Hubble finds hints of a planet oddly far-flung from its star

A mysterious gap in a star’s dusty shell of debris could be the signature of a young planet circling its sun at twice the distance of Pluto’s orbit. If it does exist, the far-flung planet’s birth may be hard for astronomers to explain.

“If this is a planet, it is extremely challenging for existing planet formation theories,” says Katherine Kretke, an astronomer at the Southwest Research Institute in Boulder, Colo.

Most planets are thought to begin their lives as small clumps of hot, rapidly moving dust and gas within vast disks of debris that orbit newborn stars. As a planet grows it behaves like a snow plow, scooping up some material to bulk up while flinging other material away, until it has cleared a smooth orbital path.
John Debes, an astronomer at the Space Telescope Science Institute in Baltimore, used the Hubble Space Telescope to study a disk around TW Hydrae, a 10-million-year-old star located about 176 light-years from Earth.

Hubble images revealed an unmistakable gap 12 billion kilometers from the star, 80 times farther than Earth is from the sun. “It’s very striking,” says Phil Armitage, an astrophysicist at the University of Colorado Boulder. “It looks like what you’d expect from a forming planet.”

If the planet’s existence is confirmed, astronomers have their work cut out for them explaining how it got there. Compared with particles in tighter orbits, ones near that gap are less densely packed and move much more slowly, Kretke says. As a result, it would be difficult for a potential planet to accrue enough material to clear its own orbit.

An alternative theory of planet formation posits that clumps of gas within a disk can rapidly collapse together in a process similar to the one that forms stars. That could account for the outer bulky planets recently discovered around the star HR 8799 (SN Online: 12/3/10). But Kretke says that process is capable only of building worlds more massive than Jupiter, while this potential planet would be the size of Neptune or a large Earth.

“No matter how you look at it, if there’s a planet there it’s going to change theories of how planets form,” Debes says. His team’s results appear June 14 in the Astrophysical Journal.

The next step is to find the planet, Debes says, which will be no easy task. Just identifying the gap in TW Hydrae’s disk was akin to seeing a groove in an LP record from six kilometers away; now astronomers hope to find a speck hidden within that groove.

Debes notes that the Hubble photos were taken by a nearly 20-year-old instrument; he is confident that next-generation telescopes will see the planet if it exists.