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

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 bestfitting 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.

The First Near-Infrared Transmission Spectrum of HIP 41378 f, a Low-Mass Temperate Jovian World in a Multi-Planet System

“…The different emergent spectra for these two planets with similar equilibrium temperatures may be due to their distinct bulk properties. Conversely, the observed atmospheric signal for K2-18 b may be caused by stellar surface inhomogeneities (Barclay et al 2021).…”

The metal-poor atmosphere of a Neptune/Sub-Neptune planet’s progenitor

“…Variability in such young stars can be attributed photospheric inhomogeneity (star spots and faculae) and fast stellar rotation. Unocculted star spots can contaminate the observed transmission spectrum [68]. We estimate the effect of stellar contamination on the transmission spectrum of V1298 Tau b following the prescription of [13].…”

Detecting the proposed CH4-CO2 biosignature pair with the James Webb Space Telescope: TRAPPIST-1e and the effect of cloud/haze

“…The only habitable zone exoplanet with a measured transmission spectrum precise enough to reveal spectral features -K2-18b, a sub-Neptune orbiting an M dwarf -appears to be cloudy, with a cloud-top pressure estimated between approximately 10-100 mbar, close to where the temperature profile crosses the H 2 O condensation curve (Benneke et al 2019). Although, it should be noted that the latter interpretation remains open to question, as Barclay et al (2021) have recently argued that stellar inhomogeneities cannot yet be ruled out as an alternative explanation for the existing K2-18b data. For other transmission spectra published to date for planets in habitable zones, the data have only been precise enough to rule out cloud-free, H 2 /He-dominated atmospheres (de Wit et al 2018) or produced inconclusive results (Edwards et al 2021).…”