When astronomers describe an exoplanet as “Earth-like,” it usually refers to size and composition—a rocky world rather than a gas giant. But as the James Webb Space Telescope (JWST) peers deeper into the cosmos, that label is becoming a point of contrast. A nearby rocky world, LHS 3844 b, may share Earth’s basic architecture, but its surface tells a story of a barren, scorched wasteland that looks far more like Mercury than our own home.
Located approximately 48.5 light-years away, LHS 3844 b is a rocky planet roughly 30 percent wider than Earth. However, any resemblance ends there. Trapped in a punishingly tight 11-hour orbit around its host star, the planet is subjected to an unrelenting barrage of radiation. New data analyzed by researchers suggests the world is a dark, airless rock, stripped of the water and tectonic activity that define Earth’s geology.
The findings, published in Nature Astronomy, mark a significant shift in how we study distant worlds. For years, the primary goal of exoplanet research has been the “hunt for atmosphere”—searching for the chemical signatures of oxygen, methane, or water vapor. But by focusing on the heat radiating from the planet’s surface, astronomers are now beginning to perform “remote geology,” determining what the ground actually feels like on worlds we will never visit.
Decoding the heat signature of a dead world
Because LHS 3844 b is too far away to be photographed in detail, the team led by Sebastian Zieba, Ph.D., at the Center for Astrophysics | Harvard & Smithsonian had to rely on infrared light. Using the Mid-Infrared Instrument (MIRI) on the Webb telescope, the team measured the “light drop” that occurs when the planet passes behind its star. This allowed them to isolate the heat emitted by the planet’s dayside from the overwhelming glare of the star.
The resulting spectrum—a map of brightness across different wavelengths—acted as a chemical fingerprint. Different minerals and rock textures absorb and emit heat in distinct patterns. When the team compared the signal from LHS 3844 b against libraries of known rocks from Earth, the Moon, and Mars, the results were clear: the planet does not possess a silica-rich, granite-like crust.
On Earth, the light-colored granite that makes up our continents is the product of water and plate tectonics, which recycle rock through the mantle. The absence of this signal on LHS 3844 b suggests a world devoid of these processes. Instead, the data fits a much darker material, leading researchers to believe the planet is either coated in basalt—a dark volcanic rock rich in iron and magnesium—or a thick layer of weathered space dust.
| Feature | Earth | Mercury | LHS 3844 b |
|---|---|---|---|
| Atmosphere | Thick/Nitrogen-Oxygen | Negligible/Exosphere | None detected |
| Surface Composition | Silicate/Granite-rich | Iron-rich/Regolith | Dark Rock/Basalt-like |
| Dayside Temp | Avg 59°F (15°C) | Up to 800°F (430°C) | ~1,340°F (727°C) |
| Tectonic Activity | Active Plate Tectonics | Global Contraction | Likely Dead/Inactive |
The “Space Weathering” effect
The comparison to Mercury is not accidental. Both worlds are rocky, cratered, and lack a substantial atmosphere to protect them from the vacuum of space. Without an atmosphere to burn up incoming meteors or wind to erode the surface, these planets undergo a process called “space weathering.”
Radiation and constant micrometeorite impacts break hard rock down into a fine, gritty powder known as regolith. Over millions of years, this process changes the chemistry and color of the surface, typically darkening it. This explains why “fresh” volcanic powder appeared too bright in the Webb calculations, while older, weathered material matched the dim signal coming from LHS 3844 b.
This suggests the planet is not just a “big Mercury,” but a world that has followed a similar evolutionary path: losing its volatile elements (like water) to the heat of its star and being slowly ground down by the environment of space.
Searching for volcanic ghosts
To determine if the planet is still geologically active, the team searched for volcanic gases. On Earth, eruptions release sulfur dioxide and carbon dioxide into the atmosphere. If LHS 3844 b had experienced recent volcanic activity, these gases should have lingered above its scorching surface.
The search came up empty. Sulfur dioxide was not detected above 10 microbars, and carbon dioxide remained below 100 millibars—roughly one-tenth of Earth’s sea-level pressure. This lack of gas supports the theory that the surface is an ancient, weathered crust rather than a landscape of fresh, gassy lava flows.
“We see a dark, hot, barren rock, devoid of any atmosphere,” said Laura Kreidberg, Ph.D., director at the Max Planck Institute for Astronomy, who contributed to the study. This conclusion narrows the possibilities for the planet’s history, painting a picture of a world that dried out and died long ago.
What comes next for planetary geology
The current data leaves one final puzzle: is the surface composed of solid, monolithic rock or a loose layer of grit? The answer lies in how heat is scattered. Solid rock and loose grains emit infrared light at slightly different angles—a technique astronomers already use to study asteroids in our own solar system.
Future observations with the JWST will attempt to measure these scattering angles to definitively separate an active rocky crust from an ancient layer of regolith. This methodology could eventually be applied to other hot, rocky exoplanets, allowing scientists to categorize distant worlds by their internal activity—sorting the living, tectonic worlds from the dead, weathered husks.
The next phase of research will involve targeted MIRI observations to refine the surface texture models, which will be shared in upcoming peer-reviewed updates as more data from the LHS 3844 system is processed.
Do you think the search for life should prioritize “Earth-like” sizes or “Earth-like” atmospheres? Share your thoughts in the comments below.
