Following the formation of the giant planets and their satellites, a wide variety of objects remained scattered around in the solar system. This chapter discusses these remnants that were not swept up into planets. Like most refugees, they have interesting tales to tell of their former history.

Pluto and Charon

The solar system does not become simpler with increasing distance from the Sun, as the Pluto and Charon pair testify [1, 2]. The initial discovery of Pluto in 1930 posed a dilemma. What was this tiny object and why was it in such a strange orbit? One might have expected a smaller cousin of Neptune to neatly round off the solar system. Instead, beyond the giant there was apparently nothing but a dwarf. The problem became more bizarre when Pluto was found to be even smaller than first thought and to be accompanied by a large satellite orbiting at a high angle to the plane of the ecliptic.

The mass of Pluto is very small (1.318×1025 g), amounting only to 18% of the mass of the Moon, 1/2000 of the mass of the Earth or 1/64,000 of the mass of Jupiter. Triton (2.14×1025 g), a close cousin to Pluto in many respects, is nearly 40% more massive, while the Moon, Io, Europa, Ganymede, Callisto and Titan are also bigger.