Using the Mid Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST), an international team led by former MPIA PhD student Sebastian Zieba (now at Harvard & Smithsonian) and MPIA Director Laura Kreidberg has peered into the surface composition of the rocky exoplanet LHS 3844 b. Moving beyond the usual atmospheric studies, this work digs into the geology of planets orbiting other stars - because why should our solar system have all the fun? The findings appear in Nature Astronomy.
LHS 3844 b is a rocky world about 30% larger than Earth that whips around a cool red dwarf star in under 11 hours. It orbits extremely close to its star - only about three stellar diameters above the surface - and is tidally locked, meaning one side permanently faces the star while the other broods in eternal darkness. The dayside averages about 1000 Kelvin (roughly 725°C or 1340°F). The system is a relatively close 48.5 light-years (14.9 parsecs) away.
"Thanks to JWST's amazing sensitivity, we can detect light coming directly from the surface of this distant rocky planet," Kreidberg said. "We see a dark, hot, barren rock, devoid of any atmosphere." So, not exactly a vacation spot.
Its dark appearance suggests it may resemble an oversized Moon or Mercury. This conclusion comes from analyzing infrared radiation emitted by the planet's hot dayside. Scientists can't directly image the planet; instead, they measure subtle brightness changes from the combined light of star and planet as it orbits.
MIRI examined infrared emission between 5 and 12 micrometers, splitting light into smaller wavelength intervals to create a spectrum - essentially a rainbow that reveals how light is distributed. Earlier data from the Spitzer Space Telescope strengthened the analysis.
The team compared their observations with computer models and libraries of known rocks from Earth, the Moon, and Mars. These comparisons showed LHS 3844 b lacks a crust like Earth's - which is typically rich in silicate minerals like granite. This isn't surprising, since Earth is unique in the solar system for having such a crust. On Earth, silicate-rich crusts form through long-term tectonic activity and water, involving repeated melting and recycling of rock.
"Since LHS 3844 b lacks such a silicate crust, one may conclude that Earth-like plate tectonics does not apply to this planet, or it is ineffective," Zieba said. "This planet likely only contains little water."
Instead of granite, the data point to a surface made of basalt or mantle-like rock, similar to volcanic material found on Earth or the Moon. The researchers found that large areas of solid basalt or magmatic rock best match the data. These rocks are rich in magnesium and iron and may contain olivine. Broken rock fragments like gravel also fit reasonably well, while fine powders or dust alone don't match - they'd be too bright.
Without an atmosphere to shield it, the planet is constantly exposed to intense radiation and meteorite impacts. These processes break down rock and alter its surface.
"It turns out, these processes not only slowly dissolve hard rocks into regolith, a layer of fine grains or powder as found on the Moon," Zieba explained. "They also darken the layer by adding iron and carbon, making the regolith's properties more consistent with the observations."
The data support two possible scenarios. First: a landscape dominated by solid basaltic rock that's relatively fresh, suggesting recent geological activity like widespread volcanism. Second: a dark surface shaped by long-term space weathering, creating extensive layers of darkened regolith similar to the Moon or Mercury - implying the planet has been geologically inactive for a long time.
These possibilities differ mainly in whether the planet is still active. On Earth, volcanic processes release gases like sulfur dioxide (SO2). If LHS 3844 b were currently active, MIRI would likely have detected this gas. No such signal was found. This absence suggests recent volcanic activity is unlikely, making the weathered, inactive surface scenario more plausible. If correct, the planet may closely resemble Mercury.
To resolve this, the team is pursuing further JWST observations. These new measurements aim to detect subtle differences in how solid rock and loose material emit and reflect light at different angles - a technique already used successfully on asteroids in our solar system.
"We are confident the same technique will allow us to clarify the nature of LHS 3844 b's crust and, in the future, other rocky exoplanets," Kreidberg concluded.
The study involved researchers from multiple institutions including Harvard & Smithsonian, University of Chicago, JPL, Caltech, Peking University, Penn State, NASA Goddard, and TU Dortmund. The JWST observations were part of GO program #1846 (PI: Laura Kreidberg, co-PI: Renyu Hu), titled "A Search for Signatures of Volcanism and Geodynamics on the Hot Rocky Exoplanet LHS 3844 b." The MIRI consortium includes ESA member states and national science organizations. The James Webb Space Telescope is an international program led by NASA, ESA, and CSA.