Halley's Comet and others may have formed around other stars

Famous comets such as Halley, Hale-Bopp and McNaught may have formed around other stars and been snatched into orbit around the sun back when it was packed closely with hundreds of other stars, according to new simulations.

Researchers modeled this process as an alternative to the standard picture of the formation of the Oort cloud, the roughly spherical body of comets that extends halfway to the next nearest star and is believed to be home to so-called long period comets.

Until now, the leading model for Oort cloud formation held that Jupiter's gravity ejected icy balls of protoplanetary material (planetesimals) from inside the solar system.

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"The current thinking is that when we see a comet it represents the stuff from which the planets formed," said study researcher Hal Levison of the Southwest Research Institute in Boulder, Colo. "According to the standard model, we should have an Oort cloud that's roughly a few percent as populous as the one we see." [Great Comet McNaught photos.]

Inefficient comet-mover

The problem with the standard model is its efficiency, Levison said.

After Jupiter ejected the planetesimals, the gravity of surrounding stars had to nudge them just right to put them into orbit around the sun. Only three to four percent of planetesimals would make the grade, or enough to explain some 10 percent of the Oort cloud's estimated 400 billion inhabitants.

After failing to come up with a way of making the standard process more efficient, Levison and his colleagues finally decided to consider an alternative model proposed 20 years ago but quickly abandoned as unnecessary and unworkable.

In this model, the young sun was part of a birth cluster of hundreds or more stars, each one with its own collection of planetesimals constantly being tugged by the gravity of the surrounding stars.

"You end up with this huge population of comets just floating among the stars in the star cluster," Levison said.

As the stars finally begin to spread apart, each one pulls comets with it like a light bulb drawing gnats.

Levison said his "duh" moment came when he realized that the radius of the Oort cloud is roughly the same size as a star-forming region in a birth cluster.

The number of comets generated by the model depends on a few factors that aren't well known, including the mass of a typical protoplanetary disk, the masses and orbits of gas giants in a typical planetary system and the average mass of a planetesimal around the gas giants.

Using our solar system as a guide, Levision and his colleagues find that the birth cluster model produces more than enough comets to fill the Oort cloud, suggesting that the 90 percent of the Oort cloud that goes unexplained in the standard model could have an extrasolar origin.

Comet origins debate

Given the uncertainties in the model, some researchers remain skeptical.

"I am not entirely convinced that this theory has obvious advantages over the more conventional one," said astronomer Julio Fernández of the Instituto de Física in Monteviedo, Uruguay.

He said it's also premature to claim there's an inconsistency between the observed number of Oort cloud objects and the number expected from standard theory.

"Right now we're scrambling to accommodate this new idea and see what new tests we can devise to help test the possibilities presented here," said planetary scientist Michael Mumma of Goddard Space Flight Center.

Mumma said that comets formed around other stars could have different chemical signatures than those formed around the Sun, as a consequence of x-rays from brighter stars penetrating deep into protoplanetary dust.

"Comets that formed within [reach] of the sun are likely to be very different from those that formed further out," he said.

• Gallery: The Great Comet McNaught: Part 1, Part 2
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• New Comet Visible in Early Morning Sky

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