This general scheme of planet formation—the building up of larger masses by the accretion of smaller ones—occurred in the outer solar system as well. Here, however, the accretion of icy planetesimals produced objects with masses 10 times that of Earth, sufficient to cause the gravitational collapse of the surrounding gas and dust in the solar nebula. This accretion plus collapse allowed these planets to grow so large that their composition approached that of the Sun itself, with hydrogen and helium the dominant elements. Each planet started with its own “subnebula,” forming a disk around a central condensation. The so-called regular satellites of the outer planets, which today have nearly circular orbits close to the equatorial planes of their respective planets and orbital motion in the same direction as the planet’s rotation, formed from this disk. The irregular satellites—those having orbits with high eccentricity, high inclination, or both, and sometimes even retrograde motion—must represent objects formerly in orbit around the Sun that were gravitationally captured by their respective planets. Neptune’s moon Triton and Saturn’s Phoebe are prominent examples of captured moons in retrograde orbits, but every giant planet has one or more retinues of such satellites.

It is interesting that the density distribution of Jupiter’s Galilean satellites, its four largest regular moons, mirrors that of the planets in the solar system at large. The two Galilean moons closest to the planet, Io and Europa, are rocky bodies, while the more-distant Ganymede and Callisto are half ice. Models for the formation of Jupiter suggest that this giant planet was sufficiently hot during its early history that ice could not condense in the circumplanetary nebula at the present position of Io. (See Jupiter: Theories of the origin of the Jovian system.)

The small bodies

At some point after most of the matter in the solar nebula had formed discrete objects, a sudden increase in the intensity of the solar wind apparently cleared the remaining gas and dust out of the system. Astronomers have found evidence of such strong outflows around young stars. The larger debris from the nebula remained, some of which is seen today in the form of asteroids and comets. The rapid growth of Jupiter apparently prevented the formation of a planet in the gap between Jupiter and Mars; within this area remain the thousands of objects that make up the asteroid belt, whose total mass is less than one-third the mass of the Moon. The meteorites that are recovered on Earth, the great majority of which come from these asteroids, provide important clues to the conditions and processes in the early solar nebula.

The icy comet nuclei are representative of the planetesimals that formed in the outer solar system. Most are extremely small, but the Centaur object called Chiron—originally classified as a distant asteroid but now known to show characteristics of a comet—has a diameter estimated to be about 200 km (125 miles). Other bodies of this size and much larger—e.g., Pluto and Eris—have been observed in the Kuiper belt. Most of the objects occupying the Kuiper belt apparently formed in place, but calculations show that billions of icy planetesimals were gravitationally expelled by the giant planets from their vicinity as the planets formed. These objects became the population of the Oort cloud.

Formation of ring systems

The formation of planetary rings remains a subject of intense research, although their existence can be easily understood in terms of their position relative to the planet that they surround. Each planet has a critical distance from its centre known as its Roche limit, named for Édouard Roche, the 19th-century French mathematician who first explained this concept. The ring systems of Jupiter, Saturn, Uranus, and Neptune lie inside the Roche limits of their respective planets. Within this distance the gravitational attraction of two small bodies for each other is smaller than the difference in the attraction of the planet for each of them. Hence, the two cannot accrete to form a larger object. Moreover, because a planet’s gravitational field acts to disperse the distribution of small particles in a surrounding disk, the random motions that would lead to accretion by collision are minimized.

The problem challenging astronomers is in understanding how and when the material making up a planet’s rings reached its present position within the Roche limit and how the rings are radially confined. These processes are likely to be very different for the different ring systems. Jupiter’s rings are clearly in a steady state between production and loss, with fresh particles continuously being supplied by the planet’s inner moons. For Saturn, scientists are divided between those who propose that the rings are remnants of the planet-forming process and those who believe that the rings must be relatively young—perhaps only a few hundred million years old. In either case, their source appears to be icy planetesimals that collided and fragmented into the small particles observed today.

Solution to the angular momentum puzzle

The angular momentum problem that defeated Kant and Laplace—why the planets have most of the solar system’s angular momentum while the Sun has most of the mass—can now be approached in a cosmic context. All stars having masses that range from slightly above the mass of the Sun to the smallest known masses rotate more slowly than an extrapolation based on the rotation rate of stars of higher mass would predict. Accordingly, these sunlike stars show the same deficit in angular momentum as the Sun itself.

The answer to how this loss could have occurred seems to lie in the solar wind. The Sun and other stars of comparable mass have outer atmospheres that are slowly but steadily expanding into space. Stars of higher mass do not exhibit such stellar winds. The loss of angular momentum associated with this loss of mass to space is sufficient to reduce the rate of the Sun’s rotation. Thus, the planets preserve the angular momentum that was in the original solar nebula, but the Sun has gradually slowed down in the 4.6 billion years since it formed.