The 0.8–4.5 μm Broadband Transmission Spectra of TRAPPIST-1 Planets

Abstract

The TRAPPIST-1 planetary system represents an exceptional opportunity for the atmospheric characterization of temperate terrestrial exoplanets with the upcoming James Webb Space Telescope (JWST). Assessing the potential impact of stellar contamination on the planets' transit transmission spectra is an essential precursor step to this characterization. Planetary transits themselves can be used to scan the stellar photosphere and to constrain its heterogeneity through transit depth variations in time and wavelength. In this context, we present our analysis of 169 transits observed in the optical from space with K2 and from the ground with the SPECULOOS and Liverpool telescopes. Combining our measured transit depths with literature results gathered in the mid/near-IR with Spitzer/IRAC and HST/WFC3, we construct the broadband transmission spectra of the TRAPPIST-1 planets over the 0.8-4.5 µm spectral range. While planets b, d, and f spectra show some structures at the 200-300ppm level, the four others are globally flat. Even if we cannot discard their instrumental origins, two scenarios seem to be favored by the data: a stellar photosphere dominated by a few high-latitude giant (cold) spots, or, alternatively, by a few small and hot (3500-4000K) faculae. In both cases, the stellar contamination of the transit transmission spectra is expected to be less dramatic than predicted in recent papers. Nevertheless, based on our results, stellar contamination can still be of comparable or greater order than planetary atmospheric signals at certain wavelengths. Understanding and correcting the effects of stellar heterogeneity therefore appears essential to prepare the exploration of TRAPPIST-1's with JWST.

Ground-based follow-up observations of TRAPPIST-1 transits in the near-infrared

“…Transmission spectra of TRAPPIST-1 b-h planets. The solid black line is the weighted mean of all measurements excluding HST presented in Ducrot et al (2018) with corresponding 1-σ confidence intervals in shades of gray and numerical values on the right. HST measurements are presented as grey points.…”

“…At first, we followed the approach to differential photometry and transit light curve analysis that is generally used for ground-based transit photometry obtained in the visible (Ducrot et al 2018). The best aperture size and combination of comparison stars were selected on the basis of the minimization of the out-of-transit (OOT) scatter of the light curve.…”

“…Both studies agree that the HST transit depths are globally deeper than those measured at other wavelengths. Nevertheless, as outlined by Ducrot et al (2018), the origins of these larger transit depths could be instrumental, as the HST/WFC3 systematic effects combined with the low-Earth orbit of HST make it very difficult to normalize transit light curves as short as those of TRAPPIST-1 planets (see Subsection 3.2 in Ducrot et al 2018). The uncertainties affecting the absolute values of the transits measured by HST in the near-IR is unfortunate, as this spectral range encompasses the peak of the spectral energy distribution of TRAPPIST-1 (see Fig.…”

Do the TRAPPIST-1 Planets Have Hydrogen-rich Atmospheres?

“…Stellar contamination effects on transmission spectra are expected to be less significant than those predicted inZhang et al (2018) (Ducrot et al 2018; Morris et al 2018a,b) …”

GJ 357: a low-mass planetary system uncovered by precision radial velocities and dynamical simulations

“…Transits from seven small rocky planets on short period orbits were found, allowing detailed studies to be conducted on the possible atmospheres and their constituents of small planets orbiting a single star, and hence with similar initial conditions (e.g. Alberti et al 2017;Ducrot et al 2018;Moran et al 2018;Miles-Páez et al 2019;Burdanov et al 2019).…”