Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1

Abstract

Seven temperate Earth-sized exoplanets readily amenable for atmospheric studies transit the nearby ultra-cool dwarf star TRAPPIST-1 1, 2 . Their atmospheric regime is unknown and could range from extended primordial hydrogen-dominated to depleted atmospheres 3-6 .Hydrogen in particular is a powerful greenhouse gas that may prevent the habitability of inner planets while enabling the habitability of outer ones [6][7][8] . An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in 2 the near-infrared detectable during transits. Observations of the innermost planets ruled out such signatures 9 . However, the outermost planets are more likely to have sustained such a Neptune-like atmosphere 10,11 . Here, we report observations for the four planets within or near the system's "Habitable Zone" (HZ)-the circumstellar region where liquid water could exist on a planetary surface 12-14 . These planets do not exhibit prominent spectroscopic signatures at near-infrared wavelengths either, which rules out cloud-free hydrogen-dominated atmospheres for TRAPPIST-1 d, e and f with significance of 8, 6 and 4σ, respectively. Such an atmosphere is instead not excluded for planet g. As highaltitude clouds and hazes are not expected in hydrogen-dominated atmospheres around planets with such insolation 15,16 , these observations further support their terrestrial and potentially habitable nature.We observed transits of TRAPPIST-1 planets d, e, f, and g with four visits of the Hubble Space Telescope (HST). Each of the visits contained two planetary transits (Figure 1), planets d and f in visits 1 (4 December 2016) and 3 (9 January 2017), and planets e and g in visits 2 (29 December 2016) and 4 (10 January 2017). The observations were conducted using the 'forward' scanning mode with the near-infrared (1.1-1.7µm) G141 grism on the wide-field camera 3 (WFC3) instrument (see Methods). We capitalized on the frequency of the transit events in the TRAPPIST-1 system to select observation windows encompassing transits from two different planets, thereby optimizing the time allocation. The time sensitivity of these observations (TRAPPIST-1's visibility window closing in January 2017) combined with our 3 "multiple-transit-per-visit" approach constrained us to perform exposures when HST crossed through the South-Atlantic Anomaly (SAA). Visits 1, 3, and 4 contain SAA crossing events which forces HST into GYRO mode, where its fine pointing ability is lost. The loss of fine pointing during and following the SAA crossing events cause the spectral position on the detector to change over time. In addition, during the SAA crossing a greater number of cosmic ray hits are introduced to the observations/exposures. We use the IMA output files from the CalWF3 pipeline and correct for this by cross-correlating each spectral read in the individual exposures and interpolating (see Methods). The raw light curves present primarily ramp-like systematics on the scale of HST orbit-induced instrumental settling discuss...

An emission spectrum for WASP-121b measured across the 0.8–1.1 μm wavelength range using the Hubble Space Telescope

“…Although methods exist to correct the WFC3 first-orbit systematics (e.g. Zhou et al 2017;de Wit et al 2018), we opted for this approach to be consistent with our previous analyses (Evans et al 2016(Evans et al , 2017 and to avoid modeling the baseline trend over the full fiveorbit visits, which is less likely to be well approximated as linear. The resulting four-orbit white light curves were fit using a similar methodology to that described in Evans et al (2017), in which the systematics are treated as a Gaussian process (GP).…”

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

“…The combined spectrum of the planets rules out cloud-free, hydrogen-dominated atmospheres, except for TRAPPIST-1 f and 1 g (de Wit et al 2018;Moran et al 2018); Wakeford et al (2019) recently suggested that a clear hydrogen-dominated atmosphere may be ruled out for TRAPPIST-1 g. Since high-altitude clouds and haze are not expected to form in hydrogen-dominated atmospheres around temperate planets (e.g. Morley et al 2015), transit spectroscopy of the TRAPPIST-1 planets suggests no atmosphere or a high-metallicity atmosphere referred to as a secondary atmosphere.…”

“…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).…”