Neptune’s dead zone is not empty

Neptune and some of its moons

Neptune and some of its moons. Stable points along Neptune's orbit, called Lagrangian points, have been found to harbour asteroids that probably were captured as they wandered through.

  • Stable points where gravity from Sun and a planet balance out
  • Asteroids and other material can gather in these points
  • First asteroid has been found in one of Neptune’s stable points

There are places in space where the gravitational tug between a planet and the Sun balance out, allowing other smaller bodies to remain stable there. These places are called Lagrangian points.

Three Lagrangian points are found inline with a planet and the Sun—one beyond the planet (called L2), one between the planet and the Sun (L1), and one on other side of the Sun (L3)

There are also two stable points along the planet’s orbit, but at an angle of 60 degrees to the Sun. These are the L4 and L5 points (see the diagram at right).

Diagram showing Neptune's Lagrangian points

Neptune's Lagrangian points, where the gravitational tug of the planet and the Sun balances out. Asteroids had been found at L4, and now one has been found at L5.

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.

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 the difficult-to-study L5 point at Neptune.

They used the discovery to estimate the asteroid population there and suggest that it is similar to the asteroid population at Neptune’s L4 point.

“The L4 and L5 Neptune Trojan stability regions lie about 60 degrees ahead of and behind the planet, respectively,” explains Sheppard. “Unlike the other three Lagrangian points, these two areas are particularly stable, so dust and other objects tend to collect there.”

“We found 3 of the 6 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.”

This means that it is very hard to pick out a faint asteroid from amongst the myriad stars in the background.

Silver lining to these dark clouds

So the scientists devised a unique observing strategy. They used images from a digitised all-sky survey to identify places in the stability regions where dust clouds in our galaxy blocked out the background stars, making it easier to spot the foreground asteroids.

Discovery images of the L5 trailing Neptune Trojan 2008 LC18

Discovery images of the L5 trailing Neptune Trojan 2008 LC18, taken at the Subaru telescope on June 7, 2008. The Trojan is seen moving from right to left near the centre of the image. Each image is separated by about one hour in time. The background stars are stationary.

They discovered the L5 Neptune Trojan using the 8.2-metre Japanese Subaru telescope in Hawaii and determined its orbit with Carnegie’s 6.5-metre Magellan telescopes at Las Campanas, 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,” Sheppard said. “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.”

Probably captured

There are fewer Neptune Trojans known simply because they are very faint since they are so far from the Earth and Sun.”

The Trojan 2008 LC18 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.

Adapted from information issued by Carnegie Institution / NASA, ESA, E. Karkoschka (University of Arizona), and H.B. Hammel (Space Science Institute, Boulder, Colorado).

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