Coupled Thermal-Chemical Evolution Models of Sub-Neptunes Reveal Atmospheric Signatures of Their Formation Location

Abstract

The observed masses and radii of sub-Neptunes are typically explained by the gas dwarf and the water world scenarios. While their evolutionary history on a population level has been proposed as a method to distinguish between these compositions, previous evolutionary models, neglected the crucial role of atmosphere-interior chemical interaction. We present a novel evolution framework for sub-Neptunes that combines the thermal evolution with the chemical coupling of the atmosphere and interior. Using this model, we examine how planets formed inside and outside the water-ice line can be observationally distinguished, with an emphasis on their atmospheric properties. Young sub-Neptunes store the majority of their volatile budget in the interior, regardless of formation location. Nevertheless, the atmospheric metallicity is a factor 4 higher for the planet formed outside the water-ice line. During cooling, hydrogen and oxygen exsolve from the interior, increasing the atmospheric mass fraction and counteracting the thermal contraction. Consequently, radius evolution alone cannot distinguish between the two formation scenarios. Instead, the primary discriminators are the abundance of carbon-bearing species and the resulting atmospheric C/O ratio. For sub-Neptunes formed beyond the water-ice line, nearly all carbon resides in the gaseous phase. We find that high molar fractions of CH${}_4$ ($>10^{-2}$) and H${}_2$O ($>5\times 10^{-2}$), and a high C/O ratio $(>5\times 10^{-1})$ are indicative of formation outside the water-ice line. In contrast, sub-Neptunes formed inside the water-ice line exhibit strongly suppressed CH${}_4$ abundances, yielding C/O ratios ranging from $10^{-7}$ to $10^{-1}$.

19 pages, 7 figures, updated to reflect published version in ApJ

References in corpus (43)

• Array Programming with NumPy
• Most 1.6 Earth-Radius Planets are not Rocky
• Growth Model Interpretation of Planet Size Distribution
• On the radiative equilibrium of irradiated planetary atmospheres
• Rayleigh scattering in the transit spectrum of HD 189733b
• The imprint of exoplanet formation history on observable present-day spectra of hot Jupiters
• Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation
• Can we constrain interior structure of rocky exoplanets from mass and radius measurements?
• Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• VULCAN: an Open-Source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres
• A generalized bayesian inference method for constraining the interiors of super Earths and sub-Neptunes
• How drifting and evaporating pebbles shape giant planets I: Heavy element content and atmospheric C/O
• Predictions of the atmospheric composition of GJ 1132b
• A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve
• HELIOS-K 2.0 Opacity Calculator and Open-source Opacity Database for Exoplanetary Atmospheres
• Self-luminous and irradiated exoplanetary atmospheres explored with HELIOS
• Condensation-inhibited convection in hydrogen-rich atmospheres: Stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune
• Earth shaped by primordial H${}_2$ atmospheres
• Water on Hot Rocky Exoplanets
• Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis
• JWST/NIRISS reveals the water-rich "steam world" atmosphere of GJ 9827 d
• The origin of the high metallicity of close-in giant exoplanets: Combined effect of the resonant and aerodynamic shepherding
• Convection and Mixing in Giant Planet Evolution
• High-temperature miscibility of iron and rock during terrestrial planet formation
• Majority of water hides deep in the interiors of exoplanets
• Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs
• Redox hysteresis of super-Earth exoplanets from magma ocean circulation
• To cool is to keep: Residual H/He atmospheres of super-Earths and sub-Neptunes
• The importance of silicate vapor in determining the structure, radii, and envelope mass fractions of sub-Neptunes
• A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18 b
• Convective inhibition with an atmosphere, I: super-critical cores on sub-Neptune/super-Earths
• Magma ocean evolution at arbitrary redox state
• Role of magma oceans in controlling carbon and oxygen of sub-Neptune atmospheres
• Atmospheric C/O Ratios of Sub-Neptunes with Magma Oceans: Homemade rather than Inherited
• Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds
• Experiments reveal extreme water generation during planet formation
• Magma ocean interactions can explain JWST observations of the sub-Neptune TOI-270 d
• Evolution of steam worlds: energetic aspects
• Competing chemical signatures in the atmosphere of TOI-270 d: Inference of sulfur and carbon chemistry
• Diversity of low-mass planet atmospheres in the C-H-O-N-S-Cl system with interior dissolution, nonideality, and condensation: Application to TRAPPIST-1e and sub-Neptunes
• Redefining interiors and envelopes: hydrogen-silicate miscibility and its consequences for the structure and evolution of sub-Neptunes
• The Effects of Non-ideal Mixing in Planetary Magma Oceans and Atmospheres

Cited by in corpus (4)

• KRONOS I: The $1-2.8\mu$m JWST Transmission Spectrum of the 23 Myr V1298 Tau c
• Oxygen left behind: Atmospheric Enrichment due to Fractionation in Sub-Neptunes using BOREAS
• JWST unveils a high mean molecular weight atmosphere for mini-Neptune TOI-1130b: Evidence for formation beyond the water ice line
• Impact of Clouds on the Atmosphere-Mantle Interface of Sub-Neptunes