A Heavy Molecular Weight Atmosphere for the Super-Earth {\pi} Men c

TL;DR

This study reports the detection of heavy ions in the atmosphere of super-Earth { extpi} Men c, indicating a thick, heavy volatile-rich atmosphere that is escaping, providing insights into the planet's composition and evolutionary history.

Contribution

First detection of C II ions in a super-Earth's atmosphere, revealing a heavy, volatile-rich atmosphere with escape signatures, advancing understanding of small exoplanet atmospheres.

Findings

Detection of C II ions during transit suggests atmospheric escape.

The atmosphere contains over 50% heavy volatiles by mass.

{ extpi} Men c may lose its atmosphere and become a rocky core.

Abstract

Strongly irradiated exoplanets develop extended atmospheres that can be utilized to probe the deeper planet layers. This connection is particularly useful in the study of small exoplanets, whose bulk atmospheres are challenging to characterize directly. Here, we report the 3.4{\sigma} detection of C II ions during a single transit of the super-Earth {\pi} Men c in front of its Sun-like host star. The transit depth and Doppler velocities are consistent with the ions filling the planet's Roche lobe and moving preferentially away from the star, an indication that they are escaping the planet. We argue that {\pi} Men c possesses a thick atmosphere with abundant heavy volatiles ($>=$ 50{\%} by mass of atmosphere) but that needs not be carbon rich. Our reasoning relies upon cumulative evidence from the reported C II detection, the nondetection of H I atoms in a past transit, modeling of the planet's interior, and the assumption that the atmosphere, having survived the most active phases of its Sun-like host star, will survive another 0.2-2 Gyr. Depending on the current mass of atmosphere, {\pi} Men c may still transition into a bare rocky core. Our findings confirm the hypothesized compositional diversity of small exoplanets, and represent a milestone toward understanding the planets' formation and evolution paths through the investigation of their extended atmospheres.