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Field Guide To New Planets - astronomers discover new planets
Discover, March, 2000 by Kathy A. Svitil
Amazing worlds beyond our own solar system
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.
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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 Bootis.
As of January, astronomers have confirmed 29 worlds around sunlike stars, along with a host of promising candidates. Three of those planets orbit a single star--the first discovery of another solar system. Astronomers have found hot planets, cool planets, planets orbiting yellow stars, planets orbiting red stars, planets orbiting two stars at once. Most intriguing of all, they've found planets occupying the not-too-hot and not-too-cold zone, planets that may be habitable or have habitable moons. Carl Sagan was right, and astronomers now expect to announce a new world every month or so.
Water-Cloud Worlds These are the coldest of the gas planets found so far, and the most like those of our own solar system. Although many fall within the tantalizing liquid-water habitable zone-the region around a star where liquid water could theoretically exist-none of these are thought to be able to support life. However, any moons they have might be habitable. Like Jupiter, these planets would have three decks of clouds: ammonium sulfide at the top of the atmosphere, then a tier of ammonia clouds, and a layer of water, water vapor, and ice clouds. The clouds probably would make the planets reflective-perhaps as much as Venus. Some mixing between the atmosphere layers is possible, as are banding, winds, cyclones, and anticyclones.
Finding Another Earth As early 2011, NASA hopes to launch what may be the most ambitious telescope ever conceived: the Terrestrial Planet Finder. Scientists hope it can be used to answer the question of whether life exists on planets beyond our solar system. "When you're asking the greatest question ever, you need a great telescope," says Charles Beichman, project scientist for the telescope. Planet Finder will consist of a football-field-sized array of four massive telescopes and a mother ship. Each telescope will train its powerful infrared eyes on a star within 50 light-years of Earth, filter out glare, and scan for pinpoint images of individual planets. Light from each of the telescopes will be beamed to the mother ship and combined into a single high-resolution image. "We'll be able to take a snapshot of the system and see individual planets orbiting around a star," says Beichman, who works at the Jet Propulsion Laboratory. The telescope won't be able to spot continents and certainly not any little green men. But its spectrometers will be able to sniff out the presence of atmospheric gases like ozone that, on Earth at least, are linked to life. "If life is an inevitable outcome of physics and chemistry, then we ought to be seeing something if we scan two or three hundred stars," Beichman says. "If we don't see anything, then maybe life is much more rare."
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