Comets don’t all start out on the fringe

New model suggests comets also come from the inner Oort Cloud

WASHINGTON — A new study of fringe comets could let the outer solar system lose some weight.

Simulations of the most distant objects in the solar system suggest that, counter to long-standing thinking, the comets that swing past Earth can originate in the inner region of the Oort Cloud, not just from its outer fringes. The new model could resolve a discrepancy: The amount of solid mass estimated to have been in the protoplanetary disk falls short of the amount needed to build the Oort Cloud in the first place, said Nathan Kaib of the University of Washington. Kaib presented his calculations on January 5 at the winter meeting of the American Astronomical Society.

Comets were thought to come from only the outer Oort Cloud, a region that defines the edge of the solar system and is 20,000 times farther from the sun than is Earth. Being so remote from the sun, outer Oort Cloud objects can feel slight gravitational pushes from distant stars and the rotation of the Milky Way. These perturbations occasionally nudge rocky, icy bodies out of the Oort Cloud and into long orbits around the sun, where they appear to Earth as comets.

The Oort Cloud is too distant to be observed directly by telescopes on Earth, so these comet visitors are the only means astronomers have to estimate the mass of the outer Oort Cloud. Current estimates place that number at about four times the mass of the Earth.

But objects in the Oort Cloud weren’t born there — they’re thought to be leftover pieces of planets that Jupiter and Saturn flung away from the inner solar system. Existing models of planet formation show that only 1 to 2 percent of these early protoplanets ended up trapped in the outer Oort Cloud. If four times the mass of the Earth is trapped in the outer Oort Cloud now, the solid mass of the protoplanetary disk would have been 300 times that of the Earth — or six times the solid mass that is calculated for the protoplanetary disk. In other words, there’s not enough stuff in the solar system to have populated its outer fringes, Kaib said.

“It’s almost impossible to line up with … the planets you have today,” he said. “It suggests there’s either a problem with how the Oort Cloud formed, or the way we’re calculating its mass.”

Unlike objects from the outer fringe, inner Oort Cloud objects — which are between about 2,000 and 20,000 times the distance from Earth to the sun — were never supposed to make it into cometlike orbits. If those objects ever strayed from the Oort Cloud, it was thought, Jupiter’s and Saturn’s gravitational oomph would have ejected the objects from the solar system.

Kaib’s simulations of how the Oort Cloud may have evolved over the last 1.2 billion years turns this notion around. The new models show that the orbits of some objects from the inner Oort Cloud are inflated by interacting with Jupiter and Saturn. Rather than fleeing the solar system, they end up looking like long-period comets that originated in the outer Oort Cloud.

“What this means is that the inner Oort Cloud is actually helping produce the comets we see today, which hasn’t been accounted for,” Kaib said. At least half and perhaps most of all comets could come from this closer population, he said. It also means that the 1 to 2 percent of the early solar system’s solid mass thought to be bound in the outer Oort Cloud is actually less than four times the mass of Earth.

Up to 15 percent of protoplanets could have been trapped in the inner Oort Cloud, bringing the calculated solid mass of the solar system to about 40 times the mass of the Earth, in better agreement with the observed mass.  

“Nate really solved the problem,” said Matija Äuk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “It was disturbing that planet formation theory could not form the Oort Cloud … Now it’s all starting to come together.”

Lisa Grossman

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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