Astronomers explore the surface composition of a nearby super-Earth – Image for illustrative purposes only (Image credits: Pixabay)
The James Webb Space Telescope recently captured the thermal emission spectrum of LHS 3844 b, a super-Earth just 49 light-years away that orbits its dim red dwarf star every 11 hours. Observations with the Mid-Infrared Instrument, or MIRI, exposed a dark, featureless surface dominated by low-silica rocks similar to basalt or olivine-rich materials.[1][2] This rocky world, 1.3 times Earth’s size with an equilibrium temperature of 805 Kelvin, offers a rare glimpse into the geology of exoplanets beyond our solar system.
Pioneering Mid-Infrared Observations
A team led by Sebastian Zieba, now at the Center for Astrophysics at Harvard and Smithsonian, and principal investigator Laura Kreidberg from the Max Planck Institute for Astronomy conducted the study using JWST’s MIRI in low-resolution spectroscopy mode. They targeted three secondary eclipses of the planet, spanning wavelengths from 5 to 12 microns, to measure its dayside thermal emission.[1] These observations built on prior Spitzer data and TESS transits to refine the planet’s ephemeris and achieve high precision.
The white-light curve showed an eclipse depth of 696 parts per million, detected at 39-sigma confidence, confirming strong dayside emission with no evidence of heat redistribution to the nightside. Independent data reductions using the Eureka! pipeline yielded consistent results, allowing robust spectral analysis against blackbody models and mineral libraries.[1]
A Surface of Dark, Low-Silica Rock
The planet’s spectrum closely matched a blackbody at 1000 Kelvin, with a brightness temperature ratio indicating a low Bond albedo of about 0.14 – similar to the Moon or Mercury. Models from spectral libraries favored dark, mafic or ultramafic compositions, such as olivine clinopyroxenite or Kilauea basalt, over brighter or silica-rich alternatives.[1]
Fresh powder surfaces, which would appear brighter and cooler, were ruled out at more than 3-sigma confidence, except for rare cases like hematite. High-silica granite, indicative of water-involved crustal differentiation on Earth, faced exclusion at over 8.9-sigma even under extreme space weathering scenarios. Space weathering with nanophase iron or carbon could darken powders to fit the data, pointing to an aged, bombarded surface.
| Surface Type | SiO2 Content (wt%) | Fit Quality |
|---|---|---|
| Olivine clinopyroxenite | 47 | ~1σ (best fit) |
| Kilauea basalt | 51 | ~1.5σ |
| Granite | >73 | >8.9σ (ruled out) |
| Fresh powders (general) | Varies | >3σ (ruled out) |
Absence of Volcanic Gases and Thick Atmosphere
Atmospheric models ruled out substantial gas layers, with 5-sigma upper limits placing carbon dioxide partial pressure below 100 millibars and sulfur dioxide below 10 microbars. Thicker atmospheres, whether CO2-dominated or oxygen-rich, mismatched the smooth spectrum at high confidence. This aligns with earlier Spitzer phase curves suggesting an airless or extremely tenuous envelope.[1]
The lack of detectable SO2 disfavors ongoing volcanism with gas accumulation, though trace outgassing could collapse into cold traps on the nightside. A potential subtle dip near 11 microns hints at olivine transparency features, but requires confirmation from future observations.
Key Constraints:
– Bond albedo: 0.14+0.13-0.14
– Dayside temperature: 1000+15-14 K
– No fresh powders or granite
– Thin or absent atmosphere
Insights into Rocky Exoplanet Evolution
LHS 3844 b’s dark, low-silica crust suggests a mantle rich in magnesium relative to silicon, potentially from prolonged volcanism without the water-driven differentiation seen on Earth. The featureless spectrum supports either an ancient, space-weathered terrain or a recently resurfaced one lacking outgassing. JWST’s success here demonstrates mid-infrared spectroscopy’s power for probing bare-rock worlds.[1]
Upcoming phase curve observations will test surface texture and heat transport, refining models of tidal locking and geology on ultra-short-period planets. These results edge closer to decoding the histories of rocky exoplanets inhospitable to life as we know it.