NASA defines an extrasolar planet as "a planet that orbits a star outside the solar system." NASA Jet Propulsion Laboratory: Planet Quest As of July 18, 2009, a total of 353 extrasolar planets have been discovered. Most of these planetary bodies have been detected through radial velocity observations and other indirect methods rather than actual imaging.

In November 2008, two groups of astronomers successfully photographed the first legitimate images of actual extrasolar planets (planets orbiting other stars). One group, led by Dr. Christian Marois, used a powerful telescope in Hawaii to capture images of planets orbiting star HR 8799, 130 light years away. The second group, led by Dr. Paul Kalas, used the Hubble Space Telescope to photograph a planet 25 light years away, orbiting the star Fomalhaut. Marois' team accomplished their feat by using a new method of optics that warps the telescope's mirrors and then uses software to render the images clearer.

Though over 300 extrasolar planets have been discovered, these were the first photographs to capture them. Extrasolar planets were previously discovered from drops in light from stars, ostensibly when a planet is passing in front of it.New York Time: First Pictures Taken of Extrasolar Planets (November 13, 2008)

1. Doppler Shift: Deduces a planet's mass and orbit by measuring the change in wavelength of the light that it reflects as it moves around its host star.European Space Agency: How to Find an Extrasolar Planet
2. Astrometric Measurement: Measures polarized light rays.European Space Agency: http://www.esa.int/esaSC/SEMYZF9YFDD_index_0.html How to Find an Extrasolar Planet]
3. Transit Method: Detects a periodic dip in brightness of a star that occurs when a planet moves between a star and an observer's direct line of sight. The observer calculates the orbit and size of the planetary companions from the period and depth of the transits. NASA Jet Propulsion Laboratory: Planet Quest
4. Gravitational Microlensing: Uses Albert Einstein's Theory of General Relativity to find planets by measuring the temporary sharp increase in brightness and change of the apparent position of the star. This occurs when a planet passes in front of a host star in an observer's line of sight NASA Jet Propulsion Laboratory: Planet Quest
5. Direct Detection: Gathering data during space missions and/or planet imaging.

Extrasolar planets do not receive names like the ones in our own Solar System. Some do have distinctive names like HD 209458 b (Osiris) and 51 Pegasi b (Bellerophon), but it is impractical because you cannot derive information from such names.Nomenclature: Planetary Nomenclature

On the other hand, referring to a planet like Gliese 581 c tells astronomers several key things. Lowercase letters follow the numbers for planets; uppercase letters are used for the stars. The letters are assigned based on the order of their discovery, not their positions. However, the letter "a" is not used, so Gliese 581 c is the second planet to be discovered in the Gl581 system.Wikipedia Planetary Nomenclature: [ http://en.wikipedia.org/wiki/Extrasolar_planet Wiki Planetary Nomenclature]

Nomenclature becomes more complicated, as shown by the planet PSR 1257+12 B. This planet, along with several others, are exceptions to the 'lowercase letter rule'.Nomenclature: Planetary Nomenclature