It’s almost exhilarating to find that the volume of space studied in new work on the Trojan asteroids near Neptune includes an area through which New Horizons will pass on its way to Pluto/Charon. This used to seem like an all but unknowable region until Voyager 2 made its Neptune pass, and although it’s been a long time since we’ve had a spacecraft there, we’re learning much more about the outer system from Earth-based resources, as the discovery of objects like Eris and Sedna makes clear. We can surely look forward to more surprises as New Horizons moves toward its 2015 flyby and pushes on into the Kuiper Belt.

The latest find, based on data from the Subaru Telescope in Hawaii and the Magellan telescopes in Chile, is the first Trojan asteroid found at Neptune’s L5 Lagrangian point. Both the L4 and L5 Lagrangian points, 60 degrees ahead of and behind the planet, are stable, meaning that objects tend to collect there over time. Six Neptune Trojans are known in the L4 region, but until now the L5 point was hard to study because from our vantage on Earth, the line of sight is near the center of the galaxy. That called for a strategy using places where galactic dust clouds black out background light, revealing foreground asteroids. The result was the object called 2008 LC18.

Image: Discovery images of the L5 trailing Neptune Trojan 2008 LC18, taken at the Subaru telescope on June 7, 2008 Universal Time. The Neptune Trojan is seen moving from right to left near the center of the image. Each image is separated by about one hour in time. The background stars are stationary. This image only shows about 1 percent of the area of one image from the telescope. Credit: Scott Sheppard/Chad Trujillo.

Scott Sheppard (Gemini Observatory), explains the result:

“We estimate that the new Neptune Trojan has a diameter of about 100 kilometers and that there are about 150 Neptune Trojans of similar size at L5. It matches the population estimates for the L4 Neptune stability region. This makes 100-km-wide Neptune Trojans more numerous than similar-sized bodies in the main asteroid belt between Mars and Jupiter. There are fewer Neptune Trojans known simply because they are very faint since they are so far from the Earth and Sun.”

So now we’ve identified another Trojan population linked to Neptune to join the L4 asteroids there and the Trojans associated with Jupiter. The objects are useful adjuncts to planet formation theories. In this case, the fact that 2008 LC18 has an orbit that is highly tilted to the plane of the Solar System parallels the similar orbits of some L4 Trojans, and suggests the objects were captured during the early years of the Solar System, when Neptune itself was moving in a different orbit than today. We have much to learn about planetary migration as the giant planets refined their orbits and a chaotic system gradually settled into place.

The paper is Sheppard and Trujillo, “Detection of a Trailing (L5) Neptune Trojan,” published online in Science Express August 12, 2010 (abstract).