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 (distant moons with unusual orbits). However, recent observations have found what appear to be three new irregular moons around Uranus, suggesting that the seventh planet from the sun is just one of the gang after all.
Last week, the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., reported that an international team of astronomers, including two Cornell researchers, had made very careful observations over the summer to find these extremely faint objects, using the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii.
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.
The Cornell researchers are Philip Nicholson, professor of astronomy, and Joseph Burns, professor of astronomy and the I.P. Church Professor of Theoretical and Applied Mechanics. The two researchers were members of the team that found Uranus' first two irregular moons in 1997, since named Caliban and Sycorax.
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 and follow paths that are severely tipped to the plane of the planet's equator.
The three newly discovered moons, temporarily referred to as S/1999 U1, S/1999 U2 and S/1999 U3, said Nicholson, are extremely faint objects no more than 20 kilometers in diameter and orbiting Uranus at a distance of 10 million to 25 million kilometers. The moons U1 and U2 apparently are orbiting Uranus in a counterclockwise direction, and U3, Nicholson said, "appears to be moving directly away from Uranus."
U1 was found at a distance even greater than that of Sycorax, which orbits the planet every 1,283 Earth days and is the most distant known satellite of Uranus. U2 appears to have an orbit about the planet similar to that of Caliban, which orbits Uranus every 579 Earth days. U3 was discovered on the opposite side of the planet from U1 and U2 and appears to have an orbit that to terrestrial observers appears almost edge-on.
Burns noted that a classification of Uranus' irregular moons might now be possible. Jupiter, for example, has eight irregular planets that have been grouped in two sets of four, each set with similar orbits. "Are there families of similar satellites around Uranus that possibly arose from a single entity?" he asked.
"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," said 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."
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," said Brett Gladman of the Observatory of Nice, France, and leader of the team. Gladman was also a member of the team that found Uranus' first two irregular moons using the Palomar Observatory.
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 charge-coupled-device detectors covering a very large patch of sky, 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.
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