A little more than 400 years ago, Italian philosopher and astronomer Giordano Bruno theorized that the universe was filled with an infinite number of stars orbited by an infinite number of worlds. For that astounding insight and others he was branded a heretic by the Catholic Church and burned at the stake.
When the late Carl Sagan made similar statements before TV audiences in the 1980s, he was spared the stake, but not the smug laughter of some of his fellow astronomers. They noted that no planet had yet been found in any galaxy beyond our own solar system. Still, Sagan's words rang convincingly in the minds of the millions who watched his Cosmos series-- there must be billions and billions of stars orbited by billions of planets. The most romantic implication of this vision was clear: Of those billions of other planets, some had to fall in the Goldilocks zone of not-too-hot and not-too-cold, about the same distance from their suns as we are from ours. There might be other Earths. Redemption finally arrived in October 1995--a bit late for Bruno and nearly too late for Sagan, who died about a year later. A Swiss team announced evidence of gravitational tugs on the star 51 Pegasi, about 50 light-years from Earth. The cause had to be a planet orbiting the star. And there was a surprise: The planet was most likely a giant ball of gas of about the same mass as Jupiter but circling eight times closer to its star than Mercury's orbit around our sun. That made it very hot--and very strange. Of course, no one actually saw the planet circling 51 Peg. Detection was indirect. But the ball was rolling. With better instruments and more eyes trained on the skies, planet discoveries soon became routine. Still, a nagging doubt remained. The evidence seemed clear, but no one had actually laid eyes on a new planet.
Then, last November 7, planet hunters Geoff Marcy of the University of California at Berkeley, Greg Henry of Tennessee State University, Paul Butler of the Carnegie Institution in Washington, D.C., and Steven Vogt of the University of California at Santa Cruz finally got proof, from an object orbiting a star called HD209458, in the constellation Pegasus. When the planet passed in front of its star, it cast a shadow on Earth, producing a small but predictable dip in HD209458's brightness. The planet's mass was calculated at 200 times the weight of Earth. A month later, there was even better news: British astronomers announced they had spotted a faint blue-green hue of light reflecting off a hot, gassy giant planet known to orbit the star Tau Boštis .
