Wanted for Theft: Sun Stole Its Comets From Other Stars
John Gleason captured this amazing shot of the comet Hale-Bopp in March 1997.
Credit: John Gleason

It turns out our sun may be a cosmic thief that's stolen most of its comets from other stars, a new study suggests.

Comets are small icy bodies that flare up when they near the sun as solar radiation vaporizes their ice to create a glowing tail.

New computer simulations of the billions of comets crisscrossing the solar system suggest that most of them originated beyond our local neighborhood, but got grabbed and pulled in by our sun's gravity later.

Such a scenario goes against the long-standing model for comet evolution, which holds that most of our local comets come from the same region where the sun and its planets formed. This region, known as the Oort cloud, encircles the solar system and extends far beyond Pluto.

According to researcher Hal Levison, an astronomer at the Southwest Research Institute in Boulder, Colo., however, "the standard model can't produce anywhere near the number of comets we see."

"This model says the comets are dregs of our own solar system's planetary formation and that our planets gravitationally booted them to huge distances, populating the cloud," Levison explained. Such a process would likely have occurred around other stars as well, with each giving rise to their own cloud of comet debris.

But stars may not have held on to their initial clouds.

Like many other stars, the sun was birthed in an open star cluster that disintegrated over time. These clusters, typically containing between ten and a thousand stars jammed into a tiny space, have an average radius not much different from the present day Oort cloud. The close proximity of stars within these clusters could have allowed stars to "steal" fledgling comets from one another.

And a star wouldn't have had to have been the biggest in order to be the most successful thief. If a comet moved far enough away from its parent star and close enough to the sun, for example, the sun's gravity could trap it even if the parent star was significantly more massive.

When it doesn't add up

The distance of the Oort cloud from Earth makes it difficult to observe ? much less pin down the exact number of comets it contains. The amount of comets that exist there must be inferred from observations of those comets that light up as they pass near the sun.

But based on this data, Levison and his team say there seem to be around 400 billion comets hovering just beyond Pluto. In comparison, the conventional model predicts only 6 billion.

"That's?a huge discrepancy," Levison said. "Too huge to be explained by mistakes in the estimates. There's no way we could be that far off, so there has to be something wrong with the model itself."

The orbits of many long-period comets seem to support that finding. Their highly oblong orbits take them far out into the depths of space.

"So they couldn't have been born in orbit around the sun," Levison said. "They had to have formed close to other stars and then been hijacked here."

Comets are generally regarded as excellent snapshots of the early solar system, because they spend much of their lives encased in ice. But if some of these comets come from outside our solar system, then they can tell us about their parent stars, as well.

"We can study the orbits of comets and put their chemistry into the context of where and around which star they formed," Levison said. "It's intriguing to think we got some of our 'stuff' from distant stars. We're kin."