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 .
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 EarthAs early as 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."Also-ran Planets
Pulsar planets were the first worlds spotted outside our solar system, in 1991. Their suns are rapidly spinning neutron stars no more than six miles or so in diameter. They emit the energy of 10 suns in the form of deadly gamma rays, X rays, and other radiation. A pulsar's strong magnetic field focuses that energy into beams that sweep through the universe like a lighthouse signal. By measuring subtle variations in the arrival time of radio pulses from pulsars, astronomers are able to detect orbiting planets. At least three such uninhabitable planets lie in the constellation Virgo, 1,000 light-years from Earth; one is in the globular cluster M4, 5,500 light-years away.
Researchers have also detected what may be two planets using the gravitational microlensing technique: When an object like a planet or a star moves in front of a star, its gravity can act as a lens, bending and amplifying the star's light. Two planets detected this way orbit near the center of the Milky Way.
Finally, astronomers have caught telltale dips in the brightness of a pair of tightly orbiting red dwarfs in the constellation Draco. They suspect the dips are caused by a planet 2.5 times bigger than Earth that may be conducive to life. Roasters
In our own solar system, gas balls like Saturn, Jupiter, Uranus, and Neptune are frigid and far from the sun. However, outside our solar system, everything found so far seems to be reversed, with hot, gassy giants rotating precariously close to their parent stars. Because astronomers think none could have formed so near their suns, it's probable that they coalesced on the cooler edges of their planetary disks and then spiraled gradually inward. The very hottest ones, dubbed roasters by astrophysicist Adam Burrows of the University of Arizona, fly by just a few million miles from their suns, locked in corotation, with one side perpetually facing an inferno.
These are hellish worlds, with temperatures up to nearly 2,500 degrees Fahrenheit. Intense ultraviolet, X-ray, and charged-particle radiation heats their atmospheres. The view upward from the "surface" of these planets would be unlike anything on Earth. Clouds made of silicate would rain rock grains and iron droplets. Deeper within the planets, intense heat and pressure would turn hydrogen into a metal, and its convection would create a powerful magnetic field. Understanding Doppler
Planet hunters spot their prey by measuring tiny variations in light emitted by distant stars. As a planet orbits a star, its gravity tugs on the star, creating a slight wobble. When the star wobbles toward Earth, the light waves it sends our way are squeezed together like an accordion, causing a subtle shift toward shorter blue wavelengths. That's called a Doppler shift. When the star wobbles away, its light waves are stretched apart, shifting the spectrum toward red. The same effect makes a train's whistle rise in pitch as it approaches and then, as it hurries away, drop off to a low-pitched howl. With Doppler, astronomers can determine how long a planet takes to orbit its star, how far away it is, and what its minimum mass might be. They can also estimate temperature. The effects can't be measured unless a star is stable, limiting the number of candidates. Our sun's velocity is braked only 27 miles per hour by Jupiter's tugs. A planet the size of Jupiter will compress and expand the light from a star by about one part in 10 million, and plucking that signal out of the spectrum of a star that's trillions of miles away requires a precision of three parts in 100 million. Today's best instruments perform three times better, says astronomer Steven Vogt: "That's equivalent to detecting the change in the length of a two-inch ruler lying on a table vs. its length when standing on its end: It is shorter standing by 1/100,000,000 of its length, due to its own weight." Clear Skies
These planets rotate from 7 million to about 80 million miles from their suns. They are too cool to have silicate clouds, but too warm for water clouds. Gas giants, they range in temperature from 900¡ F down to a nearly tolerable 170¡ F, estimates modeler Burrows. They may have clear or hazy skies of sulfides and chlorides, including table salt. If such a planet orbits a star like ours, its red wavelengths might be absorbed by the atmosphere, and blues would scatter.
By Josie Glausiusz
In the beginning our solar system was a gigantic whirling disk of gas and dust surrounding a primitive sun. Solid minerals condensed out of the gas and clumped together to form proto-planets. Small ones like Earth emerged close to the center; giant planets, big enough to grab gases in the disk, formed further out. The orbits in which they were born, some 4.6 billion years ago, have remained the same ever since.
Until recently that was the accepted scenario. But now the detection of extra-solar planets has forced astronomers to re-examine such notions, because they present us with a paradox. Many are so monstrous in size, and hug their stars so closely, that they could not have formed in their present positions. The searingly hot stars around which they circle would have melted their rocky cores before they got started. Instead, it's assumed that they coalesced some distance away, then barreled inward over millions of years. And if such chaos characterizes the birth of extra-solar planets, could not similar disorder have reigned closer to home?
That's exactly what astronomers are proposing. Instead of staid and steady motion from the start, they see turmoil. During the early years of our solar system, they say, giant planets were born, bounced about, swung past one another, and were flung apart before settling into their present orbits. Computer modeling by Martin Duncan of Queen's University in Ontario, Canada, suggests that the massive icy planets Uranus and Neptune formed in close proximity to gassy Jupiter and Saturn, then barged past the behemoths into the far reaches of the solar system. There isn't enough matter that far from the sun for such planets to have grown so huge within the life span of the solar system.
Modeling by astrophysicist Philip Armitage of the Max Planck Institute for Astrophysics near Munich suggests that the emergence of a Jupiter-sized planet during the early years of a solar system can trigger chaos, birthing punier planets, then ejecting them in all directions. "The discovery of these extra-solar planetary systems has caused considerable change in our understanding of how planets form," says Armitage. "It suggests that the whole idea of planets moving around and migrating is definitely needed to explain these extra-solar systems. And that has motivated people to think about what it is in our own solar system that might benefit from similar explanations."
The Extrasolar Planets Encyclopedia is a compendium of the latest information about extrasolar planets and planet searches: www.obspm.fr/encycl/encycl.html.
To learn the latest from the planet search team of Geoff Marcy, Paul Butler, and their colleagues: www.physics.sfsu.edu/ ~gmarcy/planetsearch/planetsearch.html.