The Featureless HST/WFC3 Transmission Spectrum of the Rocky Exoplanet GJ 1132b: No Evidence for a Cloud-free Primordial Atmosphere and Constraints on Starspot Contamination

Abstract

Orbiting an M dwarf 12 pc away, the transiting exoplanet GJ 1132b is a prime target for transmission spectroscopy. With a mass of 1.7 M ⊕ and radius of 1.1 R ⊕, GJ 1132b’s bulk density indicates that this planet is rocky. Yet with an equilibrium temperature of 580 K, GJ 1132b may still retain some semblance of an atmosphere. Understanding whether this atmosphere exists and its composition will be vital for understanding how the atmospheres of terrestrial planets orbiting M dwarfs evolve. We observe five transits of GJ 1132b with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). We find a featureless transmission spectrum from 1.1 to 1.7 μm, ruling out cloud-free atmospheres with metallicities <300× solar with >4.8σ confidence. We combine our WFC3 results with transit depths from TESS and archival broadband and spectroscopic observations to find a featureless spectrum across 0.7 to 4.5 μm. GJ 1132b therefore has a high mean molecular weight atmosphere, possesses a high-altitude aerosol layer, or has effectively no atmosphere. Higher-precision observations are required in order to differentiate between these possibilities. We explore the impact of hot and cold starspots on the observed transmission spectrum GJ 1132b, quantifying the amplitude of spot-induced transit depth features. Using a simple Poisson model, we estimate spot temperature contrasts, spot covering fractions, and spot sizes for GJ 1132. These limits, as well as the modeling framework, may be useful for future observations of GJ 1132b or other planets transiting similarly inactive M dwarfs.

Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation

“…In the presence of hazes, the spectrum is significantly muted, in line with existing observations of sub-Neptune atmospheres (Knutson et al 2014;Kreidberg et al 2014;Libby-Roberts et al 2020). These models are for a 3 M ⊕ planet, and existing spectroscopic data on atmospheres currently do not detect discernible atmospheric features toward these lower-mass planets (de Wit et al 2018;Diamond-Lowe et al 2020;Libby-Roberts et al 2022). The expectation of our model results is that similar features would be anticipated for slightly more massive planets (e.g., 5 M ⊕ ).…”

High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations

“…Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Gressier et al 2022), GJ 1132b(Diamond-Lowe et al 2018Mugnai et al 2021;Libby-Roberts et al 2022), the L 98-59 system (Damiano et al 2022;Zhou et al 2023), LTT 1445Ab (Diamond-Lowe et al 2022, and LHS 3488b (Kreidberg et al 2019;Diamond-Lowe et al 2020). However, many of these observations do not preclude higher mean molecular weight secondary atmospheres for these small planets (Moran et al 2018;Damiano et al 2022).…”

Constraining the Thickness of TRAPPIST-1 b’s Atmosphere from Its JWST Secondary Eclipse Observation at 15 μm

“…Observationally, to date there are no studies that definitively confirm the presence of an atmosphere on a rocky exoplanet. Flat transmission spectra are the norm (e.g., de Wit et al 2018;Diamond-Lowe et al 2018Mugnai et al 2021;Libby-Roberts et al 2022;Lustig-Yaeger et al 2023), and the few studies that have claimed detections of atmospheric spectral features for terrestrial exoplanets have been called into question or have ambiguous interpretation (e.g., Southworth et al 2017;Swain et al 2021;Moran et al 2023). Thermal emission measurements of the planets LHS 3844b (Kreidberg et al 2019) and GJ 1252b (Crossfield et al 2022) have found dayside temperatures that are consistent with the no-atmosphere limit, the former by way of a full-orbit phase curve.…”