The Near-infrared Transmission Spectra of TRAPPIST-1 Planets b, c, d, e, f, and g and Stellar Contamination in Multi-epoch Transit Spectra

Abstract

The seven approximately Earth-sized transiting planets in the TRAPPIST-1 system provide a unique opportunity to explore habitable zone and non-habitable zone small planets within the same system. Its habitable zone exoplanets -due to their favorable transit depths -are also worlds for which atmospheric transmission spectroscopy is within reach with the Hubble Space Telescope (HST) and with the James Webb Space Telescope (JWST). We present here an independent reduction and analysis of two HST Wide Field Camera 3 (WFC3) near-infrared transit spectroscopy datasets for six planets (b through g). Utilizing our physically-motivated detector charge trap correction and a custom cosmic ray correction routine, we confirm the general shape of the transmission spectra presented by de Wit et al. (2016Wit et al. ( , 2018. Our data reduction approach leads to a 25% increase in the usable data and reduces the risk of confusing astrophysical brightness variations (e.g., flares) with instrumental systematics. No prominent absorption features are detected in any individual planet's transmission spectra; by contrast, the combined spectrum of the planets shows a suggestive decrease around 1.4 µm similar to an inverted water absorption feature. Including transit depths from K2, the SPECULOOS-South Observatory, and Spitzer, we find that the complete transmission spectrum is fully consistent with stellar contamination owing to the transit light source effect. These spectra demonstrate how stellar contamination can overwhelm planetary absorption features in low-resolution exoplanet transit spectra obtained by HST and JWST and also highlight the challenges in combining multi-epoch observations for planets around rapidly rotating spotted stars.

The Influence of Stellar Contamination on the Interpretation of Near-infrared Transmission Spectra of Sub-Neptune Worlds around M-dwarfs

“…For a fixed fs, increasing the temperature contrast alone causes an increased stretching effect at longer wavelengths. Figure 1 shows example contamination spectra for both spots and faculae with various temperature contrasts and covering fractions (e.g., similar to Rackham et al 2017;Zhang et al 2018) for realistic stellar spectra (using PHOENIX models from Allard et al 2003Allard et al , 2007Allard & Freytag 2009). Consistent with the preceeding analytical result, with realistic spectra we also see that increasing the spot covering fraction f s from a small 0.9% to a large 12% value, produces a scaling effect at longer wavelengths.…”

“…(Han et al 2014) c From Lecavelier Des Etangs et al 2008. Haze cross section given by σ λ = σ0(λ0/λ) β w ith λ0=0.43 µm and σ0 given in units relative to H2 rayleigh scattering (2×10 −27 cm 2 ) d Initial Values are specific to cases of spot and faculae covering fractions, listed in Table 2, also given in Rackham et al 2018;Zhang et al 2018 porated into our assumed photometric precision, which may not necessarily be true. The posterior probability distributions of f s (fractional coverage of the spot region on the photosphere), T s (Temperature of the spot region), and T p (Temperature of the photosphere) retrieved from the diskintegrated spectrum under both approaches are presented in figure 9 top panel.…”

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) …”

“…Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) measurements exhibit no prominent absorption features at near-infrared wavelengths in transmission spectra of six of the TRAPPIST-1 planets (de Wit et al 2016(de Wit et al , 2018Zhang et al 2018;Burdanov et al 2019)…”

O2- and CO-rich Atmospheres for Potentially Habitable Environments on TRAPPIST-1 Planets

“…Orbiting small stars, these planets provide an accelerated path toward finding and characterizing potentially habitable worlds. The TRAPPIST-1 planets already have transmission spectra measured by Hubble (de Wit et al 2018;Zhang et al 2018). With >7 times more collecting area and infrared instruments, the James Webb Space Telescope (JWST) will be capable of providing a more detailed look into the atmosphere of these cold exoplanets (Beichman et al 2014).…”