Extrasolar Planets
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What is an extrasolar planet?

An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of 763 such planets (in 611 planetary systems and 101 multiple planetary systems) have been identified as of April 14, 2012. Estimates of the frequency of systems strongly suggest that more than 50% of Sun-like stars harbor at least one planet. In a 2012 study, each star of the 100 billion or so in our Milky Way galaxy is estimated to host "on average ... at least 1.6 planets." Accordingly, at least 160 billion star-bound planets may exist in the Milky Way Galaxy alone. Unbound free-floating planetary-mass bodies in the Milky Way may number in the trillions with 100,000 objects larger than Pluto for every main-sequence star.

For centuries, many philosophers and scientists supposed that extrasolar planets existed, but there was no way of knowing how common they were or how similar they might be to the planets of the Solar System. Various detection claims made starting in the nineteenth century were all eventually rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Due to improved observational techniques, the rate of detections has increased rapidly since then. Some exoplanets have been directly imaged by telescopes, but the vast majority have been detected through indirect methods such as radial velocity measurements.

Most known exoplanets are giant planets believed to resemble Jupiter or Neptune. That reflects a sampling bias, since massive planets are much easier to observe. Some relatively lightweight exoplanets, only a few times more massive than Earth (now known by the term Super-Earth), are known as well; statistical studies now indicate that they actually outnumber giant planets while recent discoveries have included Earth-sized and smaller planets and a handful that appear to exhibit other Earth-like properties. There also exist planetary-mass objects that orbit brown dwarfs, and there exist others that "float free" in space not bound to any star, however the term "planet" isn't always applied to these objects.

The discovery of extrasolar planets, some of which orbit in the habitable zone where life as we know it can exist, have intensified interest in the search for extraterrestrial life. Thus, the search for extrasolar planets also includes the study of planetary habitability, which considers a wide range of factors in determining an extrasolar planet's suitability for hosting life.

Source: http://en.wikipedia.org/wiki/Exoplanets