sciencehabit shares a report from Science.org: Astronomers say they have witnessed a planet being born from a disk of gas and dust swirling around a young star. Such claims have been made before, but the team comes to an even more controversial conclusion: that this planet is forming from gas that is collapsing under its own gravity, a mechanism known as gravitational or disk instability. That stands in contrast to a more widely accepted theory of planet formation, in which dust and rocks stick together, slowly building up a planetary core with enough gravity to pull in gas from the disk. If true, the planetary system would be the strongest evidence to date for disk instability. “This system stands alone right now,” says team leader Thayne Currie of the Subaru Telescope in Hawaii. That conclusion is already dividing theorists. “This system certainly looks like it’s [undergoing] disk instability,” says Alan Boss of the Carnegie Institution for Science, a longtime advocate of the theory. But Anders Johansen, a theorist at the University of Copenhagen who helped develop the rival theory of core accretion, is not convinced. “This could be either mechanism,” he says. Although more than 5000 exoplanets have been discovered, only a few tens have been imaged directly, and none in the act of being born. Currie and colleagues were intrigued by the nearby star AB Aurigae because it was young — somewhere between 1 million and 4 million years old — and because its disk contains kinked, spiral features that could indicate protoplanets. But showing that some of the light from its disk was from a glowing-hot new planet rather than reflected starlight was no easy task. “We sat on this result for 5 years,” Currie says. “I did not believe it was a planet until fairly recently.” The team published their findings in the journal Nature Astronomy.Read more of this story at Slashdot.