Astronomers using NASA’s James Webb Space Telescope have discovered a giant planet outside our solar system, called an exoplanet, lurking inside one of the most intensively studied planetary systems in our 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 imaged directly, and Beta Pictoris c. The newly identified Beta Pictoris d makes it the second known planetary system to contain at least three planets depicted. 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 fingerprints of its atmosphere – a method that could change the search for worlds around other stars.
“This discovery adds another layer to an already fascinating planetary system,” said Aidan Gibbs, lead author of the new study published Wednesday in the Astrophysical Journal Letters and a postdoctoral fellow 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.”
This artist’s concept depicts the Beta Pictoris system with the discovery of the giant exoplanet Beta Pictoris d on the right. It has the widest orbit of the three known exoplanets in the system.
Illustration: NASA, ESA, CSA, STScI, Ralph Crawford (STScI)
Located 63 light-years from Earth and about 23 million years old, Beta Pisces is a nearby system in the Milky Way that offers a rare opportunity to see the interaction 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’s mass is likely at least twice that of Jupiter, making it the smallest of the three known giant planets in the system. Modeling suggests it likely orbits its star at a distance of about 30 astronomical units, comparable to the region occupied by Neptune in our solar system. This is the widest orbit of the three known planets, but is still located within the inner edge of the debris disk.
Although astronomers were not searching for another planet with Webb, Beta Pictoris d emerged when the team used the NIRSpec (Near-Infrared Spectrograph) telescope 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 the image.
“We weren’t looking for a new planet,” Gibbs said. “We were trying to understand one that we already knew existed. “Then that telltale signal showed up in the data where we didn’t expect it.”
This signal was a series of peaks and valleys in the spectroscopic data, where the team expected to see a smooth spectrum of light reflecting off the dust. It was a distinctive pattern of carbon monoxide absorption lines, scattered like a bar code, an expected feature of the atmospheres of giant planets.
Because 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 are consistent with something orbiting Beta Pictoris, rather than a background star or brown dwarf with carbon monoxide in its atmosphere.
“The Integral Field Unit’s imaging process revealed an unexpected bright light source, but we learned not to trust bright spots in the images,” said Jean-Baptiste Ruffio, a postdoctoral fellow at the University of California, San Diego, and principal investigator of Webb’s original observations that made the discovery. “They could be instrumental artifacts or other structures in the debris disk. By acquiring the spectrum at the same time as the image, we were able to quickly confirm our suspicions.”
Subsequent observations with Webb’s MIRI (Mid Infrared Instrument) at the director’s request in his spare time discovered water vapor and methane, further confirming the planet’s identity and providing a richer understanding of the planet’s atmosphere.
Unlike traditional imaging, the spectroscopic approach allowed researchers to identify the planet and begin studying its atmosphere from the very first observation.
“The spectrum contains an incredible amount of information,” Ruffio said. “You don’t just learn that something is a planet, you immediately start learning about its temperature, its chemistry and its motion.”
A separate imaging study by Ben Sutliffe of the University of Edinburgh and Marcus Bones 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.
The researchers used the built-in NIRSpec (near-infrared spectrograph) field unit of NASA’s James Webb Space Telescope to map the chemical composition of the Beta Pictoris system. As a result, they discovered a third planet, Beta Pictoris d, orbiting the young star.
Image: NASA, ESA, CSA, STScI, Lia Hustak (STScI); Science: Aidan Gibbs (UC San Diego), Jean-Baptiste Ruffio (UC San Diego), Alexis Bidot (STScI); Image Processing: Alyssa Pagan (STScI)
The newly discovered third planet orbiting Beta Pictoris, Beta Pictoris d, is visible in reconstructed NIRSpec (Near Infrared Spectrograph) images from NASA’s James Webb Space Telescope.
Image: NASA, ESA, CSA, STScI; Science: Aidan Gibbs (UC San Diego), Jean-Baptiste Ruffio (UC San Diego); Image Processing: Alyssa Pagan (STScI)
Beta Pictoris d remained hidden for years because it resides within one of the brightest known debris disks.
The dust disk acts like a fog, scattering the star’s light, making it difficult to distinguish planets from surrounding structures using traditional imaging techniques. The Webb team’s spectroscopic method effectively ignored this 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 distinct inner edge and other mysterious structures. In fact, astronomers have already predicted the existence of a planet like Beta Pictoris d to explain the unusual disk structure.
In addition to expanding our understanding of Beta Pictoris, this discovery demonstrates a powerful new way to search for exoplanets.
It is the first planet to be directly imaged, discovered primarily using moderate-resolution spectroscopy. This shows that astronomers can identify worlds in complex environments by their atmospheric fingerprints, rather than relying solely on traditional coronagraphic images.
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 the most iconic planetary systems in astronomy.
The James Webb Space Telescope is the world’s largest space science observatory. Webb unravels the mysteries of our solar system, peers into distant worlds around other stars, and explores the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA and its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
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