Mauna Kea - September 20, 1999 - Until just a few years ago, many astronomers believed the planet Uranus was a bit strange. That's because, unlike the other giant members of the Solar System, Uranus did not appear to have any so-called irregular satellites, or, distant moons with unusual orbits.
However, recent observations have found what appear to be three new irregular moons around Uranus, thus suggesting that the seventh planet from the Sun is just one of the gang after all.
Using the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, an international team of astronomers made very careful observations over the summer to find these extremely faint objects.
If confirmed, and tallied with two other irregular satellites discovered in 1997, Uranus would have 16 regular and five irregular moons, making it the most populated planetary satellite system known.
Irregular satellites do not follow the normal, near-circular orbits of most satellites, such as the Earth's Moon. Instead, these irregular objects either travel in highly elliptical orbits, or follow paths that are severely tipped to the plane of the planet's equator.
"The discovery of these irregular satellites is very important because it means that Uranus is not some oddball, but rather is just like Neptune, Saturn, and Jupiter," says Matthew Holman, a planetary scientist at the Harvard-Smithsonian Center for Astrophysics and a member of the team that made the discovery.
"It might also help us better understand how the irregular satellites of the giant gas planets originated and how they've evolved," added Holman.
These newly discovered objects are being referred to as "candidate" irregular satellites because further observations are necessary to absolutely confirm that these bodies are not comets or asteroids on planet-encountering orbits. However, based on the data so far, the team is confident these are true moons of Uranus.
"Given how these bodies are following the planet exactly, it is highly unlikely that these are some sort of Solar System interlopers," says Brett Gladman of the Observatory of Nice, France, and leader of the team. Gladman and his colleague J.J. Kaveleaars of McMaster University, Canada, were both members of the team that found Uranus's first two irregular moons in 1997.
The three new candidate satellites were discovered in a search using the world-class wide-field imaging camera, known as CFH12K, which is a mosaic of CCD detectors covering a very large patch of sky (currently 35x28 arcmin, or roughly the area of the full moon).
This instrument allowed the team to explore more than 90 percent of the region around Uranus in which satellite orbits are stable and to find these extremely faint objects, which are no more than 20 kilometers in diameter and orbit Uranus at a distance of 10 to 25 kilometers.
Other members of the discovery team include Jean-Marc Petit and Hans Scholl (Observatory of Nice, France), and P. Nicholson and J. A. Burns (Cornell University.)
Follow-up observations were obtained at the Mount Palomar 5-meter and Kitt Peak 4-meter telescopes, the latter in conjunction with D. Davis and C. Neese of the Planetary Science Institute in Tucson, AZ.
Brian Marsden and Gareth Williams of the International Astronomical Union's Minor Planet Center computed preliminary orbits for the reported objects.
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