James Webb Space Telescope discovers its first exoplanet

Based on new evidence from the James Webb Space Telescope, the exoplanet LHS 475 b is rocky and almost precisely the same size as Earth. Illustration / NASA, ESA, CSA

Researchers using the James Webb Space Telescope have confirmed the presence of an exoplanet, a planet that orbits another star, for the first time.

Formally classified as LHS 475 b, the planet is almost exactly the same size as our own, at 99 per cent of Earth’s diameter.

Although the data show that this is an Earth-sized terrestrial planet, they do not yet know if it has an atmosphere. It is possible the planet has a completely carbon dioxide atmosphere but this will be very hard to detect.

The exoplanet LHS 475 b. It is rocky and almost precisely the same size as Earth, and whips around its star in just two days, Illustration / Nasa, ESA, CSA

The research team, led by Kevin Stevenson and Jacob Lustig-Yaeger, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, zeroed in on the area of space with the telescope after reviewing data from Nasa’s Transiting Exoplanet Survey Satellite (TESS) which hinted at the planet’s existence.

Webb’s Near-Infrared Spectrograph (NIRSpec) captured the planet easily and clearly with only two transit observations.

“There is no question that the planet is there. Webb’s pristine data validate it,” said Lustig-Yaeger.

“The fact that it is also a small, rocky planet is impressive for the observatory,” Stevenson said.

“These first observational results from an Earth-sized, rocky planet open the door to many future possibilities for studying rocky planet atmospheres with Webb,” Mark Clampin, Astrophysics Division director at Nasa Headquarters in Washington, confirmed.

“Webb is bringing us closer and closer to a new understanding of Earth-like worlds outside the Solar System, and the mission is only just getting started.”

A light curve from the James Webb Space Telescope’s Near-Infrared Spectrograph shows the change in brightness from the LHS 475 star system over time as the planet transited the star on August 31, 2022. Illustration / Nasa, ESA, CSA

Among all operating telescopes, only Webb is capable of characterising the atmospheres of Earth-sized exoplanets. The team attempted to assess what is in the planet’s atmosphere by analysing its transmission spectrum.

“The observatory’s data are beautiful,” said Erin May, with the Johns Hopkins University Applied Physics Laboratory. “The telescope is so sensitive that it can easily detect a range of molecules, but we can’t yet draw any definitive conclusions about the planet’s atmosphere.”

Although the team can’t conclude what is present, they can be sure what is not present.

“There are some terrestrial-type atmospheres that we can rule out,” explained Lustig-Yaeger. “It can’t have a thick methane-dominated atmosphere, similar to that of Saturn’s moon Titan.”

The team believes it’s possible the planet has no atmosphere but some atmospheric compositions have not been ruled out, such as a pure carbon dioxide atmosphere.

“Counterintuitively, a 100 per cent carbon dioxide atmosphere is so much more compact that it becomes very challenging to detect,” said Lustig-Yaeger. Even more precise measurements are required for the team to distinguish a pure carbon dioxide atmosphere from no atmosphere at all. The researchers are scheduled to obtain additional spectra with further observations this summer.

Webb also revealed that the planet is a few hundred degrees warmer than Earth, so if clouds are detected it may lead the researchers to conclude that the planet is more like Venus, which has a carbon dioxide atmosphere and is perpetually shrouded in thick cloud.

“We’re at the forefront of studying small, rocky exoplanets,” Lustig-Yaeger said. “We have barely begun scratching the surface of what their atmospheres might be like.”

The researchers also confirmed that the planet completes an orbit in just two days, information that was almost instantaneously revealed by Webb’s precise light curve. Although LHS 475 b is closer to its star than any planet in the Solar System, its red dwarf star is less than half the temperature of the Sun, so the researchers project it still could support an atmosphere.

The researchers’ findings have opened up the possibility of pinpointing Earth-sized planets orbiting smaller red dwarf stars.

“This rocky planet confirmation highlights the precision of the mission’s instruments,” Stevenson said. “And it is only the first of many discoveries that it will make.” Lustig-Yaeger agreed: “With this telescope, rocky exoplanets are the new frontier.”

LHS 475 b is relatively close, at only 41 light-years away, in the constellation Octans.

The discovery was presented at a press conference of the American Astronomical Society on January 11.