Large planetoids may have formed hundreds of times farther from the Sun than previously thought, new calculations reveal. The work suggests a vast population of undetected icy bodies - like the recently discovered Sedna - may skirt the fringes of the solar system.

All of the objects in the solar system are thought to have condensed from a disc of dust and gas, starting about 4.5 billion years ago. But the process was far from orderly.

Observations of comets suggest some objects that formed in the disc - probably around the orbit of Saturn - were flung outwards within 100 million years. These scattered bodies are currently believed to make up a shell called the Oort Cloud that surrounds the solar system like a bubble.

The astronomers who discovered the large object Sedna in November 2003 think it may be one of these scattered objects. The planetoid is nearly the size of Pluto and about three times as far away.

But now, Alan Stern, director of space studies at the Southwest Research Institute in Boulder, Colorado, US, says Sedna may have formed at its current distance - or perhaps even farther out.

Cut-off point

This suggestion is based on computer simulations that assume the solar system's original gas and dust disc extended beyond the Kuiper Belt, a ring of icy objects beyond Neptune. This belt, first discovered in 1992, seems to cut off abruptly at 50 astronomical units (1 AU being the distance between the Sun and Earth). The cut-off delineates an apparent "edge" to the main portion of the solar system.

But when Stern tested models in which the disc extends out to 500 AU, those containing relatively high densities of material produced planetoids like Sedna in just 100 million years.

"The implication is that our solar system's planet-forming disc was really large," Stern says. "The outer edge of the Kuiper Belt may actually be the inner edge of a gap in the disc." Such a gap may have been carved out early in the solar system's history by a passing star or a Mars-sized planet being ejected into space.

Compact disc

He adds that dusty discs have been seen around other stars and tend to be big, with diameters of up to 3000 AU. "It's rare to find a compact disc like the Kuiper Belt," he told New Scientist. "Instead of trying to explain why ours is different, I thought: what if it isn't?"

Stern says Sedna was probably born in a circular orbit, possibly as far away as 500 AU. But he agrees with other astronomers who say that a passing star or unseen planet must have knocked it into its current, elongated trajectory, which stretches from about 75 to 900 AU.

The simulations suggest many hundreds of objects like Sedna may have formed at those distances. Stern admits that finding the extremely faint objects will be incredibly hard, but says his theory would be bolstered if deep surveys turned up far-distant objects in circular orbits.

But Renu Malhotra, an astronomer at the University of Arizona in Tucson, US, has some reservations about the work. She says dust thins out toward the outer edges of a disc, so building distant objects as big as Sedna within the age of the solar system "is pretty hard even in very large discs".

And quick timing is crucial. If a passing star did cause Sedna's extreme orbit, that suggests the planetoid must have formed in solar system's first 100 million years - the time it took to assemble the Oort Cloud. If the star passed by after that time, it would have disturbed and dissipated the cloud.

She also questions Stern's suggestion that other planetoids would lie in circular orbits. Any passing star that disturbed Sedna's orbit would probably also affect its siblings, she told New Scientist. But she says the discovery of the Kuiper Belt's edge at 50 AU in recent years has puzzled theorists and says Stern's paper "suggests we should keep our minds open to the possibility that what we're seeing is indeed a gap".

Journal reference: Astronomical Journal (vol 129, p 526)

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