PRESS RELEASE, July 10 2001 -- EMBARGOED UNTIL July 12/2001
Since mid-1997 the number of distant irregular satellites of the giant planets has almost quadrupled. Jupiter has gone from 8 to 20 such moons, Uranus from 0 to 5, and Saturn from 1 to 13. A paper appearing in the July 12/2001 issue of Nature discusses the discovery of the 12 new saturnian satellites and the conclusions the authors draw about what the orbital distribution of the satellites tells us about their origin and the formation of the giant planets.

The satellites were discovered between August and December 2000 by an international team of observers including: Brett Gladman and Jean-Marc Petit of the Observatoire de la Cote d'Azur, France; JJ Kavelaars of McMaster University, Canada; and Matthew Holman and Timothy Spahr of the Harvard-Smithsonian Center for Astrophysics, USA. Twelve faint bodies were spotted between August and November of 2000 by these observers using several telescopes around the world (see the web page below for more details).

These moons are what astronomers refer to as `irregular' moons because they are far from their planet and were likely captured into orbit after the planet formed. In contrast, the `regular' moons of the giant planets, which commonly have nearly circular equatorial orbits nested close to the planet, are thought to have formed out of a disk of dust and gas that surrounded each planet as it formed. Saturn's only previously-known irregular satellite Phoebe was discovered in 1898 by W. Pickering at a Peruvian observatory. Saturn's total count of 30 moons currently surpasses all other planets. The new moons of Saturn have diameters ranging from 5-40 kilometers (about 3-25 miles), in line with the sizes of other irregular moons.

Although the saturnian discoveries were reported in the fall of 2000, the current progress revolves around the tracking of these and other irregulars moons. In the period after their initial discovery, the team of astronomers, expanded to include P. Nicholson and J. Burns of Cornell University, T. Grav and K. Aksnes of the University of Oslo, Norway, as well as Carl Hergenrother and collaborators at the University of Arizona, USA, tracked the moons for several months. Each observation show how the moon had advanced along its orbit around the planet. The measurements of these positions allowed others to tackle the computation of the size and shapes of the orbits around the planet.

The orbital calculations were performed by Brian G. Marsden (IAU Minor Planet Center, Harvard-Smithsonian Center for Astrophysics, USA), Robert Jacobson (Jet Propulsion Laboratory), and William Gray (Project Pluto). They independently calculated the evolving estimates of the orbits and aided during the tracking observations by telling the observers where best to look for the moons. The final best fits to the orbits recently derived by these workers show that the orbits of the moons are not at all randomly distributed around the planet, but rather they fall in tightly packed groups (see the figure below).

The conclusion to be drawn from the orbital distribution of all the irregular moons of the giant planets is that most of the moons are the fragments of larger satellites that were broken up by passing asteroids or comets after the planet has been formed. The authors propose the following sequence of events in their July 12 Nature paper: As the giant planets formed 4.5 billion years ago they were each surrounded by a cocoon of gas which later collapsed onto the planet (the giant planets have very massive atmospheres of hydrogen and helium in the form of methane and ammonia; other planets lacked such cocoons and therefore couldn't capture irregular moons). Other small bodies (often called 'planetesimals') formed on independent orbits around the Sun, and these planetesimals were constantly passing near the planet. Although not the only theory, it is thought that some of the passing planetesimals were `captured' into orbit by friction with a gas cocoon, through which they pass at speeds approaching a kilometer per second. Large planetesimals are too massive to feel the effects of this gas drag, and pass by unaffected. Very small planetesimals which penetrate close to the planet are nearly stopped by the drag, and fall into the planet. The intermediate size range between about 10-100 km is thought to be favoured for capture. After the gas cocoon dispappeared (soon after the planets formed), satellite capture ceased to be an effective process.

Once captured into orbit, the irregular moons are subject to bombardment by passing comets over the rest of the age of the solar system (in the case of Jupiter passing asteroids may also be important). The clumped orbital distribution of the moons is consistent with this idea, because the spread in relative speeds within a given clump are about the same as the gravitational 'escape speed' of the largest member of the clump. The authors suggest that the breakups must have occurred after the dissipation of the planet's gas cocoon, for if not the smallest satellites within a given clump would have spiralled systematically closer to to planet, something which is not observed. Therefore, the breakups could have occurred anytime between 4.5 billion years ago and today. The fact that almost all the irregular satellites are observed to be in groups of fragments attests to the number of passing impactors that have passed over the age of the Solar System.

It is likely that all the giant planets harbour more irregular satellites, as yet unseen due to their small size and therefore faintness. More new satellites are likely to be discovered this starting in August 2001 as astronomers focus their attention back on the giant planets.

Web page (with contacts):

ORBITS OF THE IRREGULAR SATELLES of the giant planets (GIF image).
Caption : This sketch compares the orbital properties of the irregular satellites of the giant planets. A different color is used for each planet's satellites. Each orbit's inclination 'I' relative to the J2000 ecliptic (approximately the planet's orbital plane about the Sun) is illustrated by the angle from the horizontal; moons in the right quadrant move in the same (direct) sense as their planets orbit the Sun while those in the left quadrant are retrograde. The radial distance from the origin to the symbol represents the orbital semimajor axis 'a' (given as a fraction of the radius of its planet's Hill sphere R_H, which is the region within satellites are retained by the planet). The length of each line illustrates the pericenter-to-apocenter variation in distance due to the orbital eccentricity. The symbol's size is proportional to the logarithm of the satellite's radius, either measured or estimated from its magnitude and an assumed albedo of 6%. Time-averaged orbital elements are used to remove the effect of solar perturbations. The region within which the dynamical Kozai mechanism removes nearly polar orbits is indicated by the dashed lines for each planet.