Planetary scientist Mike Brown has had plenty of practice finding objects at the fringes of the solar system. In sky images spaced an hour apart, he and his colleagues have identified several of the solar system's most distant denizens, revealed by their motion relative to the background of fixed stars. Early in 2004, soon after his team began using a new version of their discovery software, Brown was in his office at the California Institute of Technology in Pasadena reviewing images on his computer screen. He came across a sequence of pictures, taken by the Samuel Oschin telescope on Palomar Mountain near Escondido, Calif., showing a faint object moving so slowly that it must have been in the outer solar system, far beyond Pluto. "I think I fell out of my chair," says Brown.
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