C/O Ratios and the Formation of Wide-separation Exoplanets

• Published in: Astrophysical journal. Letters Vol. 969; no. 1; p. L21
• Main Authors: Bergin, Edwin A., Booth, Richard A., Colmenares, Maria Jose, Ilee, John D.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01-07-2024; IOP Publishing
• Subjects: Accretion disks; Astronomical models; Atmospheric composition; Carbon content; Circumstellar disks; Cloud formation; Depletion; Estimates; Exoplanet atmospheric composition; Exoplanet formation; Extrasolar planets; Gas giant planets; Gravitational instability; Jupiter; Metallicity; Planet formation; Planetary atmospheres; Planetary composition; Planets; Protoplanetary disks; Radio telescopes; Ratios; Separation; Solid state
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ad5839

The gas and solid-state C/O ratios provide context to potentially link the atmospheric composition of planets to that of the natal disk. We provide a synthesis of extant estimates of the gaseous C/O and C/H ratios in planet-forming disks obtained primarily through analysis of Atacama Large Millimeter/submillimeter Array observations. These estimates are compared to atmospheric abundances of wide-separation (>10 au) gas giants. The resolved disk gas C/O ratios, from seven systems, generally exhibit C/O ≥ 1 with subsolar, or depleted, carbon content. In contrast, wide-separation gas giants have atmospheric C/O ratios that cluster near or slightly above the presumed stellar value with a range of elemental C/H. From the existing disk composition, we infer that the solid-state millimeter/centimeter-sized pebbles have a total C/O ratio (solid cores and ices) that is solar (stellar) in content. We explore simple models that reconstruct the exoplanet atmospheric composition from the disk, while accounting for silicate cloud formation in the planet atmosphere. If wide-separation planets formed via the core-accretion mechanism, they must acquire their metals from pebble or planetesimal accretion. Further, the dispersion in giant planet C/H content is best matched by a disk composition with modest and variable factors of carbon depletion. An origin of the wide-separation gas giants via gravitational instability cannot be ruled out, as stellar C/O ratios should natively form in this scenario. However, the variation in planet metallicity with a stellar C/O ratio potentially presents challenges to these models.