The word planet means "wanderer" in Greek. It derives from the fact that planets within our solar system seem generally to wander eastward about 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, which are the material remnants of burned out ancient stars whose masses where too small to form white dwarfs or collapse completely, forming black holes in the manner that stars much larger than ten solar masses, or ten times the mass of our sun
Generally, planets are defined as a body that emits no light or other energy of its own, but orbits a star, reflecting its light. A more technical definition of a planet relies on its size relative to the mass of Jupiter or "Mjup's." According to this description, a planet is larger than Pluto and smaller than thirteen Mjup's, which is approximately the minimum mass of a body that is capable of radiating energy, either by nuclear reactions or by burning Deuterium (Kolb).
There are three types of planets within the nine known planets within our
Solar System. The four planets closest to the Sun are Mercury, Venus, Earth and Mars, known as terrestrial planets because they are solid; Jupiter, Saturn, Uranus and Neptune are known as "Jovian" planets, which are giant, gaseous bodies much larger than the terrestrials. Pluto is the only one of the nine known planets that falls within neither designation, partly owing to its much smaller size and partly because its orbit crosses the plain of that of Neptune's, whereas the other eight planets all occupy the same plane about the sun. Furthermore, Pluto seems to violate the other observed rule that the smaller terrestrial planets are 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 much farther away than the other eight. Pluto's moon Charon is also far bigger in proportion to Pluto than any other moon in relation to its host planet
Sagan). Finally, Pluto is composed mainly of ice, which combined with the fact that it is nearly far enough from the Sun to fall within the Kuiper belt, where most comets (which primarily consist of ice) visible from Earth originate. Consequently, some astronomers have always maintained that Pluto's size, composition, distance from the Sun and orbital plane indicate that it is more likely a dormant comet rather than a bona-fide planet (Engelbert).
The Search for Extra-Solar Planets:
Five known planets within our Solar System are capable of being viewed with the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. The 1781 discovery of Uranus by William Herschel was an accident, and originally mistaken for a comet.
Neptune is seventeen times larger than Earth, but so far away that it was discovered only indirectly, when several different teams of astronomers suspected its existence, merely because irregularities in the orbit of Uranus suggested the existence of another planetary body with a gravitational field sufficient to explain those observed orbital irregularities (Engelbert).
Extra-solar planets, or 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 planets orbiting them (Lemonick). Nevertheless, astronomers have developed other powerful tools for locating and identifying suspected extra-solar planetary bodies indirectly -- in the manner that Neptune and Pluto were discovered "mathematically" -- as well as directly, by examining data gathered by extraterrestrial telescopes along other spectra besides visible light. Infrared, for example, lowers the ratio between host and orbital planet radiation from ten billion-to-one to ten million-to-one, making the detection of extra-solar planets a thousand times easier via infrared radiation than by the visible light spectrum (Lemonick).
Despite some of the obvious advantages to using infrared radiation instead visible light, the former also presents difficulties that must be broached, such as the interstellar dust clouds beyond Mars that obscure infrared detection from Earth-based telescopes. Solutions to this particular problem include positioning telescopes in space beyond the range of the interstellar dust clouds and building sufficiently large terrestrial telescopes capable of resolving the very faint visible light from distant stars.
Techniques intended to solve the problem of eliminating the stellar glare that obscures the much fainter radiation from their orbiting planets include sophisticated light filtering systems, which were largely ineffective (Lemonick).
In 1995, scientists at the Geneva Observatory in Switzerland announced the first confirmed detection of an extra-solar planetary system approximately thirty- five to forty-five light years from Earth, orbiting star number 51 in the Constellation
Pegasus via sophisticated spectroscopic techniques. The most surprising aspect of this discovery was the proximity of the planet to its host star, because what we know about planet formation within our Solar System seems to preclude the formation of such a large gaseous planet so close to its sun (Butler). Other planetary bodies orbiting stars within the new planetary system were detected shortly thereafter by astronomers at San Francisco using conventional optical telescopes to measure spectral changes in the visible light radiated from the host star about which suspected planets orbit (Sagan).
The particular technique relies on the Doppler effect, which enables scientists to measure changes in the motion of distant stars. The technique employs the exact same principle that accounts for changes in the audible pitch of the sound waves emitted by a car horn (or siren) which varies in frequency and changes depending on whether its direction of travel is toward or away from us. In the case of astronomical observations, the Doppler effect is applied to the visible light from distant stars.
Certain variations in these spectral changes indicate the presence of massive bodies orbiting the stars under observation, even yielding preliminary data as to their size and composition (Sagan).
In 2000, astronomers at Princeton University experimented with variations of known technique that used multiple telescopes trained on a star simultaneously, which has the effect of nullifying the star's glare to some degree. The multiple scope or multiple mirror) principle led to the development of a much simpler method of achieving similar results using a mask in the shape of a cat's eye over an ordinary telescope. The mask filter generates an interference pattern (familiar to quantum physicists studying wave-particle duality) which can be reflected onto a film. In the case of the pattern generated in this manner, the reflected pattern is called an Airy pattern. The Princeton team found that a system employing opaque filters and the cat's eye slit results in a method of identifying possible planets within the dark areas of the reflected Airy pattern generated in that fashion (Lemonick).
The most recent advances in locating extra-solar planetary objects makes use of gravitational lensing, a technique suggested by Albert Einstein's General Relativity and exploited for years by astronomers analyzing the light from distant galaxies. In principle, gravitational lensing refers to the warping of space around any sufficiently massive object. In some instances, the lensing effect magnifies the visible light emanating from a very distant source in the same manner as a lens within a telescope, although the effect is a temporary one that depends on the proper alignment of stars and planets (Engelbert).
A derivative technique called gravitational microlensing was also first suggested as a way of finding extra-solar planets by astronomers at Princeton
University in 1991, but successfully employed for he first time only this year.
According to the Princeton researchers, appropriate microlensing opportunities arise in the field of extra-solar planet detection in connection with only one out of 100 million stars, and the effect typically lasts only a few minutes at a time (Reuters).
In 2004, the Princeton team used the gravitational microlensing technique to discover a large planet slightly larger than Jupiter approximately 17,000 light years away from Earth in the constellation Sagittarius by analyzing the light from an even more distant star that was magnified by the lensing effect of the newly discovered planet's gravity in conjunction with that of its host star (Reuters).
Earthlike Extra-solar Planets:
Ultimately, scientists hope to determine how common (or how rare) a phenomenon within the universe it is that planets exist that seem potentially capable of supporting the carbon-based life forms similar to life on Earth. Even with the many trillions of known stars comprised within the billions of galaxies in the universe, only an exceedingly small number of planets orbiting those stars are potentially capable of supporting anything that we might recognize as biological life.
The reason is the extreme scarcity of stars that exist for the necessary period of time to enable life to develop on nearby planets. Stars much more massive than our sun, for example, burn out or explode in powerful supernova explosions much too soon for life to evolve as it presumably did over billions of years on earth (Davies).