“Not too long ago, it was easy for an armchair astronomer to keep up to speed on the moons of the solar system. There was the Moon, of course, and the four Jovian satellites spotted by Galileo, those two around Mars, and some odd ones here and there — that weird fractured cue ball orbiting Uranus, for instance.

These days, though, it is tough to tell the moons without a scorecard. In the past six years, dozens of satellites have been discovered around the giant planets, more than doubling the total in the solar system. Jupiter is the current leader, with 61, followed by Saturn, Uranus and Neptune. The tally for these four planets is 124 (the other five planets have only four among them), but that number is sure to change in the next year or two.

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(S)cientists say these moons offer some of the only clues to the early years of the solar system. They are a window into the past, some 4.5 billion years ago, when the planets formed from a swirling nebular disk of gas and dust.”