Astronomers have found the first Trojan asteroid in Neptune's difficult-to-detect gravitationally stable point known as Lagrangian 5.

At certain points in a planet's orbit the gravitational tug between the planet and the Sun balance out, allowing smaller bodies, like asteroids or spacecraft, to remain stable. There are five such locations (known as Lagrangian points); or the case of the Earth L1 and L2 tend to be prime locations for satellites – the Solar and Heliospheric Observatory (SOHO) is located at L1 and space observatories including Planck and Herschel are situated at L2.

The population of so-called Trojan asteroids share an orbit with a planet but remain fixed in the stable L4 and L5 positions 60 degrees ahead and behind the planet on its orbit. Many Trojan asteroids are known in stable L4 and L5 locations around Jupiter and now the first Trojan asteroid in Neptune's L5 point has been found by astronomers Scott Sheppard at the Carnegie Institution’s Department of Terrestrial Magnetism and Chad Trujillo at the Gemini Observatory.

“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 areas are particularly stable, so dust and other objects tend to collect there," explains Sheppard. "We found three of the six known Neptune Trojans in the L4 region in the last several years, but L5 is very difficult to observe because the line-of-sight of the region is near the bright centre of our Galaxy.”

The astronomers used images from the digitized all-sky survey to identify places in the stability regions where dust clouds in the Milky Way blocked out the background starlight from the Galaxy’s plane, providing an observational window to the foreground asteroids. The discovery of asteroid 2008 LC18 was then enabled using the 8.2-metre Japanese Subaru telescope in Hawaii, while its orbit was determined using Carnegie’s 6.5-metre Magellan telescopes at Las Campanas in Chile.

“We estimate that the new Neptune Trojan has a diameter of about 100 kilometres and that there are about 150 Neptune Trojans of similar size at L5,” says Sheppard. “It matches the population estimates for the L4 Neptune stability region. This makes 100-kilometre-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.”

Identifying Trojan asteroids helps shed light on the evolution of the Solar System. Neptune's L5 Trojan is on a very tilted orbit with respect to the plane of the Solar System, just like many at the L4 point, suggesting that they were captured into these orbits. This likely happened early in the Solar System's history, perhaps as Neptune migrated into its present orbit, grabbing asteroids and trapping them in these gravity 'voids'.

The research is published in the 13 August issue of Science.