Astronomers have achieved a groundbreaking discovery by capturing the first image of a growing planet outside our solar system, nestled within a cleared gap of a multi-ringed disk of dust and gas. This remarkable find was made by a team led by University of Arizona astronomer Laird Close and Richelle van Capelleveen, an astronomy graduate student at Leiden Observatory in the Netherlands. Utilizing the University of Arizona’s MagAO-X extreme adaptive optics system at the Magellan Telescope in Chile, along with the Large Binocular Telescope in Arizona and the Very Large Telescope at the European Southern Observatory in Chile, their findings have been published in The Astrophysical Journal Letters.
For years, astronomers have observed planet-forming disks surrounding young stars, often noting gaps in these disks that suggested the presence of nascent planets, or protoplanets. However, only a handful of young protoplanets have been identified, all situated in cavities between a host star and the inner edge of its adjacent disk. Until now, no protoplanets were observed in the conspicuous disk gaps, which appear as dark rings.
Astronomical Breakthrough
“Dozens of theory papers have been written about these observed disk gaps being caused by protoplanets, but no one’s ever found a definitive one until today,” said Close, a professor of astronomy at the University of Arizona. He describes the discovery as a “big deal” because the absence of planet discoveries in expected locations had led some in the scientific community to propose alternative explanations for the ring-and-gap pattern in protoplanetary disks.
“It’s been a point of tension, actually, in the literature and in astronomy in general, that we have these really dark gaps, but we cannot detect the faint exoplanets in them,” he said. “Many have doubted that protoplanets can make these gaps, but now we know that in fact, they can.”
Approximately 4.5 billion years ago, our solar system began as a similar disk. As dust coalesced into clumps, absorbing surrounding gas, the first protoplanets started to form. The precise mechanics of this process remain largely unknown, prompting astronomers to study other young planetary systems, known as planet-forming disks or protoplanetary disks, to glean insights.
Technological Advancements in Astronomy
Close’s team leveraged an advanced adaptive optics system, developed by Close, Jared Males, and their students. Males, an associate astronomer at Steward Observatory, is the principal investigator of MagAO-X, which stands for “Magellan Adaptive Optics System eXtreme.” This system significantly enhances the sharpness and resolution of telescope images by counteracting atmospheric turbulence, the phenomenon responsible for the twinkling and blurring of stars.
Suspecting the presence of invisible planets within the gaps of protoplanetary disks, Close’s team examined these disks for a specific emission of visible light known as hydrogen alpha or H-alpha.
“As planets form and grow, they suck in hydrogen gas from their surroundings, and as that gas crashes down on them like a giant waterfall coming from outer space and hits the surface, it creates extremely hot plasma, which in turn, emits this particular H-alpha light signature,” Close explained. “MagAO-X is specially designed to look for hydrogen gas falling onto young protoplanets, and that’s how we can detect them.”
Using the 6.5-meter Magellan Telescope and MagAO-X, the team investigated WISPIT-2, a disk recently discovered by van Capelleveen with the VLT. In H-alpha light, a dot of light appeared within the gap between two rings of the protoplanetary disk around the star. Additionally, a second candidate planet was observed inside the “cavity” between the star and the inner edge of the disk.
Implications and Future Research
“Once we turned on the adaptive optics system, the planet jumped right out at us,” said Close, who considers this one of the most significant discoveries of his career. “After combining two hours’ worth of images, it just popped out.”
The planet, designated WISPIT 2b, is a rare example of a protoplanet actively accreting material. Its host star, WISPIT 2, is similar in mass to the sun. The inner planet candidate, CC1, has about nine Jupiter masses, whereas the outer planet, WISPIT 2b, is about five Jupiter masses. These masses were partly inferred from thermal infrared light observed by the University of Arizona’s 8.4-meter Large Binocular Telescope with the assistance of U of A astronomy graduate student Gabriel Weible.
“It’s a bit like what our own Jupiter and Saturn would have looked like when they were 5,000 times younger than they are now,” Weible said. “The planets in the WISPIT-2 system appear to be about 10 times more massive than our own gas giants and more spread out. But the overall appearance is likely not so different from what a nearby ‘alien astronomer’ could have seen in a ‘baby picture’ of our solar system taken 4.5 billion years ago.”
Close noted that the MagAO-X adaptive optics system is uniquely optimized for the H-alpha wavelength, allowing for the separation of bright starlight from faint protoplanets. “Around WISPIT 2 you likely have two planets and four rings and four gaps. It’s an amazing system,” he said.
CC1 might orbit at about 14-15 astronomical units (AU), which would place it halfway between Saturn and Uranus if it were part of our solar system. WISPIT-2b, the planet creating the gap, is farther out at about 56 AU, which would position it beyond Neptune’s orbit, near the Kuiper Belt’s outer edge.
A second paper, led by van Capelleveen and the University of Galway, details the detection of the planet in the infrared light spectrum and the discovery of the multi-ringed system using the 8-meter VLT telescope’s SPHERE adaptive optics system.
“To see planets in the fleeting time of their youth, astronomers have to find young disk systems, which are rare,” van Capelleveen said, “because that’s the one time that they really are brighter and so detectable. If the WISPIT-2 system was the age of our solar system and we used the same technology to look at it, we’d see nothing. Everything would be too cold and too dark.”
This research was supported in part by a grant from the NASA eXoplanet Research Program. MagAO-X was developed with funding from the U.S. National Science Foundation and the generous support of the Heising-Simons Foundation.
