The dehydration of water worlds via atmospheric losses

TL;DR

This study develops a multi-fluid MHD model to quantify water ion atmospheric losses from exoplanets, revealing how space weather and stellar activity influence water depletion on water-rich worlds.

Contribution

Introduces a novel multi-species MHD model with atmospheric chemistry to accurately estimate water ion losses from exoplanets under various stellar wind conditions.

Findings

Water ion loss rates are significantly higher during ancient and extreme space weather events.

Ion escape rates can reach up to 10^28 s^-1 during intense space weather.

Active stellar environments can accelerate atmospheric water depletion on exoplanets.

Abstract

We present a three-species multi-fluid MHD model (H$^+$, H$_2$O$^+$ and e$^-$), endowed with the requisite atmospheric chemistry, that is capable of accurately quantifying the magnitude of water ion losses from exoplanets. We apply this model to a water world with Earth-like parameters orbiting a Sun-like star for three cases: (i) current normal solar wind conditions, (ii) ancient normal solar wind conditions, and (iii) one extreme "Carrington-type" space weather event. We demonstrate that the ion escape rate for (ii), with a value of 6.0$\times$10$^{26}$ s$^{-1}$, is about an order of magnitude higher than the corresponding value of 6.7$\times$10$^{25}$ s$^{-1}$ for (i). Studies of ion losses induced by space weather events, where the ion escape rates can reach $\sim$ 10$^{28}$ s$^{-1}$, are crucial for understanding how an active, early solar-type star (e.g., with frequent coronal mass ejections) could have accelerated the depletion of the exoplanet's atmosphere. We briefly explore the ramifications arising from the loss of water ions, especially for planets orbiting M-dwarfs where such effects are likely to be significant.