# On the Method to Infer an Atmosphere on a Tidally-Locked Super Earth Exoplanet and Upper limits to GJ 876d

## TL;DR

This paper presents a method to detect or rule out atmospheres on tidally-locked super Earths using thermal phase curves, demonstrated with Spitzer observations of GJ 876d, and discusses its potential with future telescopes.

## Contribution

The study introduces a new observational approach to infer planetary atmospheres from thermal phase curves, applicable to non-transiting super Earths with upcoming telescopes.

## Key findings

- No conclusive atmosphere detection for GJ 876d due to stellar variability.
- Achieved a 3 sigma upper limit of 5.13e-05 on phase curve amplitude.
- Method shows promise for future observations with James Webb Space Telescope.

## Abstract

We develop a method to infer or rule out the presence of an atmosphere on a tidally-locked hot super Earth. The question of atmosphere retention is a fundamental one, especially for planets orbiting M stars due to the star's long-duration active phase and corresponding potential for stellar-induced planetary atmospheric escape and erosion. Tidally-locked planets with no atmosphere are expected to show a Lambertian-like thermal phase curve, causing the combined light of the planet-star system to vary with planet orbital phase. We report Spitzer 8 micron IRAC observations of GJ 876 taken over 32 continuous hours and reaching a relative photometric precision of 3.9e-04 per point for 25.6 s time sampling. This translates to a 3 sigma limit of 5.13e-05 on a planet thermal phase curve amplitude. Despite the almost photon-noise limited data, we are unable to conclusively infer the presence of an atmosphere or rule one out on the non-transiting short-period super Earth GJ 876d. The limiting factor in our observations was the miniscule, monotonic photometric variation of the slightly active host M star, because the partial sine wave due to the planet has a component in common with the stellar linear trend. The proposed method is nevertheless very promising for transiting hot super Earths with the James Webb Space Telescope and is critical for establishing observational constraints for atmospheric escape.