A team of astronomers has detected for the first time a growing planet outside our solar system, embedded in a cleared gap of a multi-ringed disk of dust and gas.

The team, led by University of Arizona astronomer Laird Close and Richelle van Capelleveen, an astronomy graduate student at Leiden Observatory in the Netherlands, discovered the unique exoplanet using the University of Arizona's MagAO-X extreme adaptive optics system at the Magellan Telescope in Chile, the U of A's Large Binocular Telescope in Arizona and the Very Large Telescope at the European Southern Observatory in Chile. Their results are published in The Astrophysical Journal Letters.

A growing exoplanet

For years, astronomers have observed several dozen planet-forming disks of gas and dust surrounding young stars. Many of these disks display gaps in their rings, hinting at the possibility that they are being "plowed" by nearby nascent planets, or protoplanets, like lanes being cleared by a snowplow. Yet, only about three actual young growing protoplanets have been discovered to date, all in the cavities between a host star and the inner edge of its adjacent protoplanetary disk. Until this discovery, no protoplanets had been seen in the conspicuous disk gaps - which appear as dark rings.

"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, professor of astronomy at the University of Arizona. He calls the discovery a "big deal," because the absence of planet discoveries in places where they should be has prompted many in the scientific community to invoke alternative explanations for the ring-and-gap pattern found in many 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."

4.5 billion years ago, our solar system began as just such a disk. As dust coalesced into clumps, sucking up gas around them, the first protoplanets began to form. How exactly this process unfolded, however, is still largely a mystery. To find answers, astronomers have looked to other planetary systems that are still in their infancy, known as planet-forming disks, or protoplanetary disks.

Close's team took advantage of an adaptive optics system, one of the most formidable of its kind in the world, developed and built by Close, Jared Males and their students. Males is an associate astronomer at Steward Observatory and the principal investigator of MagAO-X. MagAO-X, which stands for "Magellan Adaptive Optics System eXtreme," dramatically improves the sharpness and resolution of telescope images by compensating for atmospheric turbulence, the phenomenon that causes stars to flicker and blur, and is dreaded by astronomers.

Suspecting there should be invisible planets hiding in the gaps of protoplanetary disks, Close's team surveyed all the disks with gaps and probed them 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."

The team used the 6.5-meter Magellan Telescope and MagAO-X to probe WISPIT-2, a disk van Capelleveen recently discovered with the VLT. Viewed in H-alpha light, Close's group struck gold. A dot of light appeared inside the gap between two rings of the protoplanetary disk around the star. In addition, the team observed a second candidate planet inside the "cavity" between ...