Webb telescope discovers hidden planet in famous star system

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

The young, nearby 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 only the second planetary system known to contain at least three imaged planets.

Unlike Beta Pictoris b and c, however, Beta Pictoris d was discovered not by identifying a bright point of light but by detecting the unique chemical fingerprint 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 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 helping us tell that story.”

Familiar system, new surprise

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

The team estimates that the newfound Beta Pictoris d is likely at least twice the mass of Jupiter, making it the smallest of the three known giant planets in the system. Modeling suggests it likely circles its star at about 30 astronomical units, comparable to the region occupied by Neptune in our solar system. It has the widest orbit of the three known planets but is still inside the inner edge of the debris disk.

Although astronomers were not searching for another planet with Webb, Beta Pictoris d emerged while the team was using 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 we already knew existed. Then, this telltale signal appeared in the data where we didn’t expect it.”

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

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

“There was an unexpected bright source of light within the Integral Field Unit imaging, but we’ve learned not to trust bright blobs in images,” said Jean-Baptiste Ruffio, a research scientist at the University of California, San Diego, and principal investigator of the first Webb observations in which the discovery was made.

“They can 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 while providing a richer look at the planet’s atmosphere.

Unlike traditional imaging, the spectroscopic approach 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 learning about its temperature, chemistry and motion.”

A separate imaging study led by Ben Sutlieff of the University of Edinburgh and Markus Bonse of the European Southern Observatory complements 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.

Seeing through cosmic fog

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

The dusty disk acts like fog, scattering light from the star and making it difficult for conventional imaging techniques to distinguish planets from surrounding structures. The team’s spectroscopic method with Webb 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 sharply defined inner edge and other puzzling structures. In fact, astronomers had already predicted the existence of a planet like Beta Pictoris d to account for the disk’s unusual structure.

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 through moderate-resolution spectroscopy, showing that astronomers can identify worlds in complex environments through their atmospheric fingerprints rather than relying solely on traditional coronagraphic imaging.

The 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.

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Sadie Harley

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Andrew Zinin

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