Neptune’s Trojan asteroids, which share the planet’s orbit, may outnumber those in the solar system’s main asteroid belt (Image: NASA)

An asteroid that is trapped in a ‘dead zone’ behind Neptune has been found for the first time. The finding suggests that the blue planet’s rock collection may outnumber objects in the main asteroid belt and may provide clues to the origin of comets.

Objects can become trapped in two gravitational dead zones around Neptune, where the forces of the sun and the planet balance out. In the last decade, astronomers have identified six asteroids – called Trojans – in the zone that moves in front of the planet along its orbit. But finding Trojans in the region trailing the planet has proved more difficult, because the faint light reflected off of objects there is washed out by brighter starlight from the plane of the Milky Way.

Now, thanks to strategically located dust clouds, Scott Sheppard at the Carnegie Institution of Washington and Chadwick Trujillo at the Gemini Observatory in Hawaii have spied the first such trailing Trojan.


Cloudy discovery

To find the asteroid – dubbed 2008 LC18 – Sheppard and Trujillo used existing images of the sky to identify dark clouds of dust and gas in our galaxy that fall along the path of the trailing Lagrangian point. These clouds blotted out the light from background stars. They used the Subaru Telescope on Mauna Kea, Hawaii, to spot the Trojan as it moved across the sky.

The team estimates that 2008 LC18 is about 100 kilometres wide. Based on the size of the region they studied, they suspect Neptune harbours as many as 150 other Trojans of this size in the trailing Lagrangian region. Together with the projected numbers in the planet’s leading Lagrangian zone, they suspect Neptune likely holds more asteroids of this size than the solar system’s main asteroid belt between Mars and Jupiter.

Neptune’s Trojans were probably captured billions of years ago, when solar system bodies shifted positions in a period of planetary upheaval, Sheppard says. Neptune, which may have been hurled outwards to twice its original orbit in the migration, could have scooped up the rocky bodies as it moved.

Trojan comets?

The Trojans could provide insights into the origin of comets. If Neptune boasts smaller Trojans, they could be a source of short-period comets, which swing through the inner solar system every few hundred years. Dislodged Trojans may account for the comets, whose origin is uncertain, but more will need to be found before their numbers can actually support this scenario.

Automated surveys that take frequent snapshots of the sky, such as the upcoming Pan-STARRS project in Hawaii, will be particularly well-suited to hunt for asteroids in Neptune’s orbit.

“What these projects are all about is making a movie of the whole sky, so they’ll get anything that moves,” says Eugene Chiang of the University of California, Berkeley, who found the first Neptune Trojan. “That’s crucial for finding objects like Trojans.”

Journal reference: Science (DOI:10.1126/science.1189666)