An international team of scientists using the James Webb Space Telescope has successfully characterized the surface composition of exoplanet LHS 3844b, including the detection of rocky features resembling the Moon or Mercury.
The team behind the historic study of a planet roughly 30% larger than Earth and 48.5 light-years away, led by researchers from the Max Planck Institute for Astronomy, said it had deciphered the geological properties of planets orbiting distant stars. is the “next step in unveiling their nature.”
Surface Details of Nearby Exoplanet Revealed Thanks to James Webb’s ‘MIRI’
According to a statement announcing the breakthrough achievement, team leader Sebastian Zieba, a former MPIA (Max Planck Institute for Astronomy, Heidelberg, Germany) PhD student and principal investigator, and MPIA Director Laura Kreidberg used JWST’s Mid Infrared Instrument (MIRI) to study the light emissions from rocky exoplanet LHS 3844b, which they used to make the surface geological characterizations.
Separating the light reflected by the planet and the much brighter light emitted by its host star is challenging in normal circumstances. But distinguishing the light reflected by LHS 3844b from its host star was even more difficult due to the planet’s proximity.
© Sebastian Zieba et al./MPIA
Unlike Earth, which requires 365 days to orbit the Sun, the planet examined by the MPIA-led team completes an orbit every eleven hours. This close proximity also means the planet is tidally locked, with one side permanently facing the star and one facing deep space.
Kreidberg said separating enough of the planet’s light from the host star to characterize individual surface features and likely compositions was only possible “thanks to the amazing sensitivity” of Webb’s instruments.
“(With MIRI), we can detect light coming directly from the surface of this distant rocky planet,” the researcher explained.
‘Dark, Hot, Barren Rock’ Likely Mirrors Moon or Mercury
Due to the extreme challenges associated with studying the planet’s “dark” side, the research team focused JWST’s MIRI instrument on the side of the planet facing its host star. Because of the constant bombardment of radiation, the initial analysis suggested that the average surface temperature of LHS 3844b is approximately 725 Degrees Celsius (1,340 degrees Fahrenheit).
To separate the planet’s light from the star, MIRI divided a portion of the infrared emission into smaller wavelength sections, ranging from 5 to 12 micrometers. Next, the research team measured the brightness per wavelength for each bin.
When the team compared this data with another data point collected by the Spitzer Space Telescope a few years earlier, a picture of the planet’s surface began to emerge. According to the study authors, the first images derived from the infrared spectral data revealed a dark surface, resembling a large version of our Moon or the planet Mercury.
“We see a dark, hot, barren rock, devoid of any atmosphere,” Kreidberg explained.
Because libraries of exoplanet geology don’t currently exist, the research team accessed libraries of data on rocks and minerals known from the Earth, the Moon, and Mars. The team used this data to determine what infrared signatures would look like if they were part of the composition of exoplanet LHS 3844b.
According to the team’s statement, this comparison “confidently ruled out a composition comparable to Earth’s crust,” which is typically made from silicate-rich minerals such as granite. Since Earth is the only planet in our solar system with this type of crust, the team said that ruling it out for LHS 3844 b was “not very surprising.” However, they noted, this information could still “reveal details” about the planet’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,” they explained.
During this extended process, the planet’s rocky material repeatedly melts and solidifies again, ultimately leaving the lighter material on the surface. Zieba said that the lack of a silicate crust on LHS 3844 b may suggest that there are either no plate tectonic activities, or if they exist, they are “ineffective.”
“This planet likely only contains little water,” the researcher added.
Two Formation Scenarios Match JWST’s Initial Results
Although the initial analysis revealed a fair amount of surface features, the team’s statistical analysis of the light spectra suggested that the planet’s surface contained “extended solid areas of basalt or magmatic rock.” This included deposits rich in magnesium and iron, and “can include” olivine.
Models with a surface containing crushed materials, such as rocks or gravel, also fit “fairly well.” Conversely, models featuring a surface with powders or grains were “inconsistent with the observations due to their brighter appearance, at least at first glance.”
Zeiba said that as a planet without a protective atmosphere, such as LHS 3844b, experiences prolonged periods of space weathering driven by stellar radiation and meteorite impacts, which slowly dissolve hard surface rocks into regolith, “a layer of fine grains or powder as found on the Moon.” These processes also darken the surface by adding iron and carbon.
The research team notes that the presence of these two elements makes the modeled regolith’s properties “more consistent with the observations.” Still, there were two competing scenarios that could have caused the planet’s appearance.
In the first scenario, LHS 3844 b has a surface composed primarily of dark, solid rock containing basaltic or magmatic minerals, such as those found in volcanoes. A scenario involving a period of widespread volcanic activity would alter surface properties much more quickly than the geological timescales required by the alternate theory.
In the second idea, the surface is shaped by longer periods of space weathering, resulting in a fine dust layer, as on the Moon. Because this process requires much longer time scales of general inactivity, the team notes that it requires conditions “opposite to the first scenario.”
Because observations have not detected sulfur dioxide outgassing, which is commonly associated with extended periods of volcanism, the team said they favor the longer, space-weathering scenario. In that scenario, the team said the planet may look less like the Moon and “much like Mercury indeed.”
Clarifying the Nature of LHS 3844 b and “Other Rocky Exoplanets” in the Future
Although the current study was unable to complete the planet’s characterization, Zieba, Kreidberg, and colleagues have already secured additional JWST observations to solve the mystery. According to the researchers, this ‘direct approach’ should finally help them discern the planet’s surface conditions by “exploiting small differences in how solid slabs and powders emit or reflect light.”
Although the concept is usually used to characterize asteroids within our Solar System, the team said that this comparison of emission angles, which depends on the target’s surface roughness, reveals how that topography affects the amount of radiation received by JWST.
“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,” Kreidberg concluded.
The study “The dark and featureless surface of rocky exoplanet LHS 3844 b from JWST mid-infrared spectroscopy” was published in Nature Astronomy.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.