IntroductionEarly in the 17th century, two astronomers competed to describe the nature of our solar system.Galileo built a telescope and found new planets and moons. Francesco Sizi ridiculed Galileo's findings. There must be only seven planets, Sizi said. After all, there are seven windows in the head - two nostrils, two ears, two eyes, and a mouth. There are seven known metals. There are seven days in a week, and they are already named after the seven known planets. If we increase the number of planets, he said, the whole system falls apart.

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