Disentangling the planet from the star in late type M dwarfs: A case study of TRAPPIST-1g

TL;DR

This study disentangles stellar and planetary signals in the TRAPPIST-1 system, demonstrating that the planetary transmission spectrum of TRAPPIST-1g is unlikely contaminated by stellar features and ruling out a solar-like atmosphere.

Contribution

It introduces a method to separate stellar and planetary signals in late M dwarf systems, applying it to TRAPPIST-1g to refine atmospheric characterization.

Findings

TRAPPIST-1 is an M8V star with significant starspot coverage.

The planetary radius of TRAPPIST-1g is 1.124 Earth radii.

A solar H2/He atmosphere is ruled out at >3-sigma confidence.

Abstract

The atmospheres of late M stars represent a significant challenge in the characterization of any transiting exoplanets due to the presence of strong molecular features in the stellar atmosphere. TRAPPIST-1 is an ultra-cool dwarf, host to seven transiting planets, and contains its own molecular signatures which can potentially be imprinted on planetary transit lightcurves due to inhomogeneities in the occulted stellar photosphere. We present a case study on TRAPPIST-1g, the largest planet in the system, using a new observation together with previous data, to disentangle the atmospheric transmission of the planet from that of the star. We use the out-of-transit stellar spectra to reconstruct the stellar flux based on one-, two-, and three-temperature components. We find that TRAPPIST-1 is a 0.08 M$_*$, 0.117 R$_*$, M8V star with a photospheric effective temperature of 2400 K, with ~35% 3000 K spot coverage and a very small fraction, <3%, of ~5800 K hot spot. We calculate a planetary radius for TRAPPIST-1g to be Rp = 1.124 R$_\oplus$ with a planetary density of $\rho_p$ = 0.8214 $\rho_\oplus$. Based on the stellar reconstruction there are eleven plausible scenarios for the combined stellar photosphere and planet transit geometry; in our analysis we are able to rule out 8 of the 11 scenarios. Using planetary models we evaluate the remaining scenarios with respect to the transmission spectrum of TRAPPIST-1g. We conclude that the planetary transmission spectrum is likely not contaminated by any stellar spectral features, and are able to rule out a clear solar H2/He-dominated atmosphere at greater than 3-sigma.