By Will Dunham
WASHINGTON, May 8 (Reuters) - Astronomers have gotten their clearest look yet at the surface of an exoplanet - a planet beyond our solar system - thanks to NASA's James Webb Space Telescope.
Webb gathered data on a rocky exoplanet with a diameter about 30% larger than Earth that indicates it is a desolate and airless world whose surface may resemble that of Mercury, our solar system's innermost planet. Its lack of a discernible atmosphere and its extreme temperatures - fiercely hot on one side and frigid on the other - seem to render it uninhabitable.
The planet is called LHS 3844 b, or Kua'kua, the word for butterfly in an indigenous language spoken in Costa Rica. It orbits a star smaller and less luminous than the sun, located about 49 light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).
"This planet is not a nice place," said astronomer Laura Kreidberg, managing director of the Max Planck Institute for Astronomy in Germany and senior author of the study published this week in the journal Nature Astronomy.
"It's a hellish, barren rock - much more similar to Mercury than it is to the Earth. There is no trace of an atmosphere. Instead we're seeing a dark surface, likely old. Picture a bare rock hurtling through space for billions of years. You wouldn't want to go there," Kreidberg said.
The observations suggest an ancient planetary surface covered by darkened regolith - loose, fragmented rocky material that covers solid bedrock, born of eons of continuous bombardment by stellar radiation and micrometeorite impacts.
Webb, which was launched in 2021 and became operational in 2022, has enabled revolutionary advances in the understanding of exoplanets. Its robust infrared observational capabilities have helped discern the chemical composition and internal dynamics of exoplanet atmospheres, even showing what kind of clouds are present.
Webb is now letting astronomers directly study the geology and surface composition of exoplanets, said astronomer and study lead author Sebastian Zieba of the Center for Astrophysics, Harvard & Smithsonian in Massachusetts.
"That was very challenging before the James Webb Space Telescope. This, therefore, also puts the Earth and the solar system as a whole into greater context, allowing us to check if processes or surface compositions familiar within the solar system are common around other stars, too," Zieba said.
"It's like we suddenly cleaned our glasses and can see the planets clearly for the first time," Kreidberg added.
The star that Kua'kua orbits is a common type called a red dwarf. Its mass is about 15% that of the sun and its luminosity about 0.3%. Kua'kua is located extremely close to the star, orbiting it once every 11 hours. It also is "tidally locked," meaning one side always faces the star and the other side always faces away, as the moon does with Earth.
The planet's "dayside" surface - perpetually broiled by the star - is about 1,340 degrees Fahrenheit (725 degrees Celsius). There was no detectable heat on the planet's "nightside."
Webb enabled the researchers to detect light - specifically the infrared region of the electromagnetic spectrum - coming directly from the planet's surface.
"Different rocks have different spectral fingerprints, just like atmospheres do. Dark volcanic rocks like basalt matched our observations much better than brighter, silica-rich rocks like granite," Zieba said.
The surfaces of Mercury and the moon are basalt-dominated.
"On Earth, widespread granite formation is linked to water and plate tectonics," Zieba said, referring to the geological process on our planet involving the gradual movement of the immense plates that make up Earth's surface. "So if you ever robustly identified granite-like surfaces on an exoplanet, that would not (automatically) mean life, but it would suggest a much more Earth-like geological history compared to other surfaces."
Another possibility matching the observations was a solid surface of relatively recent volcanic rock, but the researchers searched for volcanism-related gases like sulphur dioxide and found none.
Without an atmosphere, there is scant protection from stellar radiation or charged particles from the star and no chance for liquid water, considered fundamental for life.
"So overall, this is almost certainly not a habitable world," Zieba said.
(Reporting by Will Dunham, Editing by Rosalba O'Brien)
