In this dead zone the sun and Neptune's gravitational pulls are roughly equal. As a result, the asteroid is held at a fixed distance behind the planet, following—but not orbiting—Neptune as it circles the sun.
These regions where the gravity of the sun and a planet balance out are called Lagrangian points. Lagrangian points L4 and L5—where the new Neptune asteroid was found—are the most stable. The small, rocky bodies caught in these zones share their host planet's orbits.
(Related: "Massive Comet Impact Detected on Neptune.")
And where there's one Trojan, there are bound to be more, said astronomer Scott Sheppard of the Carnegie Institution for Science in Washington, D.C., who led the research.
Neptune probably has hundreds of asteroids trailing it, he said—outnumbering even those in the asteroid belt, which circles the sun between the orbits of Mars and Jupiter.
(Related: "Water Discovered on an Asteroid—A First.")
Trojans Tough to Spot
Neptune's Trojans are difficult to study, simply because they're so far away.
Until now, scientists had discovered six Neptune Trojans, in the L4 zone, which orbits in front of Neptune. Trojans in the trailing, L5 zone eluded researchers because the Milky Way's bright light makes it hard to discern the space rocks.
But, using Japan's Subaru Telescope in Hawaii, Sheppard and colleagues found places in the sky where dust clouds block the Milky Way's light. In one such region the astronomers found the first L5 Trojan, an asteroid estimated at 60 miles (100 kilometers) wide, which they named 2008 LC18.
Trojans May Be Clues to Early Solar System Conditions
"The Neptune Trojans can tell us a lot about how the giant planets formed," Sheppard said.
The Trojans have an inclined orbit, which means they must have fell in line in the solar system's early days, before Neptune's orbit had shifted. (See "Uranus, Neptune Swapped Spots, New Model Says.")
"People have been saying that the giant planets were very chaotic," that they formed closer together, in unusual orbits, and migrated outward, Sheppard said. The Neptune Trojans' odd orbits "supports that theory."
For all the clues revealed by Neptune's Trojans, their compositions and origins are largely unknown, he said.
They could be objects from the Kuiper belt—a region beyond Neptune strewn with small icy bodies, including Pluto—which moved inward. Or the Trojans could represent some sort of "transition object," neither frozen Kuiper objects nor rocks from the asteroid belt, Sheppard said.
NASA's New Horizons spacecraft, currently en route to Pluto, will pass through Neptune's L5 point in about five years and could answer some of these questions.
New Horizons won't get close enough to Trojan 2008 LC18 to observe it. But there's a slight chance another, as yet undiscovered Neptune Trojan lies in the craft's path. "We're still searching the region," Sheppard said.
The Neptune Trojan asteroid study will be published tomorrow in the journal Science.