Webb images 30% larger exoplanet

On May 4, researchers led by Sebastian Zieba of the Center for Astrophysics | Harvard & Smithsonian and Laura Kreidberg of the Max Planck Institute for Astronomy reported that the James Webb Space Telescope had directly constrained the surface of the rocky exoplanet LHS 3844 b. The team said the data point to a dark, hot, airless world with a surface more like Mercury or the Moon than Earth. The findings were published in *Nature Astronomy* and described by the researchers as a step beyond the usual exoplanet work of measuring atmospheres. The planet sits 48.5 light-years from Earth and is about 30% larger than Earth, according to the research team. LHS 3844 b circles a cool red dwarf star in roughly 11 hours and is tidally locked, leaving one hemisphere in permanent daylight. That dayside reaches about 1,000 Kelvin, or about 725 degrees Celsius, the researchers said. (cfa.harvard.edu) ### How did Webb study a planet it cannot resolve as a disk? Webb used its Mid-Infrared Instrument, or MIRI, to measure thermal emission from the planet's dayside across wavelengths from 5 to 12 micrometers. The researchers said they could not image the planet directly, but instead tracked changes in the combined light from the star and planet as the planet orbited. (cfa.harvard.edu) An earlier Spitzer Space Telescope measurement was added to the analysis, according to the Center for Astrophysics release. That combination let the team compare the observed spectrum with models for different rock types and with scenarios that included atmospheric gases. ### What did the spectrum say about the surface? (eurekalert.org) The spectrum was best matched by a dark, low-silica surface such as basalt or other olivine-rich material, according to the paper summary and the institute release. The team said the data ruled out a fresh powdery surface and instead fit rock that had been altered by space weathering from irradiation and impacts. (cfa.harvard.edu) Laura Kreidberg said in the release that Webb's sensitivity allowed the team to detect light coming directly from the surface of the distant rocky planet. She said the observations showed "a dark, hot, barren rock" with no atmosphere. ### Why do researchers say there is no atmosphere? The *Nature Astronomy* paper reported upper limits on carbon dioxide and sulfur dioxide, with the data disfavoring even trace concentrations of those gases. (nature.com) The authors said those constraints rule out a thick carbon-dioxide atmosphere and show no evidence of accumulated volcanic gases. The Max Planck Institute for Astronomy said the lack of volcanic-gas signatures also suggests prolonged geological inactivity on LHS 3844 b. (eurekalert.org) That interpretation came from the institute's summary of the paper rather than from a direct claim that the planet is geologically dead. ### Why does this world look more like Mercury than Earth? LHS 3844 b orbits extremely close to its star, passing just three stellar diameters above the star's surface, the researchers said. (nature.com) That tight orbit and constant stellar heating leave the dayside intensely hot and help explain why the planet appears barren rather than Earth-like. The team said its dark surface and lack of atmosphere make the planet resemble a larger version of Mercury or Earth's Moon. (mpia.de) The comparison came from the observed infrared signature and the inferred surface composition, not from a visible image of the planet itself. ### What comes next for this line of research? Sebastian Zieba and Laura Kreidberg said the work extends Webb exoplanet studies from atmospheres to geology on rocky worlds. (eurekalert.org) The paper in *Nature Astronomy* provides a template for using thermal-emission spectra to test surface composition and to search for gases on other hot terrestrial exoplanets. The study is already published in *Nature Astronomy*, and the named next step in the researchers' framing is applying similar Webb observations to other rocky planets. The instrument used here was MIRI, and the comparison dataset included earlier Spitzer observations. (cfa.harvard.edu)