[Paper Review] A JWST transmission spectrum of a nearby Earth-sized exoplanet

Jacob Lustig‐Yaeger, G. Fu+21 more|arXiv (Cornell University)|Jan 10, 2023

Stellar, planetary, and galactic studies9 citations

TL;DR

The paper reports JWST/NIRSpec G395H transmission spectroscopy of LHS 475b, finding a featureless spectrum that rules out several hydrogen-dominated and methane atmospheres, and constrains possible atmospheres to high-MMW or airless scenarios.

ABSTRACT

The critical first step in the search for life on exoplanets over the next decade is to determine whether rocky planets transiting small M-dwarf stars possess atmospheres and, if so, what processes sculpt them over time. Because of its broad wavelength coverage and improved resolution compared to previous methods, spectroscopy with JWST offers a new capability to detect and characterize the atmospheres of Earth-sized, M-dwarf planets. Here we use JWST to independently validate the discovery of LHS 475b, a warm (586 K), 0.99 Earth-radius exoplanet, interior to the habitable zone, and report a precise 2.9-5.3 um transmission spectrum. With two transit observations, we rule out primordial hydrogen-dominated and cloudless pure methane atmospheres. Thus far, the featureless transmission spectrum remains consistent with a planet that has a high-altitude cloud deck (similar to Venus), a tenuous atmosphere (similar to Mars), or no appreciable atmosphere at all (akin to Mercury). There are no signs of stellar contamination due to spots or faculae. Our observations demonstrate that JWST has the requisite sensitivity to constrain the secondary atmospheres of terrestrial exoplanets with absorption features <50 ppm, and that our current atmospheric constraints speak to the nature of the planet itself, rather than instrumental limits.

Motivation & Objective

• Validate the planetary nature and atmospheric characteristics of LHS 475b, a nearby Earth-sized exoplanet.
• Obtain and analyze a precise transmission spectrum in the 2.9–5.3 μm range with JWST/NIRSpec G395H.
• Rule out broad classes of primordial and methane atmospheres and assess alternative atmospheric scenarios.
• Assess possible stellar contamination and instrumental noise to ensure atmospheric interpretations reflect the planet.
• Demonstrate JWST’s capability to constrain small absorption features (<50 ppm) for terrestrial exoplanets.

Proposed method

• Two JWST transits of LHS 475b were observed with NIRSpec G395H (2.87–5.14 μm) in BOTS mode.
• Data were reduced independently with three pipelines (Eureka!, FIREFLy, Tiberius) and cross-validated, with FIREFLy used for final interpretation.
• Wavelength-dependent transit depths were extracted at detector resolution and co-added into a 56-point spectrum (R ~ 100).
• Limb-darkening used 3D stellar models; a joint white-light fit provided system parameters (Rp/Rs, P, a/Rs, i).
• Bayesian retrievals were performed for a five-component atmosphere (H2O, CO2, CH4, CO, and an inert bulk gas) with variable mean molecular weight and apparent surface pressure.

Experimental results

Research questions

• RQ1 What atmospheres are consistent with the observed transmission spectrum of LHS 475b given JWST's sensitivity?
• RQ2 Does the JWST transmission spectrum show features indicative of hydrogen-dominated, methane-rich, or high-MMW atmospheres?
• RQ3 Is there evidence for stellar contamination (spots/faculae) affecting the transmission spectrum?
• RQ4 What are the practical constraints JWST places on absorption feature amplitudes (<50 ppm) for Earth-sized exoplanets?
• RQ5 Can the data distinguish between a thin/tenuous atmosphere, a thick cloudy atmosphere, or an airless world?

Key findings

• The transmission spectrum is featureless, consistent with a flat line (no clear molecular features).
• The data rule out hydrogen-dominated atmospheres with compositions from 1×–100× solar metallicity at >5σ.
• Pure methane atmospheres ≥1 bar are ruled out at >5σ due to CH4 band constraints in the 2.9–5.3 μm range.
• High-MMW compositions (e.g., 1000× solar) and pure water atmospheres are weakly disfavored, while a pure CO2 atmosphere or an airless body are favored as consistent with the data.
• There is no evidence for stellar contamination from unocculted spots or faculae in the transmission spectrum.
• The study demonstrates JWST sensitivity to absorption features smaller than 50 ppm for an Earth-sized planet, with 3σ limits at 61 ppm (H2O), 38 ppm (CH4), 49 ppm (CO2), and 62 ppm (CO).
• Retrievals indicate preference for high-MMW, compact atmospheres with low apparent surface pressures (<0.01–1 bar at 1–2σ) if an atmosphere exists, or an airless/very tenuous scenario.