Among the most exciting and challenging questions facing 21^st century astronomy are these: Is our solar system typical of the planetary systems around stars? In what sense? And how does this affect the chances for life elsewhere?

So does this mean that we are stuck with only one familiar planetary system for now? In two companion papers to be published in Icarus, we suggest otherwise: in a real sense planets can be thought of as moons of the Sun, and the regular satellites of the giant planets (Jupiter, Saturn, Uranus and Neptune) formed in a similar way to that of the planets themselves.

The genesis of this idea goes as far back as 1610, when Galileo discovered the four moons of Jupiter and urged other astronomers to study them. In those days, planetary orbits were generally perceived as infallible celestial machinery, with the Earth at the center. But Jupiter offered clear evidence that not everything revolved around the Earth. Still, the orbits of the Galilean satellites, like the orbits of the planets, were remarkably well behaved (nearly circular, lying on the equatorial plane of the planet and periodic). Indeed, it was for this reason that the satellites were at first regarded as small-sized planets.

Since Galileos days many more satellites have been discovered, some regular and some irregular (these terms refer to their orbits), and the music of the spheres has given way to something more akin to a planetary jazz, with order and structure existing side-by-side with chaos and happenstance. When satellites with orbital characteristics quite unlike that of the planets were found, such as the 1898 discovery of retrograde Phoebe by William Pickering (a retrograde satellite orbits in the opposite sense to a planets rotation, much like driving on the wrong side of the road), the paradigm of satellite formation begun to shift to a more random, catastrophic process. In fact, some of the planets themselves were known to be subject to chaotic behavior given sufficiently long time-spans. Thus, a new view of worlds in collision seized the imagination of the lay public and of astronomers alike, culminating with the spectacular collision of comet Shoemaker-Levy with Jupiter in 1994.

But just because catastrophic events are known to take place does not mean that order cannot lay hidden amid the chaos; only that one needs to exercise caution when ascribing particular properties to specific causes. While it is probably true that Earths Moon, Plutos moon Charon, and perhaps the moons of Mars formed in the aftermath of the collision of two large solid objects, our study suggests that the regular satellites of the giant planets should be seen as an ordered, even predictable family of celestial objects.

We argue that there is a sequence that relates the orbital properties and other characteristics of the observed satellites to the mass of the giant planet and of its long-gone, associated protosatellite disk. Also, within each satellite system there is a tendency for larger satellites to be found farther away from the planet (see Figure 1).

It is likely that such regular behavior is at least in part due to the presence of gas during the formation of the regular satellites. Since giant planets also formed in a gas disk, their satellites must contend with some of the same issues encountered in planet formation. In particular, the interaction of planets and satellites with the gas disk results in the inward migration of their orbits. This evolution may take place in a time shorter than the time it takes for the gas to dissipate. This means that a satellites orbit could keep shrinking until finally the satellite is lost by accretion onto the giant planet (just as the giant planet itself might accrete onto the central star).

So how was this loss prevented? We argue that sufficiently large satellites will form a gap in the gas disk (a similar process has been suggested for the giant planets). Once such a gap forms, satellite migration may stall. Moreover, by efficiently dispersing gas in the disk the large, gap-opening satellites may have allowed smaller, neighboring satellites to survive.

In looking at Figure 1, it may be appropriate to imagine that we are looking at a planetary distribution of objects, with the larger regular satellites, such as the Galilean satellites (Io, Europa, Ganymede and Callisto), Titan and Titania, akin to the giant planets of each system (in the sense that they were able to open a gap) and smaller regular satellites, such as those inside the orbit of Titan, perhaps analogous to the terrestrial planets (in the sense that they formed as gas dispersal was taking place). Our own solar system contains four giant planets (Jupiter, Saturn, Uranus and Neptune). But indications so far are that extrasolar planetary systems may contain one, or more than one, giant planet in them. So it is perhaps fitting that Titan stands alone as the only large satellite in Saturns satellite system, whereas Jupiters system has four large satellites.

Where does this leave us? Although planetary jazz is difficult to grasp at first, it does not mean that it is arbitrary or without discernable structure. We are lucky to have a chance to listen to a few more pieces before we venture to guess what its rules might be and what form such music may take. There is hope that our basic notions regarding its general outline may be correct, and that knowing which dials to tweak we may in time tune into the music while learning to suppress the noise and random hiss of the recording.

If so, it may indeed be possible to develop reliable models to constrain the diversity of extrasolar planetary systems and of their satellites, and begin to meaningfully address the question of whether such systems may harbor life.

Of course, much more work will be required to verify our hypothesis. An upcoming visit of the Cassini spacecraft to Saturn's satellite system, especially to its enigmatic, cloudy moon Titan, may afford an opportunity to test it. This moon has a thicker atmosphere than that of Earth; it is only slightly smaller than Mars; and it may have produced the kind of organic chemistry that on Earth gave rise to life. The sirens of Titan beckon. An island world awaits.