Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds

Aaron Werlen, Caroline Dorn, Remo Burn, Hilke E. Schlichting, Simon L. Grimm, Edward D. Young

TL;DR

The paper investigates whether water-rich Hycean planets can form when sub-Neptunes possess magma oceans, by coupling a global chemical-equilibrium network with a birth population from the New Generation Planetary Population Synthesis. It finds that interior–atmosphere equilibration destroys most of the accreted water, yielding final H2O mass fractions of at most $\sim$1.5 wt% and none reaching the Hycean threshold of $10$–$90$ wt%. A subset of planets develops H2O-dominated envelopes only if they formed inside the snow line and cooled into hydrogen-poor, carbon-poor regimes, but even these remain hydrogen-dominated by the bulk gas and show no stable surface oceans due to full miscibility with H2 at AMOI conditions. The results challenge the conventional link between ice accretion and water-rich atmospheres, highlighting interior processes as the dominant determinant of observable water in sub-Neptunes and informing interpretation of exoplanet atmospheres in the JWST era.

Abstract

Recent claims of biosignature gases in sub-Neptune atmospheres have renewed interest in water-rich sub-Neptunes with surface oceans, often referred to as Hycean planets. These planets are hypothesized to form beyond the snow line, accreting large amounts of H$_2$O (>10 wt%) before migrating inward. However, current interior models often neglect chemical equilibration between primordial atmospheres and molten interiors. Here, we compute global chemical equilibrium states for a synthetic population of sub-Neptunes with magma oceans. Although many initially accrete 5-30 wt% water, interior-atmosphere interactions destroy most of it, reducing final H$_2$O mass fractions to below 1.5 wt%. As a result, none meet the threshold for Hycean planets. Despite that, we find H$_2$O-dominated atmospheres exclusively on planets that accreted the least ice. These planets form inside the snow line, are depleted in carbon and hydrogen, and develop small envelopes with envelope mass fractions below 1%, dominated by endogenic water. In contrast, planets formed beyond the snow line accrete more volatiles, but their water is largely converted to H$_2$ gas or sequestered into the interior, resulting in low atmospheric H$_2$O mass fractions. Most H$_2$O-rich envelopes are also fully miscible with H$_2$, making a separate water layer unlikely. Our results topple the conventional link between ice accretion and water-rich atmospheres, showing instead that H$_2$O-dominated envelopes emerge through chemical equilibration in hydrogen-poor planets formed inside the snow line.

Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds

TL;DR

The paper investigates whether water-rich Hycean planets can form when sub-Neptunes possess magma oceans, by coupling a global chemical-equilibrium network with a birth population from the New Generation Planetary Population Synthesis. It finds that interior–atmosphere equilibration destroys most of the accreted water, yielding final H2O mass fractions of at most 1.5 wt% and none reaching the Hycean threshold of – wt%. A subset of planets develops H2O-dominated envelopes only if they formed inside the snow line and cooled into hydrogen-poor, carbon-poor regimes, but even these remain hydrogen-dominated by the bulk gas and show no stable surface oceans due to full miscibility with H2 at AMOI conditions. The results challenge the conventional link between ice accretion and water-rich atmospheres, highlighting interior processes as the dominant determinant of observable water in sub-Neptunes and informing interpretation of exoplanet atmospheres in the JWST era.

Abstract

Recent claims of biosignature gases in sub-Neptune atmospheres have renewed interest in water-rich sub-Neptunes with surface oceans, often referred to as Hycean planets. These planets are hypothesized to form beyond the snow line, accreting large amounts of HO (>10 wt%) before migrating inward. However, current interior models often neglect chemical equilibration between primordial atmospheres and molten interiors. Here, we compute global chemical equilibrium states for a synthetic population of sub-Neptunes with magma oceans. Although many initially accrete 5-30 wt% water, interior-atmosphere interactions destroy most of it, reducing final HO mass fractions to below 1.5 wt%. As a result, none meet the threshold for Hycean planets. Despite that, we find HO-dominated atmospheres exclusively on planets that accreted the least ice. These planets form inside the snow line, are depleted in carbon and hydrogen, and develop small envelopes with envelope mass fractions below 1%, dominated by endogenic water. In contrast, planets formed beyond the snow line accrete more volatiles, but their water is largely converted to H gas or sequestered into the interior, resulting in low atmospheric HO mass fractions. Most HO-rich envelopes are also fully miscible with H, making a separate water layer unlikely. Our results topple the conventional link between ice accretion and water-rich atmospheres, showing instead that HO-dominated envelopes emerge through chemical equilibration in hydrogen-poor planets formed inside the snow line.

Paper Structure

This paper contains 13 sections, 26 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Chemical Thermodynamics
  4. New Generation Planetary Population Synthesis (NGPPS) model
  5. Coupling the NGPPS and the chemical network
  6. Simulation Setup & Population
  7. Results
  8. H2O Accreted vs. H$_\textit{2}$O Equilibrated
  9. Envelope H$_\textit{2}$O Content and Formation Location
  10. Miscibility of H2–H2O in the Envelope
  11. Discussion
  12. Conclusion
  13. Chemical Network

Figures (5)

  • Figure 1: Comparison between the accreted and equilibrated water mass fractions of sub-Neptunes. The black dashed line indicates the 1:1 correlation; in the absence of chemistry, all planets would lie along this line. The grey shaded region denotes the 10–90 wt% water mass fraction range proposed for Hycean planets by madhusudhan_habitability_2021. All planets in our sample exhibit significant water depletion following equilibration and fall well below the Hycean threshold.
  • Figure 2: Envelope H2O mass fraction as a function of envelope mass fraction, using the same dataset as in Figure \ref{['fig:H2O_comparison']}. The colorbar indicates the molar bulk C/O ratio. Planets with low envelope mass fractions tend to retain a higher proportion of H2O in the gas phase. Moreover, planets with low C/O ratios consistently show higher H2O content compared to their carbon-rich counterparts.
  • Figure 3: Envelope H2O mass fraction as a function of semi-major axis. The same dataset as in Figure \ref{['fig:H2O_comparison']} is shown. The left panel shows planets that predominantly formed inside the water ice line; the right panel shows those that formed outside. Classification is based on the accreted H2O mass fraction, with a threshold set at 5% of the total planetary mass. The colorbar indicates the molar bulk C/O ratio. Planets formed inside the ice line are systematically depleted in carbon due to the lack of volatile ice accretion and exhibit higher envelope H2O mass fractions. In contrast, planets formed beyond the ice line retain lower H2O content despite higher bulk volatile abundances. Each pie chart shows the mean mass fraction of hydrogen in H2 (gas), H (metal), H2 (silicate), H2O (gas), and H2O (silicate), normalized to the total mean hydrogen inventory for each population. Only components contributing more than 5% are labeled. Planets that formed beyond the ice line store most hydrogen as H2 (gas) and H (metal), while those that formed inside the ice line retain a larger share of hydrogen in H (metal) H2 (silicate) and H2O (gas + silicate).
  • Figure 4: Atmosphere–magma ocean interface (AMOI) temperature versus AMOI pressure. The same dataset as in Figure \ref{['fig:H2O_comparison']} is shown. The colorbar indicates the mass fraction of H2O in the envelope. The grey dashed region marks the domain where pure H2O exists in the supercritical state yang_subcritical_2007. The dashed and dotted lines indicate the H2–H2O solvus at $0.68$ wt% H2 and $0.17$ wt% H2, respectively. The latter corresponds to the critical composition where the solvus in pressure–composition space reaches its minimum gupta_miscibility_2025. Planets to the left of the solvus fall within the one-phase regime, where H2 and H2O are fully miscible. Planets to the right lie in the two-phase regime, where the two species coexist as separate phases. Most planets with high H2O gas mass fractions lie within the one-phase regime.
  • Figure 5: Equilibrated H2O mass fraction as a function of total planet mass, using the same dataset as in Figure \ref{['fig:H2O_comparison']}. The colorbar indicates the molar bulk C/O ratio. The grey dashed lines mark the lower and upper estimate of Earth's water mass fraction, corresponding to 1 and 11 Earth oceans (EO) respectively. This estimate agrees well with our results, suggesting that the H2O content of sub-Neptunes is broadly consistent with Earth's upper bound. A negative correlation is visible between planet mass and H2O content for planets with high C/O ratios.