Majority of water hides deep in the interiors of exoplanets (2401.16394v1)

Abstract: Water is an important component of exoplanets, with its distribution, i.e., whether at the surface or deep inside, fundamentally influencing the planetary properties. The distribution of water in most exoplanets is determined by yet-unknown partitioning coefficients at extreme conditions. Our new first-principles molecular dynamics simulations reveal that water strongly partitions into iron over silicate at high pressures and thus would preferentially stay in a planet's core. Furthermore, we model planet interiors by considering the effect of water on density, melting temperature, and water partitioning. The results shatter the notion of water worlds as imagined before: the majority of the bulk water budget (even more than 95%) can be stored deep within the core and the mantle, and not at the surface. For planets more massive than ~6 Earth's mass and Earth-size planets (of lower mass and small water budgets), the majority of water resides deep in the cores of planets, Whether water is assumed to be at the surface or at depth can affect the radius by up to 25% for a given mass. This has drastic consequences for the inferred water distribution in exoplanets from mass-radius data.

• The paper demonstrates that ab initio molecular dynamics simulations reveal water’s preferential partitioning into iron cores under pressures of 250-1000 GPa and temperatures of 6500-13000 K.
• The paper quantifies that water distribution can alter exoplanet radii by up to 25%, particularly in planets exceeding six Earth masses.
• The paper implies that revised planetary models are needed to accurately assess exoplanet structure and habitability by accounting for significant water sequestration in metallic cores.

Insights on Water Distribution in Exoplanetary Interiors

The research presented in this paper provides a comprehensive exploration of water distribution within exoplanets, focusing on the partitioning of water under extreme conditions found in super-Earths and sub-Neptunes. Utilizing ab initio molecular dynamics simulations, the paper reveals significant insights into water partitioning favoring metallic cores over silicate mantles, which challenges prevailing notions surrounding the structure and characterization of these celestial bodies.

The methodology involves investigating water's affinity between iron and silicate melts, a critical factor influencing exoplanetary interiors. The simulations initiate at pressures ranging from 250 to 1000 GPa and temperatures from 6500 to 13000 K. The results show that water tends to be siderophile, significantly partitioning into iron at these conditions, with partition coefficients demonstrating the pressure and temperature dependence of this behavior.

Significant numerical results highlight the stark differences in expected exoplanetary parameters when internal water partitioning is considered. For instance, model scenarios reveal that a planet's radius can vary by up to 25% when accounting for water stored in core and mantle, underscoring the need for accurate interpretative models in astronomical observations. The findings particularly emphasize that for planets over approximately six Earth masses, a majority of water is predicted to be locked within the core. Moreover, the influence of pressure-sensitive partitioning shifts the bulk water distribution in planets, with core dominance for more massive varieties.

The implications of these findings extend to both theoretical and practical domains in the field of planetary science. Theoretically, the research suggests a need to re-evaluate existing models for water-rich exoplanets, which have traditionally assumed surface water presence due to limited observations of internal water dynamics. Practically, this work could influence how we interpret mass-radius relationships obtained from telescopic data, potentially revising estimates of water abundances on exoplanets. Such insights could impact our criteria for habitability assessments, altering our understanding of where life-sustaining conditions might exist beyond Earth.

Speculatively, the research could pave the way for investigating atmospheric composition impacts, as less surface water implies different atmospheric dynamics and possibly cooler surface temperatures. Future developments in this field could include higher-resolution simulations and targeted observational missions, offering deeper insight into the complex interplays of water, minerals, and volatile elements within exoplanetary bodies.

In conclusion, this paper sheds light on significant aspects of planetary interiors, particularly focusing on the sequestration of water in metallic cores, which holds profound implications for our understanding of planetary formation, evolution, and potential habitability.