Star's oversized debris ring challenges theories of planet formation

By Nadia Drake

Web edition: June 15, 2012
Print edition: July 14, 2012; Vol.182 #1 (p. 11)

ANCHORAGE, Alaska — About 80 light-years away, an enormous, dusty ring swirls around a sunlike star, with a defined inner edge that is probably sculpted by a planet orbiting at 140 times Earth’s distance from the sun.

A planet located so far from a sunlike star presents an astronomical conundrum.

“How do you get a planet out that far? We don’t know how to form something out there,” astronomer Karl Stapelfeldt of NASA’s Goddard Space Flight Center in Greenbelt, Md., said on June 14 at the 220th Meeting of the American Astronomical Society.

Stapelfeldt and a team led by John Krist of the Jet Propulsion Laboratory in Pasadena, Calif., used the Hubble Space Telescope to study 10 stars suspected of hosting large debris disks. Hubble  saw a ring around only one, HD 202628, which resides in the southern constellation Microscopium.

Everything about the dusty circle — the biggest ever observed around a solar cousin — is huge. In places, the ring itself is 70 times wider than the Earth-sun distance, called an astronomical unit. From end to end, the debris disk is roughly 400 astronomical units long — larger than the well-known ring surrounding the star Fomalhaut. Like that ring, the newly observed one is also groomed by a celestial gardener — the planet lives about 140 astronomical units from HD 202628.

This planet hasn’t yet been observed, but Stapelfeldt speculates that it might be several times more massive than Jupiter. “You can’t get a structure like this, and have it last for a long period of time, unless something keeps all the particles’ orbits lined up,” he said.

Such distantly orbiting planets challenge theories describing planet formation, especially around sunlike stars. Debris disks around these stars don’t contain enough material to grow a planet so far away. “The formation of wide-orbit planets is particularly not well-understood,” says astrophysicist Aaron Boley of the University of Florida, who studies the Fomalhaut system and its far-flung ringscaping planets. “To build a complete understanding of planet formation, we must understand the architecture of planetary systems, which includes planets that are at very large stellar separations.”

Both Boley and Krist’s team suggest that the planet living around HD 202628 may have formed closer to the star, then migrated outward. But, Stapelfeldt says, that process would have had to be unusually gentle. “If you did something violent and you threw the planet out there, its gravity would tear up the ring,” he said. “You wouldn’t expect to have something like this left over.”