BioEd Online

A family of small objects locked into the same orbit as Neptune has been spotted, lending support to the theory that the giant planets of our Solar System migrated over huge distances before settling into their current orbits.

The bodies, known as Trojan asteroids (although it is not clear that they are rocky, rather than icy), travel around the Sun at the same distance from the Sun as Neptune. They sit about 60° ahead of the planet on its circular orbit, settled into a gravitational sweet spot known as a Lagrangian point.

Every planet has these sweet spots; in Jupiter's case, there are more than 1,800 asteroids in them. Those asteroids in the sweet spot 60° ahead of the planet are named after Greek warriors from Homer's Iliad; those 60° behind after the Trojans.

The first of Neptune's Trojans was discovered in 2001, and since 2004 Scott Sheppard of the Carnegie Institution of Washington and Chad Trujillo at the Gemini Observatory in Hawaii have found three more, which they discuss in this week's Science¹. The bodies are between 80 and 140 kilometres wide.

Out of synch

One of them (dubbed 2005 TN₅₃) travels along an orbit that is tilted about 25° away from the plane in which Neptune and the other big planets orbit, and the astronomers say that this inclination with respect to the rest of the Solar System is consistent with the idea that Neptune was once much closer to the Sun.

"Ten years ago this idea would have been extremely shocking," says Sheppard; it was assumed that planets pretty much stayed in the orbit they started in. But the discovery of planets around other stars which had clearly migrated from their orbits of origin has led astronomers to doubt this maxim.

In the past few years, computer models attempting to reconstruct the history of the Solar System have found that certain features, such as the inclination of the Trojans associated with the outer planets, can only be explained if the giant planets swapped places at some point in their history.

Last year, astronomers proposed²,³,⁴ that the current position of the planets and asteroids could be explained if Neptune had formed closer to Saturn, before being flung out beyond Uranus by gravitational tides (see ' Planetary billiards answer Solar System riddle').

Sheppard explains that 2005 TN₅₃could not have formed in its current inclination, assuming it formed during the same time as the giant planets, because the new object would have crossed the paths of thousands of other chunks of rubble in more level orbits, leading to high-speed collisions that would have quickly destroyed it or sent it spiralling into deep space.

This suggests that the Trojan was captured into Neptune's Lagrangian point after the planet had assumed its current orbit. The slim chances of finding 2005 TN₅₃ in the area of sky they scanned suggests that there may be many other rocks travelling on similarly tilted courses, adds Sheppard.

Origin unknown

The fact that there has as yet been no Trojans found in the orbits of Saturn and Uranus fits with the current theories about planetary migration. What the Trojans actually are, though, remains a matter of supposition. Sheppard and Trujillo say that the four neptunian Trojans have the same pale red colour as Jupiter's, suggesting that they are from the same reservoir of rubble left over from the formation of the Solar System. But the question of whether that rubble is rocky or icy is not yet settled.

But with only four Neptune Trojans to work with, there are still plenty of uncertainties about their message for astronomers, says Francesco Marzari of the University of Padova, Italy. "We need to know more about who they are, and where they formed," he says. Finding more of them should help to determine whether they were indeed picked up by Neptune after its early travels.

So Sheppard and Trujillo are heading back to their telescope in October to catch some more, and estimate there may be as many as 500 Trojans wider than 100 kilometres just waiting to be discovered.

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