Astronomers have discovered a giant planet outside the solar system, known as an exoplanet, hidden within one of the most intensely studied planetary systems in the Milky Way galaxy.
Using NASA’s James Webb Space Telescope, astronomers discovered Beta Pictoris d orbiting the nearby star Beta Pictoris, a planetary system that scientists have been studying for decades. The system was already known to host two giant planets: Beta Pictoris b, one of the first exoplanets ever to be directly imaged, and Beta Pictoris c.
The discovery of Beta Pictoris d marks a significant milestone because, unlike Beta Pictoris b and c, it was not identified as a bright point of light. Instead, astronomers detected the unique chemical fingerprint of its atmosphere, a technique that could transform the search for planets orbiting other stars, according to NASA.
“This discovery adds another piece to an already fascinating planetary system,” said Aidan Gibbs, lead author of the study published on 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 helping us tell that story.”
Beta Pictoris, located 63 light-years from Earth and about 23 million years old, is a nearby system in the Milky Way that offers a rare glimpse of the interactions between newborn planets and the disk of dust and debris left behind from their formation.
Accidental discovery
Beta Pictoris d was not a planned discovery, but an accidental one. Astronomers were using the Webb telescope’s NIRSpec (Near-Infrared Spectrograph) to study the atmosphere of Beta Pictoris b, when they accidentally discovered the new exoplanet.
Specifically, they used NIRSpec’s Integral Field Unit, which obtains both an image and a spectrum from each pixel in an image, NASA said.
“We weren’t looking for a new planet,” said Gibbs. “We were trying to understand one we already knew existed. Then, this telltale signal appeared in the data where we didn’t expect it.”
How was it discovered?
Webb’s NIRSpec Integral Field Unit (IFU), which astronomers were using to study Beta Pictoris b, not only captures an image but also records a spectrum for every pixel in that image. In simple terms, it splits light into its different wavelengths, allowing scientists to determine properties such as how hot an object is and what elements and compounds it is made of without physically sampling it. This technique is known as spectroscopy.
While trying to study Beta Picroris b, astronomers expected to see a smooth spectrum from light reflecting off dust because the Beta Pictoris system contains a vast cloud of dust. Instead, they observed a series of peaks and troughs in the spectroscopic data.
What these peaks and troughs were a distinctive pattern of carbon monoxide absorption lines, spread out like a barcode. – It means that the dips occurred at exactly the wavelengths where carbon monoxide (CO) absorbs light. It is an expected feature in giant planet atmospheres.
However, the signal alone was not enough to confirm the presence of a planet, as it could also have come from a background star or a brown dwarf containing carbon monoxide in its atmosphere. To be sure of what it was, astronomers measured its radial velocity using the same spectroscopic data – which not only reveals chemical composition, but the motion of an object.
Using this information, the team calculated the object’s speed and determined that its position, motion, and alignment with the system’s debris disk were all consistent with a planet orbiting Beta Pictoris, rather than a background star or brown dwarf.
“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 University of California, San Diego and principal investigator of the first Webb observations where 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.”
A follow up observation 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 atmosphere of the planet.
Unlike traditional imaging, the spectroscopic approach allowed researchers to identify the planet and begin studying its atmosphere from the very 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.”
The discovery was further confirmed by a separate imaging study led by Ben Sutlieff of the University of Edinburgh and Markus Bonse of the European Southern Observatory, using data from the European Southern Observatory’s Very Large Telescope and Webb’s NIRCam (Near-Infrared Camera).
About Beta Pictoris d
Beta Pictoris d is likely at least two times the mass of Jupiter, making it the smallest of the three known giant planets in the system. Modeling suggests it likely circles around its star at about 30 astronomical units, comparable to the region occupied by Neptune in our own solar system. It’s the widest orbit of the known three planets, but still located inside the inner edge of the debris disk.
It remained hidden for years because it lies within one of the brightest debris disks known, according to NASA. The dusty disk acts like fog, scattering light from the star, making it difficult for conventional imaging methods to spot planets. However, Webb’s spectroscopic technique was able to filter out the dust and detect the unique chemical signatures of a planetary atmosphere.
Scientists believe the newly discovered planet could help explain the debris disk’s sharply defined inner edge and other unusual features. In fact, astronomers had long predicted the existence of a planet like Beta Pictoris d to account for the disk’s peculiar structure.
New way to find exoplanets
The discovery marks the first time an exoplanet has been identified primarily through moderate-resolution spectroscopy, demonstrating that astronomers can find planets by detecting the unique chemical signatures of their atmospheres instead of relying only on conventional imaging techniques.
Researchers now plan to study Webb’s observations further to learn more about Beta Pictoris d, including its temperature, atmospheric composition, and orbit. They say the discovery opens up a powerful new way to search for exoplanets, particularly those hidden in complex environments.
