Planetary System in the Making? Stellar eclipse hints at planet-forming debris

Astronomers this week reported the first evidence that a young star is periodically eclipsed by a stream of debris that could be an orbiting cluster of asteroids. The debris may be held in place by a massive, unseen planet, the researchers say. If this interpretation proves correct, scientists may have caught a planetary system during its creation.

About 5 years ago, William Herbst of Wesleyan University in Middletown, Conn., began supervising undergraduates studying the star KH 15D. After analyzing the students’ accumulating data, Herbst and one of his collaborators realized that the star was winking on and off like clockwork.

Many stars appear to wax and wane on a strict timetable, typically when they have a stellar partner that periodically passes between them and Earth. But the eclipse of one star by another doesn’t last more than about half a day. In contrast, KH 15D, which starts to fade every 48.3 days, remains faint for about 18 days.

Only a collection of dust grains, rocks, or perhaps asteroids orbiting in a strung-out arc can account for the lengthy eclipses, assert Herbst, Catrina Hamilton of Connecticut College in New London and their collaborators. Follow-up observations by several teams have revealed that two such debris groupings may be orbiting the star.

Herbst presented the findings in Washington, D.C., on June 19 at a meeting on extrasolar planets.

The extended arcs of dust or rock eclipsing KH 15D could be part of the star’s protoplanetary disk, a doughnut-shaped distribution of gas, dust, and ice that surrounds many young stars and contains the raw material to form planets, comets, and asteroids (SN: 5/4/02, p. 280: Dusty Disks May Reveal Hidden Worlds). For the debris to eclipse the star, the disk must be oriented edge-on relative to Earth, Herbst says. Because the star lies 2,400 light-years from Earth, neither the disk nor the clumps within it can be imaged.

The presence of even a single grouping of debris in a protoplanetary disk hints that a more massive object, possibly a planet, might also reside there, Herbst notes. Theory suggests that a cluster within a disk can’t survive for long unless a heavier body gravitationally shepherds the material, keeping the dust grains or rocks organized into a clump. A good analogy, notes Herbst, is the small moons of Saturn keeping the planet’s rings of icy particles intact.

The shepherding body could be as light as a planet or as heavy as a low-mass star, says Herbst. Finding such an object in the disk would entail detecting subtle to-and-fro motions that the object induces in KH 15D. If a fledgling planet were detected around this young star, it would indicate the birth of a planetary system whose evolution astronomers could track over periods as short as months.

However, Dana E. Backman of Franklin and Marshall College in Lancaster, Pa., cautions that the data don’t require the presence of a planet. The debris generating the eclipses of KH 15D may weigh no more than Earth. If so, an object as massive as a planet wouldn’t be needed to shepherd arcs of material, he says.

Recent observations of the star hint not only that there are two clumps of debris but also that each eclipse period is gradually lengthening. Backman says the data suggest that Herbst and his students were lucky enough to discover a star “literally a few years” after an orbiting body at least as large as an asteroid had shattered and begun to spread its fragments around the star.

Says Herbst, who notes his team is still analyzing its trove of data, “At this point, anything is possible.”

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