In the early 1600's Galileo Galilei (1564-1642) made extensive observations with one of the first astronomical telescopes. He discovered several significant things: (1) the phases of Venus; (2) blemishes in the heavens { sunspots, lunar craters, lunar maria ("seas"); (3) Saturn had "ears"; (4) the four large satellites of Jupiter (the Galilean satellites). The smallest (Europa) is comparable in size to the Moon, the largest (Ganymede) is a little bigger than Mercury! The Galilean moons have densities that decrease with increasing distance from Jupiter and so the Galilean moons form a system that mimics the inner Solar System; (5) ... . All of these discoveries are interesting but here we consider only the Galilean moons (Io, Europa, Ganymede, and Callisto)

The Galilean moons are the four largest moons of Jupiter appearing like a miniature Solar System indeed obeying Kepler's Laws of Planetary Motion. We could calculate P2 and a3 using the above numbers, however things would not be obvious. Let's scale the results just as we did for the Solar System. For the Solar System, we chose to use the Earth as our preferred object. That is, we chose to measure the P's and a's in terms of the Earth's period and orbital size. Let's do something similar; we measure everything in terms of Io. To do so, we divide all periods by the period for Io, P = 1.769 days and all orbital sizes by the orbital size for Io, a = 421,600 km. This leads to As advertised, the Galilean moons obey Kepler's Laws of Planetary Motion.

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This implies that solar system formation is a natural process and, therefore, planetary systems are likely to be common.