JWST Detects Hidden Giant Planet in Beta Pictoris System Through Spectroscopy, Not Direct Imaging
Webb found Beta Pictoris d by reading its atmosphere's chemical fingerprint, a first for exoplanet discovery, while a separate imaging study independently confirmed it.
Overview
Astronomers using NASA’s James Webb Space Telescope have found a giant planet hiding within one of the most intensely studied planetary systems in the galaxy, according to NASA. The new world, Beta Pictoris d, was not spotted as a point of light the way its two known siblings were. Instead, researchers detected the chemical fingerprint of its atmosphere buried inside spectroscopic data, according to NASA.
What We Know
Beta Pictoris was already known to host two giant planets, Beta Pictoris b and Beta Pictoris c, and the addition of Beta Pictoris d makes it only the second planetary system known to contain at least three imaged planets, according to NASA. The system sits 63 light-years from Earth and is about 23 million years old, according to NASA.
The discovery was serendipitous. A team led by Aidan Gibbs, a postdoctoral researcher at the University of California, San Diego, was using Webb’s NIRSpec Integral Field Unit to study the atmosphere of the previously known planet Beta Pictoris b when an unexpected signal appeared, according to NASA. “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,” according to NASA.
The signal turned out to be a distinctive pattern of carbon monoxide absorption lines, described by the team as “spread out like a barcode,” an expected feature of giant planet atmospheres, according to NASA. Because spectroscopy captures motion as well as chemistry, the team also extracted the object’s radial velocity, finding its speed, position, and alignment with the system’s debris disk were consistent with a planet orbiting Beta Pictoris rather than a background star or brown dwarf, according to NASA.
Jean-Baptiste Ruffio, a research scientist at the University of California, San Diego and principal investigator of the Webb observations where the discovery was made, said the team initially treated the signal with caution. “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,” Ruffio said. “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,” according to NASA. Follow-up observations using Webb’s MIRI instrument, obtained through a Director’s Discretionary Time request, detected water vapor and methane, further confirming the planet’s identity, according to NASA.
In the underlying study, published in The Astrophysical Journal Letters, Gibbs and colleagues report that Beta Pictoris d “was serendipitously detected in JWST/NIRSpec IFU observations,” with a second epoch of NIRSpec and MIRI/MRS data confirming the find, according to the paper. The extracted spectrum showed clear methane, carbon monoxide, and water absorption features, and the planet’s radial velocity was consistent with its orbital position, according to the paper. Combining radial velocity, astrometry, and orbital stability simulations, the team estimated a semi-major axis greater than 30 astronomical units and a mass of 2 to 4 Jupiter masses, according to the paper. The authors describe Beta Pictoris d as “the first planet discovered using spectral template matching with moderate-resolution spectroscopy,” according to the paper.
NASA’s summary of the mass and orbit estimates put Beta Pictoris d at “at least two times the mass of Jupiter,” making it the smallest of the system’s three known giant planets, with modeling suggesting an orbit of about 30 astronomical units — comparable to Neptune’s distance from the sun in our own solar system, according to NASA. That makes it the widest orbit among the three known planets, though still inside the inner edge of the system’s debris disk, according to NASA.
A separate, independent study led by Ben Sutlieff of the University of Edinburgh and Markus Bonse of the European Southern Observatory confirmed the planet’s existence through direct imaging, using data from the European Southern Observatory’s Very Large Telescope and Webb’s NIRCam instrument, according to NASA. That study reports detecting the planet in “non-coronagraphic observations obtained with VLT/ERIS as well as multi-epoch archival datasets from JWST/NIRCam and VLT/SPHERE,” with astrometric measurements spanning an 11-year baseline demonstrating orbital motion consistent with a gravitationally bound object, according to the imaging paper. Joint orbit fits across all three known planets in the system yielded a semi-major axis of 26.0 astronomical units and an inclination of 89.0 degrees for planet d, with an estimated effective temperature of 600 Kelvin and a mass of 2.4 plus or minus 0.6 Jupiter masses, according to the imaging paper. That paper also reports a distinct atmospheric signature: a red F410M-F444W color that the authors say indicates strong carbon dioxide absorption, suggesting an atmosphere enhanced in metals compared with free-floating planetary-mass objects, according to the imaging paper.
Beta Pictoris d had gone undetected for years because it sits inside one of the brightest debris disks known, according to NASA. The disk’s dust scatters starlight like fog, making it hard for conventional imaging to separate a planet from surrounding structure, while Webb’s spectroscopic method isolated only the narrow molecular signatures unique to a planetary atmosphere, according to NASA. Scientists say the planet’s presence may help explain the sharply defined inner edge of the debris disk, noting that astronomers had already predicted a planet like Beta Pictoris d to account for the disk’s unusual structure, according to NASA.
“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,” according to NASA.
What We Don’t Know
NASA says researchers plan to continue analyzing Webb’s observations to further refine the planet’s temperature, atmospheric composition, and orbit, according to NASA. The two independent teams arrived at somewhat different mass and semi-major-axis estimates — 2 to 4 Jupiter masses and a semi-major axis greater than 30 astronomical units from the spectroscopy team, versus 2.4 plus or minus 0.6 Jupiter masses and 26.0 astronomical units from the imaging team — reflecting the different modeling approaches each group used, according to the spectroscopy paper and the imaging paper. Neither study yet offers a combined, reconciled figure.
Analysis
The discovery method may prove more significant than the planet itself. NASA describes Beta Pictoris d as “the first directly imaged planet discovered primarily through moderate-resolution spectroscopy,” a result that demonstrates astronomers can identify worlds in complex, dusty environments through atmospheric fingerprints rather than relying solely on traditional coronagraphic imaging, according to NASA. The Sutlieff and Bonse-led imaging team frames its own result in similar terms, writing that their ground-based detection “highlights the deep sensitivity achievable with ground-based imaging in the mid-infrared and the discovery potential of future high-contrast observations with the Extremely Large Telescope,” according to the imaging paper. Two independent teams, using two different techniques, converging on the same previously hidden planet in the same well-studied system within weeks of each other suggests both spectroscopic template-matching and next-generation ground-based imaging could become standard tools for finding planets obscured by bright debris disks around other stars.