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John Johnson remembers the night he found his first blue lion. He was observing at the Subaru telescope on top of Mauna Kea in Hawaii. Earlier that evening, before he had driven up the rubble road to the volcanic summit, his collaborator Josh Winn had phoned from Massachusetts and joked, as he always did: "Tonight's the night we find a retrograde planet."

A retrograde, or backwards, planet is one that orbits its parent star in the opposite sense to the star's rotation. No planet in our solar system does this, and few astronomers seriously expected to find one around another star. "It's like going on a safari and stumbling across a blue lion," says Johnson, who is at the California Institute of Technology in Pasadena.

Yet ever since the first extrasolar planet was discovered in 1995, nature appears to have supplied us with green zebras, orange rhinos and purple buffalos. For a start, there are the hot Jupiters, gas-giant planets a hundred times closer to their parent star than Jupiter is to the sun. Then there are bloated planets far too big for their masses; planets more massive than Earth but less than Neptune, which are conspicuously absent in our solar system; and planets in orbits so elongated that they are more reminiscent of comets. "No one ever guessed that when we discovered other solar systems they would be so extraordinarily different from our own," says Johnson.

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