in disk chemistry and condensation processes propagate into atmospheric composition predictions and formation inferences?
• RQ5 What are the key model-dependent biases in inferring planet formation history from atmospheric abundances, and how can they be quantified?

Key findings

• Including chemical evolution in the protoplanetary disk reduces the required radial migration for HR 8799e to achieve its observed atmospheric composition, suggesting migration may be less critical than previously assumed.
• Pebble accretion models can reproduce the atmospheric composition of HR 8799e, but many setups predict atmospheric metallicities that are too low compared to observations.
• Even when accounting for potential enrichment from evaporating pebbles, some pebble accretion scenarios still fail to match the observed metallicity, indicating a persistent tension in the models.
• The study demonstrates that the inversion from atmospheric composition to formation history is highly sensitive to model assumptions, particularly regarding disk chemistry and accretion efficiency.
• Despite challenges in definitive inference, the framework enables a qualitative understanding of how different formation pathways—such as migration or pebble accretion—affect atmospheric signatures.
• The results highlight that uncertainties in atmospheric characterization may soon be outpaced by uncertainties in formation models, necessitating improved modeling of disk chemical evolution and condensation processes.