Stellar surface inhomogeneities as a potential source of the atmospheric signal detected in the K2-18 b transmission spectrum

TL;DR

This study investigates whether stellar surface inhomogeneities could mimic atmospheric signals in the transmission spectrum of exoplanet K2-18 b, suggesting stellar contamination as an alternative explanation to planetary atmosphere detection.

Contribution

The paper introduces a time-variable stellar spectral model to assess the impact of star spots on transmission spectroscopy data of K2-18 b, highlighting the need for cautious interpretation of atmospheric signals.

Findings

Stellar inhomogeneities can produce signals similar to planetary atmospheres in transmission spectra.

40% of simulated stellar variability scenarios could mimic the observed atmospheric detection.

Stellar contamination remains a plausible alternative explanation for the water absorption signal.

Abstract

Transmission spectroscopy of transiting exoplanets is a proven technique that can yield information on the composition and structure of a planet's atmosphere. However, transmission spectra may be compromised by inhomogeneities in the stellar photosphere. The sub-Neptune-sized habitable zone planet K2-18 b has water absorption detected in its atmosphere using data from the Hubble Space Telescope (HST). Herein, we examine whether the reported planetary atmospheric signal seen from HST transmission spectroscopy of K2-18 b could instead be induced by time-varying star spots. We built a time-variable spectral model of K2-18 that is designed to match the variability amplitude seen in K2 photometric data, and used this model to simulate 1000 HST data-sets that follow the K2-18 b observation strategy. More than 1% of these provide a better fit to the data than the best-fitting exoplanet atmosphere model. After resampling our simulations to generate synthetic HST observations, we find that 40% of random draws would produce an atmospheric detection at a level at least as significant as that seen in the actual HST data of K2-18 b. This work illustrates that the inferred detection of an atmosphere on K2-18 b may alternatively be explained by stellar spectral contamination due to the inhomogeneous photosphere of K2-18. We do not rule out a detection of water in the planet's atmosphere, but provide a plausible alternative that should be considered, and conclude that more observations are needed to fully rule out stellar contamination.