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There are places in space where the gravitational tug between a planet and the Sun balance out, allowing other smaller bodies to remain stable. These places are called Lagrangian points. So-called Trojan asteroids have been found in some of these stable spots near Jupiter and Neptune. Trojans share their planet's orbit and help astronomers understand how the planets formed and how the solar system evolved. Now Scott Sheppard at the Carnegie Institution's Department of Terrestrial Magnetism and Chad Trujillo at the Gemini Observatory have discovered the first Trojan asteroid, 2008 LC18, in a difficult-to-detect stability region at Neptune, called the Lagrangian L5 point. They used the discovery to estimate the asteroid population there and find that it is similar to the asteroid population at Neptune's L4 point. The research is now published in the Journal Science.
Figure 1: Discovery images of the first known 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.
The L4 and L5 Neptune Trojan stability regions lie about 60 degrees ahead of and behind the planet, respectively. Unlike the other three Lagrangian points, these two are particularly stable, so dust and other objects tend to collect there. Sheppard and Trujillo found several of the first known Neptune Trojans in the L4 leading region in the last several years, but the Neptune L5 trailing region is very difficult to observe because the line-of-sight of the region is near the bright center of our galaxy.
Figure 2: This schematic shows the five Lagrangian regions around Neptune's orbit. Six Neptune Trojans are known in the leading L4 region and for the first time one is now known in the trailing L5 region. The discovery demonstrates that Neptune hosts similar clouds of Trojans at either of two points 60 degrees away from the planet where the gravitational forces of the planet and the Sun combine to lock the asteroids into a stable, synchronized orbit. The Neptune Trojans likely outnumber the main belt of asteroids between Mars and Jupiter.
The scientists devised a unique observing strategy. Using images from the digitized all-sky survey they identified places in the stability regions where dust clouds in our galaxy blocked out the background starlight from the galaxy's plane, providing an observational window to the foreground asteroids. They discovered the L5 Neptune Trojan using the 8.2-meter Japanese Subaru telescope in Hawaii and determined its orbit with Carnegie's 6.5-meter Magellan telescopes at Las Campanas, Chile.
"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," Sheppard said. "It matches the population estimates for the L4 Neptune stability region. This makes the Neptune Trojans, in this size range, more numerous than those 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."
Figure 3: The recovery images of 2008 LC18 from Carnegie's Magellan Telescope. Recovery was difficult since the Trojan had moved out of the obscuring dust cloud region used for discovery observations and back into the stellar confusion in the galactic plane.
The L5 Trojan has an orbit that is very tilted to the plane of the solar system, just like several in L4. This suggests they were captured into these stable regions during the very early solar system when Neptune was moving on a much different orbit than it is now. Capture was either through a slow, smooth planetary migration process or as the giant planets settled into their orbits, their gravitational attraction could have caught and "frozen" asteroids into these spots. The solar system was likely a much more chaotic place during that time with many bodies stirred up onto unusual orbits.
The region of space surveyed also included a volume through which the New Horizon spacecraft will pass after its encounter with Pluto in 2015.
Figure 4: This image shows the edge of a dust cloud near the plane of the Milky Way Galaxy. The top region of the image shows very few stars since the dust blocks the light from the background stars. The bottom region shows where the dust is not as thick and is thus less effective at obscuring the light from the background stars. Foreground moving objects such as the Neptune Trojans are easier to identify when the background stellar light is minimal.
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