The VLA detected a concentration of rocky particles about the size of pebbles in the disk of material around HL Tau, which suggested a planet might be forming.
Evidence for the protoplanet was supported by longer-wavelength telescope readings at Jodrell Bank and by computer models.
The study team says the discovery bolsters the controversial theory that gas giants can form due to a rapid collapse of gas around a dense region in the dusty disk surrounding a star.
Known as gravitational instability, this process may have been kick-started in the case of HL Tau b by a close encounter with another young star about 1,600 years ago, the team reports.
(Related news: "Evidence of Huge Planetary Collision Found" [January 10, 2008].)
The new finding represents "a nice observation using complementary observational facilities," said Tom Hartquist, head of the Astrophysics Group at the University of Leeds in the U.K., who was not involved in the work.
"There's been a lot of debate recently about the role of gravitational instability in planet formation, so from that point of view [the new study] might be controversial," Hartquist added.
But given that the team has a theoretical model that explains their data, he said, "I think it's an important contribution to showing that gravitational instability does play some role [in planet formation]."
More should become clear as increasingly powerful telescopes provide better images of the protoplanet, Hartquist added.
"For instance, if gravitational instability is important, there should be a spiral structure that develops in the [star's] disk," he said. Previous models have suggested that instability in the disk would cause its gases to form into spiral arms.
What is certain, the astrophysicist said, is that the planet will never support life.
"It's just too far out [from its sun], too cold, and too massive," he said.
"It's almost massive enough to become a brown dwarf star," a type of star that ignites but is not massive enough to maintain hydrogen fusion after a few tens of millions of years.
Hartquist also noted that the planetary discovery underlines the international importance of astronomical facilities in the U.K. at a time when Jodrell Bank is threatened with closure due to funding cuts.
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