eso0036 — Photo Release
More Saturnian Moons
26 October 2000
Four new moons have been discovered orbiting Saturn, raising the number to twenty-two. This means that Saturn is now the planet in the Solar System with most known satellites. The previous record holder, Uranus, has twenty-one known satellites. Two of the new objects were first seen with the Wide-Field Imager (WFI) camera at the ESO La Silla Observatory (Chile). They have diameters of 10 to 50 km, and calculations have shown that they are almost certainly small satellites (moons) that accompany Saturn on irregular orbits.
Saturn takes the lead
Following the discovery of at least four additional moons of that planet, Saturn has again taken the lead as the planet with the greatest number of known natural satellites. A corresponding announcement was made today by an international team of astronomers  at a meeting of the Division for Planetary Sciences (DPS) of the American Astronomical Society (AAS) in Pasadena (California, USA).
The four new faint bodies were spotted during observations in August-September 2000 at several astronomical telescopes around the world. Subsequent orbital calculations have indicated that these objects are almost certainly new satellites of the giant planet.
Two Saturnian moons found at La Silla
The observations of the first two objects are described on a Circular of the International Astronomical Union (IAU) that was issued today .
The images of these new moons were first registered on exposures made on August 7, 2000, with the Wide Field Imager (WFI), a 67-million pixel digital camera that is installed at the 2.2-metre MPG/ESO Telescope at ESO's La Silla Observatory (Chile).
When analyzing the many images in a sky area near the location of the planet Saturn, Brett Gladman (who works for the "Centre National de Recherche Scientifique (CNRS)", France) realized that two faint, moving objects seen near the brilliant glare of Saturn might well be hitherto unknown satellites of that planet.
On September 23 and 24, Brett Gladman and his colleague JJ Kavelaars were observing at the Canada-France-Hawaii 3.5-m telescope on Mauna Kea (Hawaii, USA). In a more extensive search, they were again able to image the two objects first discovered at La Silla. They also detected two more candidates, also announced on an IAU Circular today . Working as fast as the images came off the telescope, they immediately alerted other teams of astronomers about these discoveries.
Additional, confirming observations soon came from (Rhiannon) Lynne Allen (University of Michigan, USA) at the 2.4-m MDM telescope (Arizona, USA), Carl W. Hergenrother and Steve Larson at the 1.5-m telescope of the Steward Observatory (Arizona, USA), as well as Alain Doressoundiram and Jorge Romon at the ESO 3.58-m New Technology Telescope (NTT) on La Silla.
Orbital calculations by Brian Marsden (IAU Minor Planet Center, Smithsonian Astrophysical Observtory, USA) proved that these objects cannot be foreground asteroids (minor planets).
Although it is currently not yet possible to completely disprove that these are comets that happen to pass near Saturn, previous experience shows that this is extremely unlikely.
Several months of continued observations will still be required to compute highly accurate orbits of these objects. This must be accomplished before the planet disappears behind the Sun in March 2001 (as seen from the Earth).
Saturn's "irregular" moons
The computations show that these moons are of the type that is referred to by astronomers as 'irregular', as they revolve around the giant planet in somewhat unstable, changing (i.e., 'irregular') orbits. They are quite far from the planet and were most probably captured into their present orbits (long) after the planet was formed.
In contrast, the `regular' moons of the giant planets - of which most have nearly circular orbits close to the planet and near its equatorial plane - are thought to have formed out of a disk of dust and gas that surrounded the planet as it formed.
Saturn's only previously-known irregular satellite is Phoebe that was discovered in 1899 by the American astronomer William H. Pickering on photographic plates obtained at the Harvard University's observing station in Peru. In contrast, Jupiter has nine known irregular satellites, one of which was discovered last year, c.f. ESO Press Photos eso0025. Neptune has two and Uranus has five (also discovered by the present team, in 1997 and 1999).
Saturn's total count of 22 moons now surpasses that of Uranus (with 21). The new moons of Saturn have diameters ranging from 10 - 50 kilometres, in line with the sizes of other irregular moons. They are almost certainly "captured" minor planets.
Possibly more moons
The team has found several other satellite candidates that are now being followed by various telescopes. When sufficient accurate positions have been measured, it will also become possible to compute the orbits of those objects.
It certainly looks as if there is a rich system of small distant moons swarming around Saturn, the beautiful 'ringed planet' of our solar system.
 The team includes Brett Gladman , Jean-Marc Petit and Hans Scholl (Observatoire de la Côte d'Azur, France), JJ Kavelaars (McMaster University, Canada), Matthew Holman and Brian Marsden (Harvard-Smithsonian Center for Astrophysics, USA), Philip Nicholson and Joseph A. Burns (Cornell University, USA).
Press releases about the new Saturnian satellites are also being issued by other organisations and institutes:
- Observatoire de la Côte d'Azur
- McMaster University
- Cornell University
- Harvard-Smithsonian Center for Astrophysics
More general information about the outer planets and their irregular satellites and some images are available on the web:
Observatoire de la Côte d'Azur Department Cassini B.P. 4229
F-06304 Nice Cedex 4, France
Cell: +1-626-403-7600 at the Marriott Hotel in Pasadena on October 26, 2000, from 15:30 to 16:30 hrs CEST = 13:30 to 14:30 UT