A nearby "super-Earth" is giving scientists a rare opportunity to peer directly into the exposed surface of a distant, rocky planet that bears little resemblance to Earth. The planet, named LHS 3844 b, is a hot, dark, atmosphere-less world with surface composition and geological conditions closer to those of the Moon or Mercury than to Earth-like planets, the latest observations show.
The scientific research team used the mid-infrared instrument MIRI on NASA's James Webb Space Telescope (JWST) to conduct detailed observations of the solar side of the planet. The research was led by Sebastian Ziba, who studied for a PhD at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, and involved Laura Kreidberg, director of MPIA and principal investigator of the project. Compared with the previous focus on exoplanet atmospheres, this work further advances the research frontier to "exoplanet geology" - trying to directly constrain the surface composition and evolutionary history of rocky planets outside the solar system. Relevant results were published in the journal Nature Astronomy.
LHS 3844 b is a rocky planet with a radius approximately 30% larger than Earth's. It orbits a cold red dwarf star at an extremely close distance, with an orbital period of only about 11 hours, and the distance between the planet and its parent star is only about three stellar diameters. Such a tight orbit causes the planet to be tidally locked, with one side always blazing toward the star and the other side permanently in darkness. Its solar surface is as hot as about 1,000 Kelvin (about 725 degrees Celsius), and the entire planetary system is only about 48.5 light-years away from Earth.
Thanks to the Webb telescope's excellent sensitivity, researchers were able to directly measure the brightness of thermal radiation coming from the rocky planet's surface. "What we saw was a dark, hot, barren rock with absolutely no detectable atmosphere," Kreidberg said of the observations. Since telescopes cannot directly resolve the circular surface of the planet, the researchers used "secondary eclipse" and "phase curve" techniques to invert the infrared radiation emitted by the planet's sun surface by tracking the weak fluctuations in the total brightness of the entire system as the orbit changes.
MIRI observes the system in the 5–12 micron band and further divides this band into finer sub-bands to obtain the mid-infrared spectral distribution of the planetary surface radiation. The team also incorporated previous data from the Spitzer Space Telescope into the analysis to improve the robustness of the spectral fit. By comparing the brightness at different wavelengths with theoretical models, researchers were able to examine a variety of potential surface material combinations, from Earth's granitic crust to lunar-style basalts and mantle-derived lavas.
The calculations explicitly rule out surface scenarios similar to Earth's continental crust. The earth's silicate-rich granitic crust is usually formed through lengthy plate tectonics and magma recycling, often requiring the participation of liquid water. Repeated melting and differentiation allow light minerals to gradually float to the surface. Ziba noted that the spectrum of LHS 3844 b shows no sign of this silicate-rich, granitic crust, which means Earth-style plate tectonics either never occurred on the planet or has long ceased to function. This also implies that the internal water content of the planet is extremely low, which is essentially different from the "Earth-like planets" in the customary sense.
Instead, observations support a "basalt-dominated" surface scenario. The model that is most consistent with the data is a large area of basalt rock formed by the solidification of mantle-derived magma, similar to the vast basalt plains on Earth or the "maria" on the moon. These rocks are typically rich in magnesium and iron and contain a variety of iron-magnesium silicate minerals such as olivine. The fit showed that coarser rock or gravel layers would also match the observations well, while a surface composed solely of fine dust would be too bright to match current observations.
Due to the lack of an atmospheric barrier, the surface of LHS 3844 b is completely exposed to radiation from the parent star and bombardment from meteoroids, and suffers from so-called "space weathering" for a long time. These processes gradually break the hard rock into tiny particles similar to lunar regolith and enrich its surface with iron and carbon, making the material darker and more endothermic. Ziba pointed out that it is this weathered dark regolith that makes the overall optical and infrared properties of the planet's surface more consistent with observations.
Based on the available data, the team proposed two possible scenarios for surface evolution. The first is that the planet's surface is extensively covered with relatively "young" basaltic rocks, suggesting that recent or ongoing volcanic activity has been delivering fresh molten rock to the surface. The second type is the "old" surface dominated by long-term space weathering: the former magma plain has been repeatedly processed by irradiation and impacts over hundreds of millions of years, and is covered by a thick dark weathering layer like the moon or Mercury. Judging from the spectral morphology, this latter "long-term silence" scene is more consistent with observations.
A key indicator to differentiate between the two scenarios is the presence of ongoing volcanic activity on the planet. On many geologically active celestial bodies, volcanoes release large amounts of gas, of which sulfur dioxide (SO₂) is one of the typical tracers. If LHS 3844 b has strong contemporary volcanic activity, MIRI should theoretically be able to identify characteristic absorption bands for SO₂ in the mid-infrared spectrum. However, observations did not detect such features, which greatly reduces the possibility of recent volcanic activity, but instead supports the explanation that its surface has been "cooling and dormant" for a long time, making it closer to Mercury in appearance.
In order to further clarify the true appearance of this planet, the research team has planned more follow-up JWST observations. One of the key tasks is to measure the thermal radiation and reflection properties of the planet's surface at different viewing angles, using the way light is scattered to distinguish rough rocky surfaces from relatively smooth or loose materials. This type of technology has been successful in studying asteroids in the solar system, and now being transplanted to the field of exoplanets, it is expected to allow scientists to determine whether the surface layer of LHS 3844 b is a whole rock slab, a lava plain, or a thick accumulation of powder and debris.
Kreidberg said the team is confident that using the same method will not only reveal the crustal properties of LHS 3844 b, but also provide "surface-level" information for more rocky exoplanets in the future. The JWST observations used here are from the General Observation Project 1846, titled "Searching for Signs of Volcanoes and Geodynamics on the Hot Rocky Exoplanet LHS 3844 b," for which she serves as chief scientist and Ren Ru serves as co-lead. Institutions participating in this research are located in the United States, Germany, China and other countries, including the Harvard-Smithsonian Center for Astrophysics, California Institute of Technology, Jet Propulsion Laboratory, Peking University, Pennsylvania State University, NASA Goddard Space Flight Center, and many European universities and research institutions.
The MIRI instrument responsible for this observation was developed by a joint team from many European countries, including Belgium, Denmark, France, Germany, Ireland, the Netherlands, Spain, Sweden, Switzerland and the United Kingdom. The relevant work was carried out with the funding of national scientific research institutions in each country. In Germany, the main funders are the Max Planck Society and the German Aerospace Center, and participating units include the Max Planck Institute for Astronomy in Heidelberg, the University of Cologne, and the Hensold Company. As one of the most important space astronomy facilities today, the James Webb Space Telescope is led by NASA, with the participation of the European Space Agency and the Canadian Space Agency, and is committed to opening up new windows in the fields of early galaxy formation, star and planet birth, and exoplanet atmosphere and surface properties. Prior to this, the Spitzer Space Telescope had laid the foundation for exoplanet research through infrared observations, and its related projects were operated by the Jet Propulsion Laboratory at the California Institute of Technology on behalf of NASA.