A JWST transmission spectrum of the nearby Earth-sized exoplanet LHS 475 b

• Published in: Nature astronomy Vol. 7; no. 11; pp. 1317 - 1328
• Main Authors: Lustig-Yaeger, Jacob, Fu, Guangwei, May, E. M., Ceballos, Kevin N. Ortiz, Moran, Sarah E., Peacock, Sarah, Stevenson, Kevin B., Kirk, James, López-Morales, Mercedes, MacDonald, Ryan J., Mayorga, L. C., Sing, David K., Sotzen, Kristin S., Valenti, Jeff A., Redai, Jéa I. Adams, Alam, Munazza K., Batalha, Natasha E., Bennett, Katherine A., Gonzalez-Quiles, Junellie, Kruse, Ethan, Lothringer, Joshua D., Rustamkulov, Zafar, Wakeford, Hannah R.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.11.2023; Nature Publishing Group
• Subjects: 639/33/34/862; 639/33/445/862; Astronomy; Astrophysics and Cosmology; Atmosphere; Earth; Mercury; Physics; Physics and Astronomy; Planets; Space telescopes; Spectrum analysis
• ISSN: 2397-3366; 2397-3366
• DOI: 10.1038/s41550-023-02064-z

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 with previous instruments, spectroscopy with the James Webb Space Telescope (JWST) offers a new capability to detect and characterize the atmospheres of Earth-sized, M-dwarf planets. Here we use the JWST to independently validate the discovery of LHS 475 b, a warm (586 K), 0.99 Earth-radius exoplanet, interior to the habitable zone, and report a precise 2.9–5.3 μm transmission spectrum using the Near Infrared Spectrograph G395H instrument. 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 show that the 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 warm Earth-sized planet LHS 475 b is validated and characterized with two transits observed by the JWST. The absence of evident spectroscopic features excludes a substantial hydrogen envelope and indicates that LHS 475 b has either little or no atmosphere or an optically thick cloud deck at high altitudes.