Astronomers using NASA’s James Webb Space Telescope have discovered a giant planet outside our solar system, called an exoplanet, hidden within one of the most intensively studied planetary systems in our Milky Way galaxy.

The nearby young star Beta Pictoris was already known to host two giant planets: Beta Pictoris b, one of the first exoplanets ever directly imaged, and Beta Pictoris c. The newly identified Beta Pictoris d makes it the second known planetary system to contain at least three imaged planets. However, unlike Beta Pictoris b and c, Beta Pictoris d was discovered not by identifying a bright point of light, but by detecting the unique chemical signature of its atmosphere, a technique that could transform the search for worlds around other stars.

“This discovery adds another piece to an already fascinating planetary system,” said Aidan Gibbs, lead author of a new study published Wednesday in the Astrophysical Journal Letters and a postdoctoral researcher at the University of California, San Diego. “Beta Pictoris has long served as a laboratory for understanding how planetary systems form and evolve, and now we have another planet to help us tell that story.”

Located 63 light years from Earth and about 23 million years away, Beta Pictoris is a nearby system in the Milky Way that offers a rare glimpse into the interactions between newborn planets and the disk of dust and debris left behind after their formation.

The team estimates that the newly discovered Beta Pictoris d is probably at least twice the mass of Jupiter, making it the smallest of the system’s three known giant planets. Models suggest that it probably orbits its star at about 30 astronomical units, comparable to the region occupied by Neptune in our own solar system. It is the widest orbit of the three known planets, but still lies within the inner edge of the debris disk.

Although astronomers were not searching for another planet with Webb, Beta Pictoris d emerged while the team used the telescope’s NIRSpec (near-infrared spectrograph) to study the atmosphere of Beta Pictoris b. Specifically, they used NIRSpec’s Integral Field Unit, which obtains both an image and a spectrum from each pixel in an image.

“We weren’t looking for a new planet,” Gibbs said. “We were trying to understand one that we already knew existed. Then this telltale signal showed up in the data where we didn’t expect it.”

This signal was a series of peaks and valleys within the spectroscopic data where the team expected to see a smooth spectrum of light bouncing off the dust. It was a distinctive pattern of carbon monoxide absorption lines, spread out like a barcode, an expected feature in the atmospheres of giant planets.

Since spectroscopy reveals not only the chemical composition but also the motion of an object, the team was also able to extract radial velocity from the data. The team determined that the planet’s speed, position and alignment with the debris disk were consistent with something orbiting Beta Pictoris rather than a background star or a brown dwarf with carbon monoxide in its atmosphere.

“There was an unexpected bright light source within the Integral Field Unit images, but we have learned not to trust the bright spots in the images,” said Jean-Baptiste Ruffio, a research scientist at the University of California, San Diego and principal investigator of the first Webb observations where the discovery was made. “They may be instrumental artifacts or other structures in the debris disk. By obtaining a spectrum at the same time as the image, we were able to quickly confirm our suspicions.”

Follow-up observations with Webb’s MIRI (Mid-Infrared Instrument) through a Director’s Discretionary Time request detected water vapor and methane, further confirming the planet’s identity and providing a richer view of its atmosphere.

Unlike traditional imaging, the spectroscopic method allowed researchers to identify the planet and begin studying its atmosphere from the first observation.

“A spectrum contains an incredible amount of information,” Ruffio said. “You don’t just learn that something is a planet; you immediately begin to learn about its temperature, chemistry, and motion.”

A separate imaging study led by Benjamin Sutlieff of the University of Edinburgh and Markus Bonse of the European Southern Observatory complemented the team’s findings with data from the European Southern Observatory’s Very Large Telescope and Webb’s NIRCam (near-infrared camera) and independently confirmed the existence of Beta Pictoris d.

Beta Pictoris d remained hidden for years because it lies within one of the brightest known debris disks.

The dust disk acts like fog, scattering light from the star, making it difficult for conventional imaging techniques to distinguish the planets from surrounding structures. The Webb team’s spectroscopic method effectively ignored that dust, isolating only the narrow molecular signatures unique to a planetary atmosphere.

Scientists say the planet’s presence may help explain why the famous debris disk has such a clearly defined inner rim and other puzzling structures. In fact, astronomers had already predicted the existence of a planet like Beta Pictoris d to explain the unusual structure of the disk.

Beyond expanding our understanding of Beta Pictoris, the discovery demonstrates a powerful new way to find exoplanets.

This is the first directly imaged planet discovered primarily using moderate resolution spectroscopy, demonstrating that astronomers can identify worlds in complex environments through their atmospheric fingerprints rather than relying solely on traditional coronagraphic images.

Researchers plan to continue analyzing Webb’s observations to better determine the planet’s temperature, atmospheric composition and orbit, providing an even more detailed view of one of astronomy’s most iconic planetary systems.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond it to distant worlds around other stars, and exploring the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

For more information about Webb, visit:

https://science.nasa.gov/webb

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