A JWST transmission spectrum of a nearby Earth-sized exoplanet (2301.04191v1)

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.

• The paper used high-resolution JWST NIRSpec data from two transits to capture LHS 475b's spectrum and rule out hydrogen-rich and cloudless methane atmospheres.
• The analysis employed multiple pipelines, including Eureka!, FIREFLy, and Tiberius, to ensure robust cross-verification of the spectral data.
• The findings set key benchmarks by constraining terrestrial atmosphere models and suggesting possible scenarios like a Venus-like cloud deck or a Mars-like thin atmosphere.

A JWST Transmission Spectrum of LHS 475b

This paper presents the transmission spectrum characterization of LHS 475b, an Earth-sized exoplanet, using the James Webb Space Telescope (JWST). The primary goal is to investigate the atmospheric composition of a rocky planet orbiting a nearby M-dwarf star. This work advances the current understanding of atmospheric properties in terrestrial exoplanets, particularly those in proximity to the habitable zone yet inside it.

Main Findings and Methodology

The researchers utilized the JWST's Near InfraRed Spectrograph (NIRSpec) to obtain a transmission spectrum ranging from 2.87 to 5.3 microns across two transits. The data excluded atmospheres dominated by primordial hydrogen or cloudless methane, indicating alternative possibilities such as a Venus-like high-altitude cloud deck, a tenuous Mars-like atmosphere, or no significant atmosphere akin to Mercury.

• Observational Setup: The transmission spectrum was composed by analyzing two transits with NIRSpec's high-resolution mode (R ~ 2700), covering atmospheric absorption bands that might reveal the presence of specific molecules.
• Data Reduction: The data were processed using multiple pipelines, including Eureka!, FIREFLy, and Tiberius, ensuring cross-verification and consistency.
• Planetry Parameters: Calculations suggest an Earth-radius (0.99 R⊕) for LHS 475b with an equilibrium temperature of 586 K, indicative of a potentially tidally locked configuration.

Implications and Theoretical Context

The featureless nature of the spectrum with current atmospheric models suggests that JWST can rigorously rule out certain lighter atmospheres. However, the uncertainty surrounding heavier gases and high-altitude clouds remains significant. The findings highlight JWST's capability to constrain terrestrial exoplanet atmospheres with sensitivity to features below 50 ppm, setting a leading edge for future explorations.

The analysis does not detect significant stellar contamination, affirming JWST's observational accuracy in distinguishing planetary atmospheric features from stellar activity. Furthermore, Bayesian retrievals of atmospheric compositions are nuanced, with mixed molecular weight scenarios allowed by the data, indicating challenges in distinguishing between them with the current dataset.

Future Directions in Exoplanet Atmosphere Studies

The presented observations pave the way for more precise investigations of similar exoplanets using JWST, especially through follow-up studies and thermal emission measurements during secondary eclipses. The methods specified in restricting atmospheric models can be instrumental in further testing hypotheses related to atmospheric retention and composition under diverse stellar environments.

Overall, the research presents essential benchmarks for interpreting JWST observations of Earth-sized exoplanets and reaffirms the technological and analytical capacity to expand the horizons of exoplanetary science, focusing on planets capable of harboring secondary atmospheres.