How are planets born? LkCa 15 could give scientists first-ever peek.

For the first time, astronomers say they have caught a fledgling gas-giant planet in the act of growing.

The researchers are cautious – dubbing it a "likely" protoplanet. Still, if the team's conclusions hold up, the observations would help fill a crucial gap in a pictorial history planetary scientists have been assembling as they strive to unravel the mysteries of planet formation.

The planet appears to be building itself from surrounding dust and gas as it orbits its host star. Such observations provide reality checks on the computer simulations astrophysicists have developed to lay out the processes that form and shape solar systems – from the collapse of enormous interstellar clouds of dust and gas to star formation to the emergence of planets.

Indeed, the observation appears to bolster one of two contending explanations for how gas giants form, other researchers say.

The announcement, released to coincide with a conference this week on solar-system formation at NASA's Goddard Space Flight Center in Greenbelt, Md., comes during a decade-long explosion in the study of planets orbiting other stars and the formation of planetary systems.

Since the mid-1990s, astronomers have detected just under 700 planets orbiting other stars, with hundreds more awaiting confirmation from projects such as NASA's planet-hunting Kepler mission. Researchers also have detected many stars surrounded by dust and gas out of which planets can emerge.

"Catching planets as they grow as been a goal for quite a while," says Adam Kraus, an astronomer at the University of Hawaii who conducted the study, along with Michael Ireland, an astronomer at Macquarie University in Sydney, Australia.

"We know that once planets form, they tend to move around," settling into orbits different than the ones they occupied at birth, Dr. Kraus explains. "You want to capture planets as close to their formation as possible so they are as good indicators of the formation process as possible."

Now, Kraus and his colleague appear to have caught a planet in the act.

The protoplanet's host star is known as LkCa 15, one in a catalog of some 160 stars surrounded by broad disks of debris, perhaps dust and gas, according to published research.

The star lies 457 light-years from Earth in the constellation Taurus. LkCa 15 is only about 2 million years old. The sun, by contrast, is about 4.6 billion years old. LkCa 15 has about the same mass as the sun.

In 2005, scientists using NASA's Spitzer Space Telescope, an orbiting observatory that can spot dust using infrared wavelengths, conducted a survey of stars with dusty disks and noted several that appeared to have gaps in the disks – including LkCa 15.

Three years later, another group published follow-up observations of LkCa 15 and confirmed a pronounced gap in the disk – just the sort of gap that might have been cleared of material by protoplanets feeding off of it.

The observation showed an inner dust ring that extended between 0.12 and 0.15 astronomical units (the distance between earth and the sun is 1 AU) from the star, followed by a more-tenuous ring out to about 5 AU. From there out to 46 AU, Spitzer saw virtually no evidence of tiny dust grains.

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