ESO Press Photos 29a-c/00
26 October 2000
For immediate release

More Saturnian Moons
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.
PR Photo 29a/00: New Saturnian satellite S/2000
S 1
PR Photo 29b/00: New Saturnian satellite S/2000
S 2
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
ESO PR Photo 29a/00
ESO PR Photo 29b/00
ESO PR Photo 29c/00
[Animated GIF: 330 x 400 pix –
Captions: The photos show the discovery
images of two new Saturnian moons, as registered on August 7, 2000,
with the Wide-Field Imager (WFI) camera at the MPG/ESO 2.2-m
telescope at the La Silla Observatory. Photo PR 29a/00
displays the faint image of the newly discovered moon S/2000 S
in the lower right corner of the field. A spiral galaxy is seen
in the upper left corner of this photo. The other objects are
(background) stars in the Milky Way. Photo PR 29b/00 is a
combination of three successive WFI exposures of the second moon,
S/2000 S 2. Because of its motion, there are three images (to
the left). Photo PR 29c/00 is an animated GIF image of the
same three exposures that demonstrates this motion.
Technical details are
found below.
The observations of the first two objects are described on a
Circular of the International Astronomical Union (IAU)
that was issued today [2].
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-m
MPG/ESO Telescope
at ESO’s La Silla Observatory
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
Follow-up observations
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 [2].
Working as fast as the images came off the telescope, they
immediately alerted other teams of astronomers about these
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
at the 1.5-m telescope of the Steward Observatory
(Arizona, USA), as well as Alain Doressoundiram and Jorge
at the ESO 3.58-m
New Technology Telescope (NTT)
on La Silla.
The orbits
Orbital calculations by Brian Marsden (IAU Minor Planet
, 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, cf. ESO PR Photos 19a-b/00. 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.
More information
Press releases about the new Saturnian satellites are also being
issued by other organisations and institutes: