No escaping helium from 55 Cnc e

TL;DR

This study used high-resolution spectroscopy to search for helium escape from 55 Cnc e, finding no evidence of atmospheric loss, which suggests the planet either never had or quickly lost a primordial atmosphere.

Contribution

First high-resolution spectral search for helium escape from 55 Cnc e, providing constraints on atmospheric mass loss rates and implications for its atmospheric history.

Findings

No helium absorption detected during transits.

Mass loss rate constrained to less than ~10^9 g/s.

Results imply the planet likely never had a primordial atmosphere.

Abstract

We search for escaping helium from the hot super Earth 55 Cnc e by taking high-resolution spectra of the 1083 nm line during two transits using Keck/NIRSPEC. We detect no helium absorption down to a 90\% upper limit of 250 ppm in excess absorption or 0.27 milli-Angstrom in equivalent width. This corresponds to a mass loss rate of less than $\sim10^9$ g/s assuming a Parker wind model with a plausible exosphere temperature of 5000-6000 K, although the precise constraint is heavily dependent on model assumptions. We consider both hydrogen- and helium-dominated atmospheric compositions, and find similar bounds on the mass loss rate in both scenarios. Our hydrodynamical models indicate that if a lightweight atmosphere exists on 55 Cnc e, our observations would have easily detected it. Together with the non-detection of Lyman $\alpha$ absorption by Ehrenreich et al 2012, our helium non-detection indicates that 55 Cnc e either never accreted a primordial atmosphere in the first place, or lost its primordial atmosphere shortly after the dissipation of the gas disk.