Trojan Asteroid Found in Neptune's Trailing Gravitational Stability Zone

Press Release for August 12, 2010:

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 published in the August 13, 2010 issue of Science.

Figure 1: 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.  Click on the
image to find out more. 


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 trailing region in 
the last several years, but the Neptune L5 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 now one is 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

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 indentify when the background stellar light is
minimal.  Click here to learn more.

To Learn about the discovery of Neptune Trojans in the L4 trailing cloud click here

### This paper includes data gathered with the Carnegie 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile. Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. The Carnegie Institution of Washington ( has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.


Scott S. Sheppard, Carnegie Institution of Washington, phone 202-478-8854
Chad Trujillo, Gemini Observatory, phone 808-974-2566


Scott S. Sheppard's Home Page
e-mail sheppard at (replace "at" with "@")