Astronomers Noticed a Mysterious Spot Appearing in Telescope Images. It Turned Out to Be the Faintest Exoplanet Ever Imaged From Earth
The gas giant, called Beta Pictoris d, had been hiding in plain sight for more than a decade, until two independent teams verified its existence
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Bright spots in astronomical images don’t always lead to new discoveries. Like the boy who cried wolf, these spots might send astronomers on a wild goose chase, only for them to realize that it was just a trick of the telescope or a run-of-the-mill object, like a background star. “We’ve learned not to trust bright blobs in images,” Jean-Baptiste Ruffio, a research scientist at University of California, San Diego, says in a statement.
But last year, Ruffio and others saw an unexpectedly bright spot in images of the star Beta Pictoris taken by the James Webb Space Telescope. In this case, the blob wasn’t alone: The team also captured a spectrum, which reveals an object’s chemical composition. Instead of the smooth pattern denoting dust that they anticipated, this data contained a surprising series of peaks and troughs. It was a telltale sign of carbon monoxide, a common molecule in the atmospheres of giant planets.
The image looked like a new exoplanet, and the spectrum offered the key data to back it up. “We were able to quickly confirm our suspicions,” Ruffio adds.
Just five days later, a separate team also found a strange spot in an image of Beta Pictoris, this time from the European Southern Observatory’s Very Large Telescope (VLT) in Chile. They, too, knew how fickle these signs could be. “Normally, when you see things like that, you work on the data some more, and these little scrappy signals go away,” Ben Sutlieff, an astronomer at the University of Edinburgh who worked on the VLT team, tells Scientific American’s Adam Kovac. “They’re not real; they’re noise and they vanish.”
Both these teams, however, verified the mysterious signal: It was a gas giant, the faintest exoplanet ever imaged from Earth. The newly discovered world, named Beta Pictoris d, is around 100 times dimmer than its neighbor Beta Pictoris b, which was the first planet to be discovered around its host star. Today, both groups published their findings about the strange object in The Astrophysical Journal Letters.
“We’ve now built a picture of this planet,” Sutlieff tells Marcia Dunn of the Associated Press, “and we are very excited to see what more can be learned about it.”
Did you know? The rarity of direct image discoveries
NASA has confirmed the existence of more than 6,000 exoplanets, but fewer than 100 of them have been identified through direct imaging. Instead, most planets are found by creating a slight drop in light when they pass in front of their star.
The star Beta Pictoris is located 63 light-years from Earth, visible in the southern constellation Pictor, the painter’s easel. Prior to this discovery, astronomers knew about two other planets in its orbit and a disk of debris that surrounded the star. That disk, however, showed some puzzling traits, like a strongly defined inner edge—and to explain these anomalies, astronomers suspected that another planet orbited Beta Pictoris, too. They just hadn’t found it yet.
Beta Pictoris d fits smoothly into that picture. The newly discovered world orbits its star once every 91 years, at a distance roughly equivalent to Neptune’s from our sun. The exoplanet is about 2.4 times the mass of Jupiter. The other planets, which orbit more closely around its star, are bigger—about ten times Jupiter’s mass.
The disk of debris acts like fog and scatters light from the central star, so the exoplanet remained hidden in its glare for years. After spotting the new gas giant, Sutlieff’s research team looked in archival images of the planetary system to try to identify the world and confirm its orbit. They ended up spotting it in images from as far back as 11 years ago, where it had previously evaded detection. “Planet d, it seems, has been playing a game of hide-and-seek with us for over a decade,” Jayne Birkby, a co-author of the VLT study and an astronomer at the University of Oxford in England, says in another statement.
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The Webb team gathered data on the planet’s atmosphere, identifying water vapor and methane alongside the carbon monoxide. They calculated the object’s position, speed and alignment with the disk of debris, which further demonstrated that it was orbiting the star rather than simply appearing in the background.
Beta Pictoris, a relatively young star of about 23 million years old, offers “probably our best look at a planetary system just after it has formed,” Aidan Gibbs, an astronomer at the University of California, San Diego, who led the Webb team, tells the AP.
The exoplanet’s discovery comes at a time in astronomy that makes it “really exciting,” says John Monnier, an astronomer at the University of Michigan who was not involved with either study, to Scientific American. The Extremely Large Telescope (ELT), which will be the world’s largest optical telescope, is expected to begin making observations in 2029. It will be able to study, and likely even directly image, rocky exoplanets. The ELT is poised to find “just a huge number more of these objects,” adds Monnier. Beta Pictoris d, he says, “is just a little bit of an appetizer.”