Too bad the perfect one-liner had already been used. When the great Columbia University physicist I. I. Rabi was confronted with news of the muon, a wholly unexpected new subatomic particle, he asked in mock horror, Who ordered that? Astrophysicists reacted pretty much the same way when University of Geneva observers Michel Mayor and Didier Queloz stood up at a conference in October 1995 to announce they’d found something their colleagues had been seeking for decades--a planet orbiting a sunlike star.
The trouble was, nobody had ordered, or even imagined, a planet quite like the object circling 51 Pegasi, a star lying 50 light-years from Earth in the constellation Pegasus. For one thing, it is huge--about half the mass of
Jupiter. Yet despite this bulk, it orbits only some 5 million miles from 51 Peg--seven times closer than tiny Mercury orbits our sun--and whips through one orbit in a scant 4.2 days.
To appreciate how bizarre this behavior is, it helps to consider the bigger planets in our solar system--Jupiter, Saturn, Uranus, and Neptune. They are all at least a hundred times farther from the sun than 51 Peg’s planet appears to be. And it takes them years--a full dozen years, in the case of Jupiter--to make a single orbit.
