The effects of snowlines on C/O in planetary atmospheres (1110.5567v1)

Abstract: The C/O ratio is predicted to regulate the atmospheric chemistry in hot Jupiters. Recent observations suggest that some exo-planets, e.g. Wasp 12- b, have atmospheric C/O ratios substantially different from the solar value of 0.54. In this paper we present a mechanism that can produce such atmospheric deviations from the stellar C/O ratio. In protoplanetary disks, different snowlines of oxygen- and carbon-rich ices, especially water and carbon monoxide, will result in systematic variations in the C/O ratio both in the gas and in the condensed phase. In particular, between the H2O and CO snowlines most oxygen is present in icy grains - the building blocks of planetary cores in the core accretion model - while most carbon remains in the gas-phase. This region is coincidental with the giant-planet forming zone for a range of observed protoplanetary disks. Based on standard core accretion models of planet formation, gas giants that sweep up most of their atmospheres from disk gas outside of the water snowline will have C/O?1, while atmospheres significantly contaminated by evaporating planetesimals will have stellar or sub-stellar C/O when formed at the same disk radius. The overall metallicity will also depend on the atmosphere formation mechanism, and exoplanetary atmospheric compositions may therefore provide constraints on where and how a specific planet formed.

• The paper presents a mechanistic model that explains deviations in C/O ratios in hot Jupiter atmospheres by examining the influence of snowlines in protoplanetary disks.
• It demonstrates that the location relative to water and CO snowlines determines whether a planet’s atmosphere maintains near-stellar or near-unity C/O ratios.
• The findings highlight that precise spectroscopic measurements of exoplanet atmospheres can reveal planet formation history and migration dynamics.

The Effects of Snowlines on C/O Ratios in Planetary Atmospheres

The paper by Öberg et al. investigates the intriguing variations in carbon-to-oxygen (C/O) ratios observed in the atmospheres of exoplanets, particularly 'hot Jupiters.' The focus is on a mechanistic model that explains deviations from stellar C/O values by considering the environment within protoplanetary disks. These deviations are observed substantially in some exoplanets like WASP-12b, suggesting fundamental processes in planet formation that differ from previously held assumptions.

Mechanism of C/O Variations

In protoplanetary disks, the snowlines of various volatiles such as water (H₂O), carbon monoxide (CO), and carbon dioxide (CO₂) play pivotal roles. The paper postulates that these snowlines cause systematic variations in the C/O ratio, both in gaseous forms and condensed ices. Within the core accretion model for planet formation, a significant point is that between the H₂O and CO snowlines, oxygen-rich ices predominantly form solid grains, while most carbon persists in gaseous form. This region overlaps with the giant-planet forming zone, supporting the notion that differing snowlines can significantly influence atmospheric compositions in emerging gas giants.

Gas giants accumulating most of their atmospheres from disk gas beyond the water snowline are predicted to exhibit C/O ratios near unity. Conversely, atmospheres substantially contaminated by evaporating planetesimals will show stellar or sub-stellar C/O ratios at comparable disk radii. The model suggests that atmospheric metallicity and C/O ratios provide constraints on the location and mechanics of a planet's formation.

Implications of the Findings

Öberg et al.'s exploration carries implications for both the theoretical understanding and observational characterization of planetary atmospheres:

• Theoretical Insights: The research suggests that device independent observational parameters like C/O and C/H ratios are reflective of specific planet formation histories. These parameters might elucidate variations in elemental distributions and chemistry across different planetary systems.
• Observational Perspectives: Determining accurate atmospheric C/O ratios becomes crucial, highlighting the need for advanced spectroscopic techniques. Successful observations can retroactively infer the location and conditions of a planet's origin in its protoplanetary disk.
• Broader Context: This analysis extends to the interpretation of gas giants in our own Solar System. While the atmospheres of distant exoplanets like WASP-12b show similar extreme C/O ratios, uncertainty in volatile mixing complicates Jupiter's bulk compositions. Understanding and comparing these aspects could constrain migration models and internal dynamics of planets both domestically and beyond.

Numerics and Contrasts

While the paper does not present exact numeric validations for individual exoplanets, it sets bold premises describing how planetesimal accretion dynamics and frozen volatile shifts imply altered C/O ratios. The contrast between atmospheres predominantly formed from gas versus those contaminated with solid materials stands out as a critical discernment of planetary evolutionary states.

Future Directions

Future developments might explore the dynamics of evolving disk profiles and chemical evolution, extending beyond static assumptions. The inclusion of detailed time-resolved dynamic modeling to account for intricate planetesimal processes and disk-planet interactions might offer refined predictions. Such advancements could enhance our understanding of how prevalent carbon-rich atmospheres are and how common they might be in diverse stellar environments.

The paper shines a light on the complex relationships between snowline positioning, accretion processes, and atmospheric compositions, laying groundwork for a deeper comprehension that blends observational astrophysics, planetary science, and astrochemistry.