Original URL: http://www.theregister.co.uk/2004/08/19/neptune_five_moons/

Neptune shows off five new moons

Gas giants battle for headlines

By Lucy Sherriff

Posted in Science, 19th August 2004 16:00 GMT

A team of international astronomers has found five new moons orbiting Neptune. Previously the planet had seven known moons, including a couple of oddities: Triton and Nereid. The moons were discovered in observations made from ground-based telescopes in Chile and Hawaii.

It is almost as if the larger planets of our solar system are trying to out-do one another: earlier this week stargazing scientists spotted two new mini-moons around Saturn.

The scientists made their observations using the 3.6-meter Canada-France-Hawaii telescope in Hawaii, and the 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile. These cameras were originally developed to hunt for Kuiper Belt objects.

Two of the new moons have prograde orbits - that is, they orbit in the same direction as the planet turns. The remaining three, however, have retrograde orbits.

The other gas giants have both kinds of moons, and they behave quite consistently. Prograde moons tend to be more regular in shape, have more circular orbits, and are thought to have formed along with their planet. Retrograde moons, by contrast, are irregular lumps, and are generally captured pieces of space junk. They have more elliptical orbits, often at inclined planes.

In the midst of all this consistent behaviour, Neptune was something of an oddity. Triton, an irregular shaped body about the size of Pluto, is in a retrograde but circular orbit, while the much smaller Nereid follows a highly eccentric orbital path in a prograde direction. Adding five more normal moons to the mix smoothes the weirdness out, a little.

The astronomers, led by Matthew Holman of the Harvard-Smithsonian Center for Astrophysics, took a long series of exposures using the wide-field mosaic CCD cameras on the two telescopes. They then digitally combined the images to look for small moving objects that were too faint to be resolved in a single shot.

Neptune's newly identified companions are all between 30km and 50km in diameters. This suggests they are most likely re-captured fragments from collisions between larger orbiting bodies, the team says. ®

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