JWST TRAPPIST-1 e/b Program: Motivation and first observations

TL;DR

The paper tackles the challenge of stellar contamination in JWST transmission spectroscopy of temperate rocky planets around M dwarfs by proposing a model-independent correction that leverages close transits, using TRAPPIST-1 b as a contamination proxy to study TRAPPIST-1 e. Simulations with an Earth-like atmosphere and hot/cold-spot contamination show that 15 close transits can yield strong evidence for an Earth-like atmosphere, with robust $CO_2$ detection (and occasionally $CH_4$) when analyzing a spectrum ratio and performing full atmospheric retrievals via POSEIDON; the significance is quantified as $Δ\ln\,Z \ge 5$. Early JWST observations demonstrate the feasibility of the approach while highlighting challenges from stellar activity and data-analysis choices, emphasizing the importance of monitoring flares and accounting for inter-planet signals in multi-planet atmospheric searches.

Abstract

One of the forefront goals in the field of exoplanets is the detection of an atmosphere on a temperate terrestrial exoplanet, and among the best suited systems to do so is TRAPPIST-1. However, JWST transit observations of the TRAPPIST-1 planets show significant contamination from stellar surface features that we are unable to confidently model. Here, we present the motivation and first observations of our JWST multi-cycle program of TRAPPIST-1 e, which utilize close transits of the airless TRAPPIST-1 b to model-independently correct for stellar contamination, with the goal of determining whether TRAPPIST-1 e has an Earth-like mean molecular weight atmosphere containing CO$_2$. We present our simulations, which show that with the 15 close transit observations, we will be able to detect this atmosphere on TRAPPIST-1 e at $Δ\ln\,Z=5$ or greater confidence assuming we are able to correct for stellar contamination using the close transit observations. We also show the first three observations of our program. We find that our ability to correct for stellar contamination can be inhibited when strong stellar flares are present, as flares can break the assumption that the star does not change meaningfully between planetary transits. The cleanest observation demonstrates the removal of stellar contamination contribution through an increased preference for a flat line over the original TRAPPIST-1 e spectrum, but highlights how minor data analysis assumptions can propagate significantly when searching for small atmospheric signals. This is amplified when using the signals from multiple planets, which is important to consider as we continue our atmospheric search.

Figures (3)

• Figure 1: The atmospheric detection significance as a function of the number of observed close transits for a full Earth-like atmosphere retrieval with POSEIDON. The 10 different noise instances are each given by a different color. With 15 close transits, we are able to achieve at least a $\Delta \,\,ln\,\,Z = 5$, "strong evidence" detection of an Earth-like atmosphere.
• Figure 2: The posterior distributions associated with the 15 close transit retrievals for each of the 10 noise instances shown in \ref{['fig:complex-detect']}. We confidently detect CO$_2$ in all noise instances, and sometimes are able to detect CH$_4$ as well, while we do not detect (and therefore do not rely on detecting) H$_2$O.
• Figure 3: Top: White light curves of observation 1, observation 3, and observation 15 from the NA reduction, both at native time resolution in black and binned to 25x in red for visibility. All transits are labeled by planet. Observation 1 also has a large flare that occurs right before the egress of planet e. Shown in the upper right is a schematic of our proposed setup, with the true overlapping planetary transit chords taken from Agol_2021. Middle: H$\alpha$ light curve from the NA reduction. Peaks in H$\alpha$ correspond to flaring events, many of which are not visible in the white light curve, at least without hints to their location from the H$\alpha$ signature. Note that the light curves are binned in time to 50x (to approximately 70 second bins) relative to that shown in the white light curve for visibility in the low signal single wavelengths shown. Bottom: Transmission spectrum of TRAPPIST-1 e (top) and TRAPPIST-1 b (bottom) for observation 1, observation 3, and observation 15 from left to right.