Astronomers Explore the Surface Composition of a Nearby Super-Earth

Webb observations constrain the properties of a rocky exoplanet’s hot crust

May 04, 2026

A partial black-and-white mosaic of the planet Mercury against a black background, captured by the Messenger spacecraft. The grey, spherical surface is heavily cratered, with impact sites of varying sizes, indicating billions of years of cosmic bombardment. The surface material is dark and basaltic, with prominent bright streaks (rays) emanating from younger craters where fresh subsurface material was ejected. The absence of atmospheric blurring produces sharp, high-contrast features, indicating a world directly exposed to the space environment and solar radiation. — This high-resolution photo of the planet Mercury probably resembles the rocky exoplanet LHS 3844 b. Results from JWST observations favour an airless rocky planet with a dark, basalt-like surface, likely space-weathered by irradiation and meteorite impacts.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington (cropped)

This high-resolution photo of the planet Mercury probably resembles the rocky exoplanet LHS 3844 b. Results from JWST observations favour an airless rocky planet with a dark, basalt-like surface, likely space-weathered by irradiation and meteorite impacts.

© NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington (cropped)

To the point

JWST Observations: The James Webb Space Telescope analysed the rocky exoplanet LHS 3844 b, revealing a dark, hot surface without an atmosphere.
Surface Composition: The analysis indicates the planet's surface is likely composed of basalt or mantle rock, ruling out a composition similar to Earth's silicate-rich crust.
Geological Activity: The findings suggest that LHS 3844 b may have undergone prolonged geological inactivity, as no signs of volcanic gases were detected.

Using MIRI (Mid Infrared Instrument) on board the James Webb Space Telescope (JWST), a team of researchers 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), analysed the surface composition of the rocky exoplanet LHS 3844 b. Beyond characterizing exoplanetary atmospheres, this kind of deciphering the geological properties of planets orbiting distant stars is the next step in unveiling their nature. The results of this investigation are now published in the journal Nature Astronomy.

A dark and airless rocky super-Earth

LHS 3844 b is a rocky planet 30% bigger than Earth and orbits a cool red dwarf star once within roughly 11 hours. Whirling just three stellar diameters above the host star’s surface, the planet is tidally locked to its orbit. This means one rotation takes just as long as one revolution. As a result, the same hemisphere of LHS 3844 b always faces its star, producing a constant dayside with an average temperature of about 1000 Kelvin (approximately 725 Degrees Celsius or 1340 Degrees Fahrenheit). The LHS 3844 system is only 48.5 light-years (14.9 parsecs) away from Earth.

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.

Laura Kreidberg, MPIA

Title: Emission spectrum of exoplanet LHS 3844b compared to geological models.
Description: The image shows a line graph plotting the Planet-to-Star Flux Ratio in parts per million (ppm) against wavelength in micrometres, from 2 to 20 micrometres on a logarithmic scale.

Key Content:
Observational Data: Individual data points represent measurements from the Spitzer Space Telescope (light-blue squares) and the James Webb Space Telescope (red circles with vertical error bars). The flux ratio increases steadily from near 0 at 2 micrometres to approximately 1100 ppm at 20 micrometres.
Geological Models: Three coloured lines represent different surface compositions. An orange solid line representing magnesium- and iron-rich mantle rock and a blue dashed line representing volcanic basalt closely follow the path of the red JWST data points. A green dashed-dotted line representing silica-rich crustal rock (granite) lies significantly below the observational data points across most of the spectrum.
Scientific Conclusion: The telescope data points align closely with models for dark, mafic, and ultramafic rocks, such as basalt and mantle rock. The model for granite, which is similar to Earth's continental crust, deviates substantially from the observed data, effectively ruling out a granitic surface for this planet. — Infrared spectrum of LHS 3844 b’s hot dayside derived from the brightness contrast to its host star in ppm (parts per million = 0.0001%) at different wavelengths. The observational data obtained from the James Webb and Spitzer Space Telescopes are consistent with mantle or lava rock, whereas they rule out an Earth-like crust.

© Sebastian Zieba et al./MPIA

Infrared spectrum of LHS 3844 b’s hot dayside derived from the brightness contrast to its host star in ppm (parts per million = 0.0001%) at different wavelengths. The observational data obtained from the James Webb and Spitzer Space Telescopes are consistent with mantle or lava rock, whereas they rule out an Earth-like crust.

© Sebastian Zieba et al./MPIA

With its dark surface, LHS 3844 b may resemble a larger version of the Moon or the planet Mercury. This conclusion is based on analysing the infrared radiation received from the planet’s hot dayside. However, when measuring this radiation, we cannot see the planet directly; instead, we register the repeating change in brightness we receive from the star and the orbiting planet combined.

MIRI divided a portion of the planet’s infrared emission, ranging from 5 to 12 micrometres, into smaller wavelength sections and measured the brightness per wavelength bin. This is what astronomers call a spectrum, a rainbow-like distribution of the light’s components. Another data point, obtained from observations with the Spitzer Space Telescope and published a few years ago, augmented the analysis.

Constraining geological activity

Similar to how exoplanetary atmosphere research has benefited from climate science, this emerging field of exoplanetary geology draws on Earth-based geologic knowledge. Zieba, Kreidberg, and their collaborators ran models and accessed template libraries of rocks and minerals known from Earth, the Moon, and Mars to see what infrared signatures they would produce under the conditions on LHS 3844 b. Comparing observation-based data with these computations confidently ruled out a composition comparable to Earth’s crust, typically silicate-rich rocks such as granite.

Although this result is not very surprising – even in the Solar System, Earth is the only planet with such a crust – it may reveal details on LHS 3844 b’s geological history. Earth-like silicate-rich crusts are thought to form through a prolonged refinement process that requires tectonic activity and typically relies on water as a lubricant. The rocky material repeatedly melts and solidifies as it is mixed with mantle material, leaving the lighter minerals on the surface.

“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.”

What can we deduce about an exoplanet's rocky surface?

A close-up black-and-white photograph of an astronaut's boot print in the fine, grey regolith of an alien surface. The footprint shows a distinct ribbed tread pattern deeply pressed into the dusty soil, which is scattered with small pebbles. Harsh shadows emphasize the texture of the ground.
The ground is composed of a thick layer of fine-grained, dark dust and small rocky debris. Positioned in the lower right quadrant, the print is crisp and well-defined, with low-angle lighting creating deep shadows within the tread ridges and behind small surface rocks. This highlights the vacuum-packed consistency of the soil on a world without wind or an atmosphere to erode such features. — A close-up view of an astronaut’s boot print in the fine-powdered lunar regolith, during the Apollo 11 extravehicular activity (EVA) on the Moon. Similar conditions may exist on the exoplanet LHS 3844 b due to prolonged space weathering by stellar irradiation and meteorite impacts.

© NASA

A close-up view of an astronaut’s boot print in the fine-powdered lunar regolith, during the Apollo 11 extravehicular activity (EVA) on the Moon. Similar conditions may exist on the exoplanet LHS 3844 b due to prolonged space weathering by stellar irradiation and meteorite impacts.

© NASA

Instead, the dark surface points to a composition reminiscent of terrestrial or lunar basalt, or of Earth’s mantle material. However, the astronomers attempted an even more detailed characterization.

A statistical analysis of how well this spectrum fits various mineral mixtures and configurations revealed that extended solid areas of basalt or magmatic rock best match the observations. They are rich in magnesium and iron and can include olivine. Crushed material, such as rocks or gravel, also fits fairly well, whereas grains or powders are inconsistent with the observations due to their brighter appearance, at least at first glance.

Without a protective atmosphere, planets are subjected to space weathering, predominantly driven by hard, energetic radiation from the host star and impacts from meteorites of various sizes.

“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.”

Geologically fresh or weathered? Two possible scenarios

This assessment left the astronomers with two scenarios for the planet’s surface that match the data equally well. One involves a surface dominated by dark, solid rock composed of basaltic or magmatic minerals. Compared to geological timescales, space weathering alters its properties quickly. Therefore, the astronomers conclude that, in this scenario, the surface should be relatively fresh, produced by recent geological activity, such as widespread volcanism.

The second scenario also proposes a dark surface, comparable to the Moon or Mercury. Still, it accounts for prolonged space weathering, which leads to extended regions covered by a darkened regolith layer, a fine powder also present on the Moon, as evidenced by the iconic photos of the astronauts’ footprints. This alternative relies on longer periods of geological inactivity, thereby requiring conditions opposite to the first scenario.

Attempts to resolve the ambiguity

These two alternatives differ in the degree of recent geological activity required. On Earth and other active objects in the Solar System, a typical phenomenon during such activity is outgassing. Sulphur dioxide (SO₂) is a gas commonly connected to volcanism. If present on LHS 3844 b in reasonable amounts, MIRI should have detected it. Still, it found nothing. Therefore, a recent period of activity seems unlikely, which leads the astronomers to favour the second scenario. If correct, LHS 3844 b may truly look much like Mercury indeed.

In order to test their idea, Zieba, Kreidberg, and their colleagues are already pursuing a more direct approach. They have obtained additional JWST observations, which should enable them to discern surface conditions by exploiting small differences in how solid slabs and powders emit or reflect light. The distribution of emission angles depends on surface roughness, which affects the amount of radiation received at a given viewing angle. This concept is successfully applied to characterizing 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.