Found: One of Neptune's Asteroid Stalkers

By Joseph Calamia | August 13, 2010 1:48 pm

neptuneAstronomers have confirmed it: Neptune has a stalker. They have spotted, for the first time, an asteroid follower that keeps a fairly constant distance behind the planet in its orbit around the sun. And there may be many more.

Asteroid 2008 LC18 can’t help itself. It’s caught in a balancing game between the gravitational tug of the sun and Neptune, and effects from its whirling course. The conflicting tugs cause the asteroid not to orbit Neptune or crash into it, but instead to follow the planet from a little distance behind (about 60 degrees on its path).

Neptune has five of the these pits–called Lagrangian points (see diagram below the fold)–but the spots ahead and behind the planet, researchers say, are best for asteroid-trapping, since the hold is particularly stable in these places. Researchers have previously spotted several asteroids in front of the planet (again by about 60 degrees), but this is the first time they’ve found one following it. The findings appeared online yesterday in Science.

24617_webSpotting followers in the place behind the planet was particularly difficult because astronomers’ line of sight to where these special Lagrangian asteroids–called Trojans–roam overlapped with the center of the Milky Way. Says lead author Scott Sheppard in a Carnegie Institute press release:

“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. 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 center of our galaxy.” [Carnegie Institution]

Using the 8.2-meter Japanese Subaru telescope in Hawaii, astronomers spotted the L5 Trojan by waiting for dust clouds in our galaxy to block out the light from the galaxy’s center. They determined its orbit using Carnegie’s 6.5-meter Magellan telescopes in Chile, and they believe finding 2008 LC18 means the planet may have many Trojan asteroids. Sheppard believes they’d be easier to spot if Neptune wasn’t so far away.

“We believe Neptune Trojans outnumber the Jupiter Trojans and the main-belt asteroids between Mars and Jupiter,” Scott Sheppard of the Carnegie Institution in Washington, D.C., told “If Neptune was where the main-belt was, we’d know thousands of these objects.” []

Sheppard also notes that Neptune’s newfound Trojan has a slightly tilted orbit, which may mean that the planet captured the asteroid when it was in a different location and trajectory–lending support to the theory that the giant planets don’t have the same orbits that they started with in the early solar system.

“The Neptune Trojans can tell us a lot about how the giant planets formed,” Sheppard said…. “People have been saying that the giant planets were very chaotic,” that they formed closer together, in unusual orbits, and migrated outward, Sheppard said. The Neptune Trojans’ odd orbits “supports that theory.” [National Geographic]

Related content:
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80beats: Did Galileo Spot Neptune Two Centuries Before Its “Discovery?”
80beats: The Earth’s Oldest Diamonds May Show Evidence of Earliest Life
Bad Astronomy: A New Ring Around Uranus
Bad Astronomy: Did Herschel See the Rings of Uranus?

Images: NASA & Scott Sheppard

CATEGORIZED UNDER: Physics & Math, Space
  • ChH

    oops – Netpune is not “much closer”
    The L4 and L5 Lagrange points are almost exactly the same distance from the sun as from the planet stabilizing that orbital slot. The three points form an equilateral triangle.

    Objects near L4 or L5 orbit the sun, but after having their orbits perterbed by massive bodies like other planets, Netpune’s weak but consistent gravitation force causes them to return to their former slot – therefore the orbit is stable.
    L1, L2 and L3, by comparison, are unstable – an object could orbit the sun at the same period as Neptune despite being closer or further – but if perterbed will not tend to return to those spots. That’s why satellites such as WIND, SOHO, ACE and Planck – all at Earth-Sun L1 or L2 – must station-keep to remain in those orbits.

  • Joseph Calamia

    ChH. Thanks so much for the correction. You are right; 2008 LC18 is almost the same distance from Neptune and the sun. I’m sorry for my mistake–I’ve updated the post above. The bodies’ motions–and their associated “apparent” forces such as the centrifugal and Coriolis effects–also enter into the balancing game to create these stable points:

    Thanks again,

  • Brian Too

    I hadn’t thought of this before, but I guess every planet has Lagrange points.

    Do major satellites also have gravitationally significant Lagrange points too? Such as the Moon, Titan, Ganymede, and so forth?

  • Eliza Strickland

    @ Brian Too :

    Yes! Two of Saturn’s moons, Tethys and Dione, have been shown to have smaller moons at their Lagrangian points.

  • Joseph Calamia

    Brian Too, thanks.

    From this Wilkinson Microwave Anistopy Probe outreach page: the L4 and L5 points are home to stable orbits so long as the mass ratio between the two large masses exceeds 24.96.

    In this case, the two masses are Neptune and the Sun. The ratio of the Sun’s mass to Neptune’s is about 19,000. The ratio of Earth’s mass to our moon’s is about 81.

  • Brian Too

    @5. Joseph Calamia,

    Your link is flawed, it generates a 404 Page Not Found error. The problem appears to be that the terminating bracket was included in the link address, whereas you meant that as formatting text in your message. I’m surmising.

    Fortunately the WMAP site gives you a search box right on the error page, you can type in Lagrange, and it takes you right there.

  • Eliza Strickland

    Thanks Brian Too — fixed the formatting & link!

    — Eliza, DISCOVER online news editor


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