A Volatile-Poor Formation of LHS 3844b based on its Lack of Significant Atmosphere

TL;DR

LHS 3844b is a terrestrial exoplanet with an extremely thin or absent atmosphere, likely due to its volatile-poor composition and geophysical history, providing insights into planet formation and atmospheric loss processes.

Contribution

This study combines observational data and geodynamical models to demonstrate that LHS 3844b's lack of atmosphere is consistent with a volatile-poor mantle, offering new constraints on its formation and evolution.

Findings

LHS 3844b likely has a volatile-poor mantle.

Atmospheric erosion mechanisms are consistent with the planet's current state.

The planet probably formed inside the system's snow-line.

Abstract

Exoplanet discoveries have reached into the realm of terrestrial planets that are becoming the subject of atmospheric studies. One such discovery is LHS 3844b, a 1.3 Earth radius planet in a 0.46 day orbit around an M4.5-5 dwarf star. Follow-up observations indicate that the planet is largely devoid of substantial atmosphere. This lack of significant atmosphere places astrophysical and geophysical constraints on LHS 3844b, primarily the degree of volatile outgassing and the rate of atmosphere erosion. We estimate the age of the host star as $7.8\pm1.6$ Gyrs and find evidence of an active past comparable to Proxima Centauri. We use geodynamical models of volcanic outgassing and atmospheric erosion to show that the apparent lack of atmosphere is consistent with a volatile-poor mantle for LHS 3844b. We show the core is unlikely to host enough C to produce a sufficiently volatile-poor mantle, unless the bulk planet is volatile-poor relative to Earth. While we cannot rule out a giant impact stripping LHS 3844b's atmosphere, we show this mechanism would require significant mantle stripping, potentially leaving LHS 3844b as an Fe-rich "super-Mercury". Atmospheric erosion by smaller impacts is possible, but only if the planet has already begun degassing and is bombarded by $10^3$ impactors of radius 500-1000 km traveling at escape velocity. We discuss formation and migration scenarios that could account for a volatile poor origin, including the potential for an unobserved massive companion planet. A relatively volatile-poor composition of LHS 3844b suggests that the planet formed interior to the system snow-line.