NASA’s Webb Identifies Its First Exoplanet — And It’s the Size of Earth

Jeremy Rehm

Jan 11, 2023

Illustration of a planet and its star on a black background. The planet is large, in the foreground at the center and the star is smaller, in the background at the upper left. The planet has no atmosphere and no clouds. The left quarter of the planet (the side facing the star) is lit, while the rest is in shadow. The star is bright yellowish-white, with no clear features. — Based on new evidence from NASA’s James Webb Space Telescope, this illustration reflects that exoplanet LHS 475b is rocky and almost precisely the same size as Earth. The planet whips around its star in just two Earth-days. Researchers will follow up this summer with additional observations with Webb, which they hope will definitively conclude if the planet has an atmosphere. LHS 475b is relatively close, 41 light-years away, in the constellation Octans. (Credit: NASA/European Space Agency/Canadian Space Agency/L. Hustak (STScI))

A team led by researchers at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, has confirmed the discovery of an exoplanet — a planet orbiting another star — by NASA’s James Webb Space Telescope. Formally logged as LHS 475b, the planet orbits a red dwarf star roughly 41 light-years away, and it’s almost exactly the size of Earth, clocking in at 99% of our planet’s diameter.

“I think rocky exoplanets are the new frontier with JWST,” said APL astrophysicist Kevin Stevenson, a co-lead on the study. “Just as NASA’s Hubble and Spitzer space telescopes paved a new field with the discovery of hot, Jupiter-sized exoplanets, JWST is going to open a whole new avenue of research into smaller, rocky worlds that were once undetectable from Earth.”

The researchers focused on the rocky world after carefully reviewing targets of interest from NASA’s Transiting Exoplanet Survey Satellite (TESS), which could only hint at the planet’s existence because of the limitations of its cameras and the small signal from the planet. Webb’s Near Infrared Spectrograph (NIRSpec), however, clearly confirmed the planet after two observations last summer of it transiting, or passing in front of its star.

The researchers presented their findings to media on Jan. 11 at the annual conference of the American Astronomical Society.

Graphic titled “Rocky Exoplanet LHS 475 b Transit Light Curve, NIRSpec Bright Object Time-Series Spectroscopy.” Behind the graph is an illustration of the planet and its star. The graph, or spectrum, shows the change in relative brightness of the star-planet system between 3:00 p.m. and 6:00 p.m. in Baltimore, Maryland, on August 31, 2022. The spectrum shows that the brightness of the system remains steady until the planet begins to transit the star. — A light curve from NASA’s James Webb Space Telescope’s Near-Infrared Spectrograph (NIRSpec) shows the change in brightness from the LHS 475 star system over time as the planet transited the star on Aug. 31, 2022. LHS 475b is a rocky, Earth-sized exoplanet that orbits a red dwarf star roughly 41 light-years away. The planet is extremely close to its star, completing one orbit in two Earth-days. The planet’s confirmation was made possible by Webb’s data. (Credit: NASA/European Space Agency/Canadian Space Agency/STScI/L. Hustak (STScI)/Johns Hopkins APL)

“Missions like Webb are among the boldest NASA projects ever undertaken, uniquely positioned to answer foundational questions about our home and its place in the cosmos,” said Jason Kalirai, the Civil Space Mission Area Executive at APL and former Webb project scientist at the Space Telescope Science Institute (STScI). “The discovery of an Earth-sized planet in Webb’s first year paves the way for a powerful new exploration of rocky exoplanets and the opportunity to place Earth in a broader context.”

Operating under NASA’s Cycle 1 General Observer program, the researchers have been assessing Webb’s ability to detect atmospheres, particularly around small, Earth-sized planets. They’re hunting for a universal line between planets that do and do not have atmospheres — a sort of cosmic shoreline, as Stevenson puts it. Such a line exists in our solar system, following mass as it grows from airless asteroids to thick atmospheric soups around the gas giants. It’s unknown whether this holds outside the solar system.

Last year, the team contributed to the first clear signal of carbon dioxide around a large exoplanet, the Jupiter-sized planet called WASP-39b. But the holy grail has been detecting atmospheres around small Earth-sized exoplanets, which only Webb can do.

Graphic titled “Rocky Exoplanet LHS 475b Atmosphere Composition, NIRSpec Bright Object Time-Series Spectroscopy.” The graphic shows the transmission spectrum of the rocky exoplanet LHS 475b, captured using Webb's NIRSpec Bright Object Time-Series Spectroscopy mode, with an illustration of the planet and its star in the background. The data points are plotted as white circles with grey error bars on a graph. — A transmission spectrum collected from the Earth-sized exoplanet LHS 475b, collected by researchers using the Near-Infrared Spectrograph (NIRSpec) instrument on NASA’s James Webb Space Telescope on Aug. 31, 2022. The spectrum shows Webb did not observe a detectable quantity of any element or molecule. The data (white dots) is consistent with a featureless spectrum representative of a planet that has no atmosphere (yellow line). The purple line represents a pure carbon dioxide atmosphere and is indistinguishable from a flat line at the current level of precision. The green line represents a pure methane atmosphere and is ruled out by the available data. (Credit: NASA/European Space Agency/Canadian Space Agency/L. Hustak (STScI)/Johns Hopkins APL)

By analyzing light from LHS 475b’s star as the planet transited in front of it, the team attempted to gauge the presence and composition of its atmosphere. The data confirmed the planet as an Earth-sized terrestrial world just a few hundred degrees warmer than our own planet, but the researchers are still uncertain if it has an atmosphere.

“The observatory’s data is beautiful,” said astrophysicist Erin May, an exoplanet astronomer and team member from APL. “The telescope is so sensitive that it can easily detect a range of molecules around the planet, but we can’t yet make any definitive conclusions about the planet’s atmosphere.”

Taking just two Earth-days to orbit its red dwarf star, LHS 475b is closer to its star than any planet in our solar system. Such proximity to the Sun would easily boil and blow off any atmosphere, but because the temperature of LHS 475b’s star is less than half that of the Sun, the researchers project the planet could still have an atmosphere.

Illustration of a planet and its star on a black, starry background. The planet is large, in the foreground at the center and the star is smaller, in the background at the upper left. A line splits the planet in half, with the left showing atmosphere and the right with an atmosphere. The left quarter of the planet (the side facing the star) is lit, while the rest is in shadow. — Based on transmission spectrum data collected by researchers using NASA’s James Webb Space Telescope, this illustration reflects the uncertainty that remains about the atmosphere of the Earth-sized exoplanet LHS 475b: It may have virtually no atmosphere like Mercury, or it may have a thick atmosphere rich in carbon dioxide like Venus. (Credit: NASA/European Space Agency/Canadian Space Agency/L. Hustak (STScI)/Matthew Wallace (Johns Hopkins APL))

The spectrum allowed the team to rule out a few possibilities, such as a hydrogen-dominated atmosphere like Jupiter’s, or a thick, methane-dominated atmosphere like Saturn’s largest moon Titan, said APL exoplanet researcher Jacob Lustig-Yaeger. But the data still pins the team between two bookend possibilities: a planet with no atmosphere, or one rich in carbon dioxide — a counterintuitive dichotomy caused by carbon dioxide’s relatively heavy mass, which prevents it from expanding enough to be easily registered, he explained.

Distinguishing between these possibilities will require measurements that are even more precise. The researchers are scheduled to collect additional spectra with a third observation of the planet next summer. They suggest that observing it during a secondary eclipse when the planet just starts to orbit behind its star would allow scientists to determine whether the planet has clouds, which could help determine if the planet’s atmosphere is like Venus’ — cloudy and rich in carbon dioxide.

“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.”