Planets Beyond Our Solar System

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Planets Beyond Our Solar System

Overview

Until the 1990s, no one had evidence of planets around Sun-like stars anywhere in the universe, except for Earth and the other eight planets circling the Sun. Then in October 1995, Michel Mayor and Didier Queloz of the Geneva Observatory changed history. They located a planet orbiting 51 Pegasi, a star located just 40 light years away from Earth. Shortly thereafter, additional astronomers chimed in, announcing evidence for other extrasolar planets. The announcements triggered a great deal of excitement in the scientific community and among the public as well. The announcements finally brought confirmation that the nine planets within our solar system are not the only ones in the universe, and begged the question, "Is there life out there?"

Background

Astronomers had long suspected that Sun-like stars had orbiting planetary systems, but they never had data to back up their assumption. They wondered whether planets were common to other stars or rare, whether other planetary systems were similar to our own, and ultimately whether Earth was the only planet in the universe that could support life as we know it.

In 1995 Michel Mayor and Didier Queloz of the Geneva Observatory announced that they had collected data verifying the existence of a planet around a star other than the Sun. The star, 51 Pegasi, is visible from Earth with the naked eye in the constellation Pegasus. The planet is at least half as large as Jupiter, and orbiting 51 Pegasi so closely that a full revolution takes only four days. With a proximity of 7 million kilometers (4,350,000 miles), the extrasolar planet travels much nearer to 51 Pegasi—closer than Mercury's trek some 58 million kilometers (36 million miles) from the Sun. Scientists have estimated the planet's surface temperature at 1,300° Celsius (2,372° Fahrenheit), much too high to support life.

Although some astronomers were skeptical, most doubts were erased when Geoffrey W. Marcy and R. Paul Butler of San Francisco State University and the University of California at Berkeley, verified the planet and shortly thereafter reported evidence of four others. Two of the planets have masses about 80 percent that of Jupiter. One orbits at 25 million kilometers (16 million miles) from its parent star and the second at about 14.5 million kilometers (9 million miles). Another of the planets is at least twice as large as Jupiter and orbits at a much less extreme 300 million kilometers (186 million miles) from its star. The fourth is the largest: a likely gaseous planet at least 6.5 times larger than Jupiter and with a great elliptical orbit around its star, 70 Virginis, in the constellation Virgo.

The evidence for each of these four planets came from measurements of the motion of the "parent" stars. According to these astronomers, the gravitational pull of the orbiting planets causes the stars to exhibit a wobbling phenomenon, which is visible from Earth as a so-called Doppler shift in the star's spectrum. They assert that this method of detection can determine the presence of a planet, its size, and its orbit.

In 1996 George Gatewood of the University of Pittsburgh, announced another extrasolar planet using a different technique but the same basic idea. Instead of scanning spectral data, he sought out the Doppler shift by watching for tiny aberrations in the path of the star Lalande 21185. Gatewood's first planet is about the same size as Jupiter, with an orbit of about 300,000 million kilometers from its parent star. The Earth, in contrast, maintains a distance of about 150 million kilometers (93 million miles) from the Sun.

During the following year astronomers continued to report extrasolar planets, including a Jupiter-sized planet in the Northern Crown constellation. The scientists suggested that other planets might share the parent star, called Rho Coronae Borealis, giving hope that a planetary system would soon be discovered. In 1999 Marcy, along with Debra Fischer of San Francisco State University, identified a multi-planet system orbiting the Sun-sized Upsilon Andromedae in the constellation Andromeda. Harvard's Robert Noyes confirmed the discovery. The three-planet system brought the total aggregate of extrasolar planets to about two dozen, a number astronomers believe will increase dramatically in the coming years.

Along with the rapid succession of planet discoveries, a scientific team led by Jane Luu of the Harvard Smithsonian Centre for Astrophysics in Massachusetts reported in 1997 that even our own solar system might contain other planets. The researchers found a 480-kilometer-wide (300 miles) object orbiting the Sun from beyond the orbit of Neptune. This miniature "planet" is larger than the comets orbiting the Sun in the Kuiper Belt, but much smaller than the system's nine known planets. Even the smallest planet, Mercury, is more than 10 times as wide as the newfound object.

Impact

The discoveries of extrasolar planets confirmed that our solar system is not unique, and apparently not even rare. These discoveries, coupled with data collected in recent years about the universe and the galaxies within it, are helping astronomers to understand how the cosmos evolved, and specifically how the Earth came to be. The discoveries have also sparked the imaginations of scientists and the lay public who wonder if life exists beyond Earth.

When Mayor and Queloz announced evidence of the first planet orbiting a Sun-like star, and other astronomers began reporting additional planets, the excitement in the scientific community was palpable. In an article in the May 1996 issue of Scientific American, Alan P. Boss of the Carnegie Institution of Washington, declared, "It is the most exciting thing I've seen in my scientific career."

The earliest discoveries described large planets mainly orbiting very close to their parent stars—a pattern much different than our solar system. Large planets with tight orbits didn't mesh well with current planet-formation hypotheses, and gave astronomers new food for thought. Some scientists proposed that the large planets were formed further from the star, but were actually migrating inward until finding stable orbits closer to the parent star. Since the discoveries, astronomical theorists have been busy reviewing data and analyzing potential planet-forming strategies to figure out what options are available in the creation of extrasolar systems.

A great deal of research also began to determine whether any of the planets could harbor life. The large planets with close orbits were poor candidates. By traveling so closely to their parent stars, their surface temperatures would be much too high to support life. The planet around 51 Pegasi, for example, actually orbits within the star's outer corona where temperatures are almost eight times that required to boil water. Planets with extreme elliptical orbits were also poor candidates, because their temperatures would vary too greatly as they circled their parent stars.

Some of the other planets, including those orbiting 70 Virginis in the constellation Virgo and 47 Ursae Majoris in Ursa Major (which contains the Big Dipper), are cool enough to allow liquid water to persist. Similar to Jupiter, these two planets are mostly gaseous, leaving little probability for the existence of life like that on Earth. The moons, however, are another story, according to Marcy, who discovered the planets. In the January 29, 1996, issue of Time, he remarked, "If they are comparable in size to the moons of Jupiter and Saturn, they could easily have rain and oceans." In the view of most scientists, water extends at least the possibility for life.

In late 1999 a research team using the Keck I Telescope in Hawaii reported finding five giant gaseous planets with similarly temperate climates. The researchers likewise suggested that their moons might support life-giving conditions.

The very presence of extrasolar planets also brings up the possibility that extrasolar systems exist. The Doppler-shift technique favors the discovery of larger planets, because their gravitational pull is strong enough to create the parent star's "wobble" that tips off astronomers to an orbiting planet. Many astronomers believe that some of the newly found planets could be part of larger systems, and that they may be part of systems with a half dozen or dozen planets.

In 1998 David E. Trilling and Robert H. Brown of the University of Arizona, detected what they believe is a Kuiper Belt around a star. They studied the star 55 Rho1 Cancri, which has an orbiting planet, and located the dusty ring of comets at about the same distance from the star as our solar system's Kuiper Belt is from the Sun. They contended that the belt suggests the system contains more than one planet.

The following year, another scientific team found evidence of the first extrasolar system surrounding a Sun-like star. Again using the Doppler-shift technique, the researchers found evidence of three planets orbiting Upsilon Andromedae in the constellation Andromeda. The scientists scrutinized the star's erratic "wobble" and found it was actually the combined effects of three large planets tugging at the star.

These discoveries have prompted the National Aeronautics and Space Administration (NASA) to begin its own investigations. NASA is planning to launch a five-year mission to search for planets as part of its medium-class Explorer (MIDEX) program. Scheduled for launch in 2004, the Full-Sky Astrometric Mapping Explorer is a space telescope that will check some 40 million stars for large, orbiting planets.

As the planetary discoveries continue, they are providing scientists with unexpected and enlightening insights into the formation of planets and planetary systems, and injecting a new air of excitement into the question: "Are we alone?"

LESLIE A. MERTZ