A backyard astronomer in Australia made a major discovery early Monday morning when he noticed a newly formed spot on Jupiter—a spot that academics and NASA astronomers have now confirmed marks a recent impact on the giant planet.

Anthony Wesley of Murrumbateman had a new 14.5-inch Newtonian telescope at his home observatory trained on Jupiter when he noticed something unusual: a dark spot on the planet's outer layers that had not been there two days before. Because its location, size and rotation speed did not jibe with any of Jupiter's moons or their shadows, nor with any of its known atmospheric features, a feasible explanation eluded him. Observed massive impacts from comets or asteroids are extremely rare—when Comet Shoemaker-Levy 9 slammed into Jupiter in 1994, it was an unprecedented event watched intently from countless observatories.

"Could it actually be an impact mark on Jupiter?" Wesley wrote in his observation report. "I had no real idea, and the odds on that happening were so small as to be laughable, but I was really struggling to see any other possibility given the location of the mark."

He continued to photograph the planet, then returned to his house to e-mail others about what he finally concluded had to be a scar from a recent impact. Leigh Fletcher, a postdoctoral astronomer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., happened to be part of a team with observing time on the NASA Infrared Telescope atop Mauna Kea in Hawaii when he and his colleagues got word of Wesley's unusual sighting. What is more, Fletcher's group was already going to train the three-meter telescope on Jupiter to observe its storms, working remotely from Pasadena.

"You can imagine the scene: We're all extremely excited, crammed around the computer screen to see those first images from the telescope facility," Fletcher says. "And there it was: an extremely bright feature on the southern hemisphere of Jupiter." It looked just like a medium-size impact from Shoemaker-Levy 9, Fletcher says, confirming Wesley's assessment.

Paul Kalas, an astronomer at the University of California, Berkeley, also had observing time booked on Mauna Kea, at the 10-meter Keck 2 telescope, when he and his colleagues read about Wesley's discovery on the blog of Kalas's Berkeley colleague Franck Marchis. Kalas and his team were using Keck to look for the exoplanet Fomalhaut b, some 25 light-years away. (Fomalhaut b was one of the first extrasolar planets whose orbit was confirmed with photographic evidence in November.)

Kalas and his colleagues took about 90 minutes away from their Fomalhaut observations to check on Wesley's purported find. What they saw was an unmistakable spot glowing brightly in the infrared where something had punched through Jupiter's outer layers. "We agreed with the amateur astronomer that this was an impact event," Kalas says.

"You have layers in Jupiter's atmosphere—you have a cloud deck, and above that is an atmosphere which absorbs infrared radiation, so in the infrared Jupiter kind of looks dark," Kalas explains. "So if you have something punching through the atmosphere, hitting those clouds, and pulling up material in a giant plume...then all of a sudden Jupiter looks bright at that spot in the infrared." Fletcher says the scar should heal over the coming days and weeks, providing astronomers a look at Jovian atmospheric meteorology working in real time.

His team and others have much work to do to analyze the data collected from the various telescopes to reverse-engineer just what kind of impact it was. "The event is so recent, I don't think people have sharpened their pencils yet to get going," Kalas says.

Whatever the culprit, the arrival of such a rare event, exactly 15 years after Shoemaker-Levy 9 was in the midst of bombarding Jupiter, is an unexpected boon to planetary scientists. "These are the only two occurrences of an impact being viewed on Jupiter," Fletcher says. "To be able to view another one in our lifetime is extremely exciting."