In 1584, when the Catholic monk Giordano Bruno asserted that there were "countless suns and countless earths all rotating around their suns," he was accused of heresy. But even in Bruno's time, the idea of a plurality of worlds wasn't entirely new. As far back as ancient Greece, humankind has speculated that other solar systems might exist and that some would harbor other forms of life.
The Earth was dethroned as a supreme entity in the cosmos early in the 16th century, when Copernicus discovered that our planet orbits the sun. His insight, while reluctantly accepted, changed Western thinking forever.
At the dawn of the 20th century, Edwin Hubble, using what was then the largest telescope in the world atop of Mount Wilson, found that the small nebulae in the sky were neighboring islands of stars far outside our own galaxy, each containing hundreds of billions of stars.
Hubble's observations proved that the potential havens for habitable planets are immeasurable in number. Still, almost an entire century went by without convincing proof of planets around even the nearest stars. On several occasions, discoveries of such extrasolar planets were announced, only to be repudiated.
Hope and Disappointment
Because planets are too small and distant to be observed directly, astronomers have sought to discern their existence by detecting their effects on the host star. During the late 1960s, astronomer Peter van de Kamp claimed to have detected two planets using this technique. However, subsequent observations failed to verify the existence of either companion around Barnard's Star, the second nearest star system to the sun.
Prospects for finding new worlds around other stars brightened in the 1980s when Dr. Bradford A. Smith of the University of Arizona in Tucson and Dr. Richard J. Terrile of the Jet Propulsion Laboratory made infrared observations of a disk of dust surrounding the normal star Beta Pictoris.
Their discovery provided the first unambiguous proof that flattened disks of matter exist around stars other than the sun. The Beta Pictoris disk appeared to be a young planetary system in the making and thus supported the standard model of solar system birth, which supposes that planets accrete from a disk of dust and gas surrounding a young star.
Truly Alien Worlds
The first true extrasolar planet discovery came in 1994, when Dr. Alexander Wolszczan, a radio astronomer at Pennsylvania State University, reported what he called "unambiguous proof" of extrasolar planetary systems.
While scientists accepted his assessment, those hoping for evidence of planetary systems similar to our own were less than elated. Wolszczan had discovered two or three planet-sized objects orbiting a pulsar, rather than a normal star, in the Virgo constellation. A pulsar is a dense, rapidly spinning remnant of a supernova explosion.
Wolszczan made his discovery by observing regular variations in the pulsar's rapidly pulsed radio signal, indicating the planets' complex gravitational effects on the dead star.
The origins of Wolszczan's unexpected pulsar planets remains a matter of debate, but there is little controversy on one point: These worlds couldn't support life as we know it. These planetary companions would be permanently bathed in high-energy radiation, leaving them barren and inhospitable.
An Onrush of Discovery
The first discovery of a planet orbiting a star similar to the sun came in 1995. The Swiss team of Michel Mayor and Didier Queloz of Geneva announced that they had found a rapidly orbiting world located blisteringly close to the star 51 Pegasi. Their planet was at least half the mass of Jupiter and no more than twice its mass. They had observed it indirectly, using the radial velocity method. (For information about radial velocity detection, see "Finding Planets.")
These announcements marked the beginning of a flood of discoveries. Three months later, a team led by Geoffrey W. Marcy and Paul Butler of San Francisco State University and the University of California at Berkeley confirmed the Swiss discovery -- and turned up two more planets. By the end of the 20th century, several dozen worlds had been discovered, many the result of months or years of observation of nearby stars.
Many, like the companion to 51 Pegasi, are bizarre, with short periods and eccentric orbits close to the star. But more recently, astronomers have found planets that more closely resemble those in our outer solar system, with circular orbits and longer orbital periods.
Astronomers attribute the abrupt surge in discoveries, in part, to technological advances in recent years. These include:
- Significant improvements in spectrometers, instruments that separate starlight into its component colors for analysis.
- Better electronic sensors that record the incoming starlight collected by telescope optics.
- The development of computer software that can reliably discern fluctuations in starlight and the motion induced by the gravitational pull of unseen companions.
Furthermore, the maturation of these technologies has led to intensified searches and data gathering.
A New Era of Exploration
None of these new worlds has actually been seen. The majority are massive, Jupiter-class planets, considered unlikely to harbor life as we know it. Many have short orbital periods. If planets like Earth exist, with smaller masses and longer orbital periods, their discovery will require more sensitive instruments and years of precise, sustained observations.
Nonetheless, the dream of other worlds waiting to be explored -- and the idea that our solar system is not unique -- has moved from philosophical speculation into reality. These discoveries harbor the potential to shift human thinking on a scale comparable to the Copernican revolution.
The next chapter in the brief history of extrasolar planet discoveries is already being written. New tools and visionary technologies currently in development will soon enable us to learn more about these nearby planetary systems. Within the next few years, missions such as NASA's Kepler and SIM PlanetQuest are expected to provide firm data on the existence or prevalence of Earthlike worlds.
