The Double Planet

Astronomy texts tell us that the Solar System contains nine planets, but this number flatters the outermost "planet" Pluto. The combined mass of Pluto and its satellite Charon is tiny: less than 5% of the mass of the next-smallest planet, Mercury. Such a feeble object may be better regarded as an extremely large comet that has lost much of its ice. Although attempts to downgrade the status of Pluto have failed — perhaps for reasons of sentiment and tradition — many planetary scientists consider that the Solar System contains only eight planets. If we do likewise, and exclude the Pluto-Charon system from the list of planets, then Earth is unique in having an exceptionally large satellite.

Note that the Moon is not the largest satellite in the Solar System. That honor belongs to Ganymede, which is one of the moons of Jupiter. Two other Jovian satellites — Callisto and Io — are also slightly larger than the Moon; and so is Titan (one of Saturn's moons). But Ganymede, Callisto, Io and Titan orbit giant planets. Compared to their parent bodies, these satellites are as grains of dust. our Moon, on the other hand, is large compared to Earth: it has 81 of the mass of our planet. The Earth-Moon system has rightly been called a "double planet." And double planets may be rare.

figure 57 Pluto and Charon combined have less than 5% of the mass of Mercury, the next-smallest planet.

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