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Planets, planets everywhere

Up until the late 1990s, astronomers couldn't be certain that any planets existed outside our solar system. These days, not only are astronomers confident that planets are out there, but new ones are being discovered all the time.
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The origins of proof II : Kepler's proofs

Johannes Kepler (1571-1630) is now chiefly remembered as a mathematical astronomer who discovered three laws that describe the motion of the planets. J.V. Field continues our series on the origins of proof with an examination of Kepler's astronomy.