The surface of a distant rocky planet about 30% bigger than the Earth may resemble a larger version of the moon or the planet Mercury, according to a new study using NASA's James Webb Space Telescope (JWST). Researchers used JWST's ability to detect radiation in mid-infrared wavelengths, such as those that emanate from the planet's hot crust, to characterize the planet's surface. The results suggest that the planet has a rocky crust that probably has not had much recent tectonic activity.
A paper describing the results, by an international team of researchers including a Penn State astronomer, appeared today (May 4) in the journal Nature Astronomy.
"The planet, known as LHS 3844 b, is about 50 light-years from Earth," said Renyu Hu, associate professor of astronomy and astrophysics in the Eberly College of Science at Penn State, an author of the paper and co-principal investigator for the JWST observing program used for the research. "It orbits a cool red dwarf star about every 11 hours and is tidally locked, meaning that the same side of the planet always faces the star. The dayside of the planet is extremely hot - averaging about 1,000 Kelvin or approximately 725 degrees Celsius or 1,340 degrees Fahrenheit."
To characterize the planet's surface, the team used the Mid Infrared Instrument (MIRI), a high-resolution camera and spectrograph on board the JWST. MIRI is tuned to analyze radiation in mid-infrared wavelengths, such as those received from the planet's hot dayside. However, when measuring this radiation, the researchers explained that they must register the repeating changes in brightness that the telescope receives from the combination of the star and the orbiting planet.
"Thanks to the amazing sensitivity of JWST, we can detect light coming directly from the surface of this distant rocky planet," said Laura Kreidberg, the leader of the research team at the Max Planck Institute for Astronomy in Heidelberg, Germany. "We see a dark, hot, barren rock, devoid of any atmosphere."
In much the same way that exoplanetary atmosphere research has benefited from climate science here on Earth, the emerging field of exoplanetary geology draws on Earth-based geologic knowledge. The team 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 silica-rich rocks such as granite.
Although this result is not very surprising - Earth is the only planet with such a crust in its solar system - it may reveal details of LHS 3844 b's geological history. Earth-like silica-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 on the surface repeatedly melts and solidifies as it is mixed with material from the mantle - the layer just below the crust - 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 these geological processes are ineffective," said first author Sebastian Zieba, a postdoctoral researcher at the Center for Astrophysics, Harvard and the Smithsonian.
Instead, the researchers said that the planet's dark surface points to a composition reminiscent of terrestrial or lunar basalt - a dark-colored volcanic rock rich in iron and magnesium - or of the Earth's mantle material.
For a more detailed characterization, the team performed statistical analysis of how well the infrared spectrum from the planet fit with the chemical signatures of Earth rocks and minerals. Their analysis revealed that the observations best match extended solid areas of basalt or magmatic rock. This includes olivine, a greenish mineral that makes up more than 50% of the Earth's mantle. The researchers said that crushed material, such as rocks or gravel, also fit the data, whereas grains or powders are inconsistent with the observations due to their brighter appearance.
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. This pummeling typically results in grains or powders that have a brighter appearance than what the researchers observed on LHS 3844 b.
"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," Zieba said. "They also darken the layer by adding iron and carbon, making the regolith's properties more consistent with the observations."
This assessment left the astronomers with two possible 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 can alter a planet's surface properties quickly. Therefore, the astronomers conclude that, in this scenario, the surface should be relatively fresh, produced by recent geological activity, such as widespread volcanic eruptions.
The second scenario also accounts for a dark surface comparable to the moon or Mercury but through prolonged space weathering, which leads to extended regions covered by a darkened regolith layer. However, this fine powder - also present on the moon, as evidenced by the iconic photographs of astronauts' footprints - requires longer periods of geological inactivity.
The two alternative scenarios differ in the degree of recent geological activity required. On Earth and other active objects in the solar system, tectonic activity produces gases like sulfur dioxide, which is commonly connected to volcanic eruptions. If sulfur dioxide is present on LHS 3844 b in reasonable amounts, MIRI should have detected it but found nothing. So, a recent period of activity seems unlikely, leading the astronomers to favor the second scenario. If correct, LHS 3844 b may truly look much like the moon or Mercury.
"Previously, JWST has been successfully used to characterize the atmospheres of planets beyond our solar system," Hu said. "The deciphering of the geological properties of planets orbiting distant stars is the next step in unveiling their nature."
Additional observations from JWST may be able to discern small differences in how solid rock and powders emit or reflect light, the researchers said, noting the same concept has been used to characterize the surface of asteroids in the solar system.
"We are confident future observations will allow us to clarify the nature of LHS 3844 b's crust, and, in the future, other rocky planets," Kreidberg said.
A full list of authors, their affiliations and funding is available in the paper.
