COMPOSITIONAL DIVERSITY IN THE ATMOSPHERES OF HOT NEPTUNES, WITH APPLICATION TO GJ 436b

Area of Science:

• Exoplanetary Science
• Atmospheric Chemistry
• Astrochemistry

Background:

• Neptune-sized exoplanets orbiting close to their stars, known as hot Neptunes, are frequently detected.
• These planets are expected to have masses dominated by heavy elements due to inefficient gas accretion, similar to Uranus and Neptune.

Purpose of the Study:

• To investigate the chemical composition of hot Neptune atmospheres under varying conditions.
• To explore the range of possible atmospheric compositions, from hydrogen-dominated to carbon dioxide-dominated.

Main Methods:

• Modeling equilibrium and disequilibrium chemistry for generic hot Neptunes.
• Analyzing atmospheric composition as a function of temperature, metallicity, and C/O ratio.
• Applying models to exoplanet GJ 436b and comparing with Spitzer eclipse data.

Main Results:

• Atmospheric composition strongly depends on temperature and bulk properties like metallicity and C/O ratio.
• Exotic atmospheres (H2O, CO, CO2, O2-dominated) are predicted for hot Neptunes.
• A high metallicity scenario explains a CO-rich, CH4-poor atmosphere for GJ 436b.

Conclusions:

• High atmospheric metallicity (10-10,000x solar) is a plausible explanation for observed hot Neptune atmospheric compositions.
• Forward models with high metallicity offer a more physically plausible explanation for GJ 436b's atmosphere than retrieval methods alone.
• High-metallicity atmospheres should be considered for GJ 436b and other hot Neptunes.