James Webb studies exoplanet surface

Sebastian Zieba and Laura Kreidberg said on May 4 that the James Webb Space Telescope had directly characterized the surface of a rocky exoplanet for the first time. Their target was LHS 3844 b, a “super-Earth” about 48.5 light-years away, and the picture that emerged was not of clouds or an atmosphere but of exposed rock. The team said Webb’s measurements point to a dark, hot, airless surface, with the dayside averaging about 1,000 kelvin, or roughly 725 degrees Celsius. The results were published in *Nature Astronomy* and released by the Center for Astrophysics | Harvard & Smithsonian and the Max Planck Society. ### Which planet did Webb study, and why this one? LHS 3844 b was already an unusual target because earlier work had suggested it lacked a substantial atmosphere. The planet is about 30% larger than Earth, circles a cool red dwarf star in roughly 11 hours, and is tidally locked, meaning the same side always faces its star. That geometry gives astronomers a permanently heated dayside that is easier to study in infrared light. (cfa.harvard.edu) Laura Kreidberg, the study’s principal investigator, said Webb’s sensitivity allowed the team to detect light coming directly from the planet’s surface. In the researchers’ description, that made LHS 3844 b a rare case where a rocky exoplanet could be studied as geology, not only as atmosphere. ### How did Webb get surface information from a world it cannot image directly? (cfa.harvard.edu) The James Webb Space Telescope used its Mid-Infrared Instrument, or MIRI, to measure thermal emission from the planet’s dayside across wavelengths from 5 to 12 micrometers. The team tracked changes in the combined light from the star and planet and used those infrared measurements to build a spectrum. A previous Spitzer Space Telescope data point was added to the analysis. (cfa.harvard.edu) Sebastian Zieba and co-authors wrote that different rock types leave distinct spectral signatures, including clues to composition and surface texture. Instead of seeing the planet as a resolved disk, the researchers inferred the surface properties from how much heat it emitted at different wavelengths. ### What did the spectrum show about the surface itself? The 5-to-12-micrometer spectrum was best matched by a dark, low-silica surface such as basalt or other olivine-rich material, the paper said. (cfa.harvard.edu) The team also said the data ruled out a fresher powdery surface unless that material had been darkened by space weathering. The Max Planck Society said the findings favor an airless rocky planet with a dark, basalt-like surface that may have been altered by irradiation and meteorite impacts. (arxiv.org) Kreidberg described the result more bluntly in the release: “We see a dark, hot, barren rock, devoid of any atmosphere.” ### Did Webb find any signs of an atmosphere or active volcanism? The paper reported upper limits on carbon dioxide and sulfur dioxide of 100 millibars and 10 microbars, respectively. (arxiv.org) Those limits, the authors said, argue against even trace accumulations of volcanic gases above the surface. The research groups said that absence of detectable gases is consistent with a world that is geologically quiet, though that interpretation was presented as an inference from the measurements rather than a direct observation of volcanic history. (mpg.de) ### Why are astronomers treating this as a step beyond atmosphere studies? Nature Astronomy and the institutional releases described the work as a move into direct surface characterization of rocky exoplanets. (arxiv.org) Most exoplanet observations so far have focused on masses, radii, or atmospheres; this study used mid-infrared spectroscopy to constrain the crust itself. The May 4 releases from the Center for Astrophysics and Max Planck framed that approach as the next stage for studying distant rocky worlds with Webb. (mpg.de) Future JWST observations using MIRI and related infrared techniques are expected to test whether other airless rocky planets also show basalt-like, space-weathered surfaces. (cfa.harvard.edu) (nature.com)