The solar system may be significantly more compact than previously thought, according to a new computer simulation of the cloud of comets that enshrouds the solar system. The work suggests the cloud may not contain as much material as once suspected, which could resolve a long-standing problem in models of how the planets formed.

Long-period comets, which take longer than 200 years to orbit the sun, come from all directions in the sky, an observation that has long led scientists to believe that they were nudged out of a diffuse halo of icy objects surrounding the solar system – the Oort Cloud.

The objects probably formed from the same disc of material that gave rise to the planets but were scattered outwards by Jupiter and Saturn a few hundred million years after their birth.

The Oort Cloud is too dim to be seen by telescopes, but astronomers believe it has two components. Long-period comets were thought to originate in an outer portion extending from 20,000 to 200,000 astronomical units from the sun (where 1 AU is the Earth-sun distance).

Solar system models also predict the existence of an inner shell that stretches some 3000 to 20,000 AU from the sun. But researchers believed that objects orbiting inside this shell would never come close enough to the sun to produce glorious cometary displays because they would be ejected into interstellar space once they approached Jupiter and Saturn.

A new computer simulation suggests this gravitational barrier might instead be "leaky", allowing a number of objects to pass inside Jupiter's orbit. Jupiter and Saturn may actually nudge the interlopers onto elongated paths that bring them closer to the sun.

Persistent mystery
The work suggests that more than half of all long-period comets could come from this unseen "inner" Oort Cloud, which would mean that the solar system is much more compact than thought.

"There may not be nearly as much stuff as far out as we thought," says Nathan Kaib of the University of Washington in Seattle, who presented the results on Tuesday at a meeting of the American Astronomical Society in Washington, DC. "The region of the Oort Cloud that is not supposed to produce any comets may be the dominant producer of comets."

That could help solve a persistent mystery about the solar system. Hundreds of long-period comets have been catalogued, and their numbers had suggested that the outer Oort Cloud might contain as much as 40 times as much mass as Earth.

Accounting troubles
Current models of the solar system cannot account for so much mass. That's because objects at such extreme distances are prone to being lost from the solar system altogether, either jettisoned into interstellar space by the gravity of passing stars and giant gas clouds or tugs from the Milky Way itself.

Just 1 to 2 per cent of the material cast outwards by Jupiter and Saturn in the early solar system should have been retained in the outer Oort Cloud.

However the inner Oort Cloud is more insulated from the gravitational tugs of passing stars and should therefore retain 10 times as much material. So if the inner cloud is the dominant source of long-period comets, far less material would need to have been kicked outwards by Jupiter and Saturn billions of years ago.

Tracing origins
Luke Dones of the Southwest Research Institute in Boulder, Colorado, who was not involved in the new research, says the large estimate for the mass of the Oort Cloud "has been a problem for a long time". This work "at least partially resolves that, because it shows that you don't need nearly as much mass in the Oort Cloud to produce what we see", he told New Scientist.

Still, testing the idea may be difficult. Kaib notes that the orbits of long-period comets are still quite uncertain. The easiest ones to observe are those that pass through the inner solar system, but their orbits have likely changed significantly over their lifetimes, making it difficult to trace their origins.

Future telescopes, such as the Pan-STARRS project in Hawaii, should be able to observe much fainter – and therefore more distant – objects, whose orbits may be relatively unaltered.