Using MIRI (Mid Infrared Instrument) on board the James Webb Space Telescope (JWST), an international team led by former MPIA (Max Planck Institute for Astronomy, Heidelberg, Germany) PhD student Sebastian Zieba (Center for Astrophysics | Harvard & Smithsonian, Cambridge, USA) and Laura Kreidberg, MPIA Director and study PI (principal investigator), investigated the surface composition of the rocky exoplanet LHS 3844 b. Moving beyond studies of atmospheres, this work focuses on the geology of planets orbiting other stars, offering deeper insight into their nature. The findings have been published in the journal Nature Astronomy.
LHS 3844 b is a rocky world about 30% larger than Earth that circles a cool red dwarf star in just under 11 hours. It orbits extremely close to its star, only about three stellar diameters above the surface. The planet is tidally locked, meaning one side permanently faces the star while the other remains in darkness. The dayside reaches an average temperature of about 1000 Kelvin (approximately 725 Degrees Celsius or 1340 Degrees Fahrenheit). The system lies relatively close to Earth at a distance of 48.5 light-years (14.9 parsecs).
“Thanks to the amazing sensitivity of JWST, we can detect light coming directly from the surface of this distant rocky planet. We see a dark, hot, barren rock, devoid of any atmosphere,” said Laura Kreidberg, MPIA.
Its dark appearance suggests it may resemble an enlarged version of the Moon or Mercury. This conclusion comes from analyzing infrared radiation emitted by the planet’s hot dayside. Scientists cannot directly image the planet. Instead, they measure subtle changes in brightness from the combined light of the star and the orbiting planet as it moves.
MIRI examined infrared emission between 5 and 12 micrometers by splitting the light into smaller wavelength intervals and measuring the intensity in each. This process creates a spectrum, which is similar to a rainbow that reveals how light is distributed across different wavelengths. Earlier observations from the Spitzer Space Telescope provided additional data that strengthened the analysis.
Ruling Out an Earth-Like Crust
Research into exoplanet geology builds on knowledge gained from studying Earth and other rocky bodies in the Solar System. The team compared their observations with computer models and libraries of known rocks and minerals from Earth, the Moon, and Mars. These comparisons showed that LHS 3844 b does not have a crust similar to Earth’s, which is typically rich in silicate minerals like granite.
This finding is not unexpected, since Earth is unique in the Solar System for having such a crust. Still, it provides clues about the planet’s past. On Earth, silicate-rich crusts form through long-term processes involving tectonic activity and often require water. Rock is repeatedly melted and recycled, allowing lighter materials to rise and form the crust.
“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,” says Sebastian Zieba. “This planet likely only contains little water.”
A Basalt-Rich Surface
Instead of granite-like material, 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 carried out a detailed statistical comparison between the observed spectrum and various possible mixtures of minerals.
They 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 minerals such as olivine. Broken rock fragments like gravel also fit reasonably well, while fine powders or dust alone do not match the observations due to their brighter appearance.
Without an atmosphere to shield it, the planet is constantly exposed to intense radiation from its star and impacts from meteorites. These processes gradually 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,” explains Zieba. “They also darken the layer by adding iron and carbon, making the regolith’s properties more consistent with the observations.”
Fresh Lava or Ancient Dust
The data support two possible explanations for the planet’s surface. One scenario suggests a landscape dominated by solid basaltic rock that is relatively fresh. In this case, recent geological activity such as widespread volcanism would have resurfaced the planet.
The second scenario also involves a dark surface, but one shaped by long-term exposure to space. Over time, weathering would create extensive layers of darkened regolith, similar to the dusty surface seen on the Moon or Mercury. This interpretation implies that the planet has been geologically inactive for a long period.
Searching for Signs of Activity
These two possibilities differ mainly in whether the planet is still geologically active. On Earth and other active worlds, volcanic processes release gases into the surrounding environment. One such gas is sulfur dioxide (SO2), which is commonly associated with volcanism.
If LHS 3844 b were currently active, MIRI would likely have detected this gas. However, no such signal was found. This absence suggests that recent volcanic activity is unlikely, making the weathered, inactive surface scenario more plausible. If correct, the planet may closely resemble Mercury.
To resolve this question, the team is pursuing further observations with JWST. These new measurements aim to detect subtle differences in how solid rock and loose material emit and reflect light. The way light is emitted at different angles depends on surface texture, which can reveal whether the surface is smooth rock or rough, powdery material. This method has already been used successfully to study asteroids in the 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,” concludes Kreidberg.
Additional Information
Laura Kreidberg is the only MPIA astronomer involved in this study.
Other researchers were: Sebastian Zieba (Center for Astrophysics | Harvard & Smithsonian, Cambridge, USA), Brandon P. Coy (Department of the Geophysical Sciences, University of Chicago, USA), Aaron Bello-Arufe (Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA [JPL]), Kimberly Paragas (Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA), Xintong Lyu (Peking University, Beijing, China), Renyu Hu (The Pennsylvania State University, University Park, USA and JPL), Aishwarya Iyer (NASA Goddard Space Flight Center, Greenbelt, USA), Kay Wohlfarth (Technische Universität Dortmund, Germany)
The JWST observations used in this study were conducted as 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 comprises the ESA (European Space Agency) member states: Belgium, Denmark, France, Germany, Ireland, the Netherlands, Spain, Sweden, Switzerland, and the United Kingdom. National science organisations fund the consortium’s work — in Germany, the Max Planck Society (MPG) and the German Aerospace Center (DLR). Participating German institutions include the Max Planck Institute for Astronomy in Heidelberg, the University of Cologne, and Hensoldt AG in Oberkochen, formerly Carl Zeiss Optronics.
The James Webb Space Telescope is the world’s leading observatory for space research. It is an international programme led by NASA and its partners ESA and CSA (Canadian Space Agency).
The Spitzer Space Telescope was operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.
