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The Planet X Saga: Science
The fact is, as Tom points out, that thousands of amateur astronomers around the world do things like detect comets when they're still outside Jupiter's orbit. Comets are just a few miles in diameter. If some planet, supposedly 4 times the size of the Earth, were only a year out, it'd be easily detectable by any amateur astronomer. Compared to a comet, this planet would be huge and easily visible. It couldn't be hidden by NASA or any other government agency, especially in the Internet age.