Extra-Solar Planets: Detection Methods and the Search for Life

Background and Definitions

The word planet means "wanderer" in Greek. It derives from the fact that planets within our Solar System appear generally to wander eastward among the so-called "fixed stars" across the zodiac constellations (Kolb). There is no clear consensus precisely defining what constitutes a planet, as distinguished from brown dwarfs—the material remnants of burned-out ancient stars whose masses were too small to form white dwarfs or to collapse completely into black holes, as stars much larger than ten solar masses tend to do (Hawking).

Generally, planets are defined as bodies that emit no light or other energy of their own, but orbit a star and reflect its light. A more technical definition relies on a planet's size relative to the mass of Jupiter, expressed in units called "Mjup's." According to this description, a planet is larger than Pluto and smaller than thirteen Mjup's—approximately the minimum mass of a body capable of radiating energy, either by nuclear reactions or by burning deuterium (Kolb).

There are three recognized types of planets among the nine bodies historically classified as planets in our Solar System. The four planets closest to the Sun—Mercury, Venus, Earth, and Mars—are known as terrestrial planets because they are solid. Jupiter, Saturn, Uranus, and Neptune are known as "Jovian" planets: giant, gaseous bodies far larger than the terrestrials. Pluto is the only one of the nine that falls within neither designation, partly owing to its much smaller size and partly because its orbit crosses the plane of Neptune's orbit, whereas the other eight planets all occupy the same orbital plane around the Sun (Sagan).

The Solar System and Pluto's Contested Status

Furthermore, Pluto seems to violate the observed rule that the smaller terrestrial planets lie much closer to the Sun than the massive, gaseous Jovian planets. Pluto is the smallest of all the known planets in our Solar System, yet it lies far beyond all the others. Pluto's moon Charon is also far larger in proportion to its host planet than any other moon in the Solar System. Finally, Pluto is composed mainly of ice, and it lies nearly far enough from the Sun to fall within the Kuiper Belt—the region from which most comets visible from Earth originate. Consequently, some astronomers have long maintained that Pluto's size, composition, distance from the Sun, and orbital inclination indicate it is more likely a dormant comet than a bona fide planet (Engelbert).

Five known planets within our Solar System are visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. The 1781 discovery of Uranus by William Herschel was accidental, and the planet was originally mistaken for a comet. Neptune is seventeen times larger than Earth, but so far away that it was discovered only indirectly, when several teams of astronomers suspected its existence because irregularities in the orbit of Uranus suggested the presence of another planetary body whose gravitational field could account for those observed irregularities (Engelbert).

Extra-solar planets—those lying outside our Solar System—cannot be detected directly by visual observation because their visible light is only a reflection of their host stars, whose light is up to ten billion times brighter than that of orbiting planets (Lemonick). Nevertheless, astronomers have developed powerful tools for locating and identifying suspected extra-solar planetary bodies both indirectly—in the manner that Neptune and Pluto were discovered "mathematically"—and directly, by examining data gathered by space-based telescopes across spectra beyond visible light. Infrared observation, for example, lowers the brightness ratio between a host star and its orbiting planet from ten billion-to-one to roughly ten million-to-one, making detection about a thousand times easier via infrared radiation than through the visible light spectrum (Lemonick).

The Search for Extra-Solar Planets

Despite obvious advantages, infrared observation also presents difficulties: interstellar dust clouds beyond Mars obscure infrared detection from Earth-based telescopes. Solutions include positioning telescopes in space beyond those dust clouds and building sufficiently large ground-based telescopes capable of resolving the very faint visible light from distant stars. Techniques intended to eliminate stellar glare—which obscures the much fainter radiation from orbiting planets—include sophisticated light-filtering systems, though many early implementations proved largely ineffective (Lemonick).

In 1995, scientists at the Geneva Observatory in Switzerland announced the first confirmed detection of an extra-solar planetary system approximately 35 to 45 light years from Earth, orbiting the star known as 51 Pegasi in the constellation Pegasus, using sophisticated spectroscopic techniques. The most surprising aspect of this discovery was the planet's proximity to its host star, because what was known about planet formation within our own Solar System seemed to preclude the formation of such a large gaseous planet so close to its sun (Butler). Other planetary bodies within the new system were detected shortly thereafter by astronomers in San Francisco, who used conventional optical telescopes to measure spectral changes in the visible light radiated from the host star (Sagan).

The spectroscopic technique relies on the Doppler effect, which enables scientists to measure changes in the motion of distant stars. The technique employs the same principle that accounts for the changing pitch of a car horn or siren: the frequency of sound waves varies depending on whether the source is moving toward or away from the listener. In astronomical observations, the Doppler effect is applied to the visible light from distant stars. Certain variations in spectral shifts indicate the presence of massive bodies orbiting the stars under observation, and can yield preliminary data about their size and composition (Sagan).