Extrasolar planets

Extrasolar planets or exoplanets are planets that orbit stars other than the sun.

Planets have no light source of their own, and most extrasolar planets are too far away to be seen by the light they reflect from their own suns, however some have been detected by examining light being blocked from the star that is being orbited.

Methods of detection

Astrometry

A star and planet orbit about a spot that is the center of gravity of the system known as the barycenter. For the Sun and Jupiter, the barycenter is 1.068 solar radii. With larger planets orbiting closer to the star, it is possible to detect the movement of the star against the distant background. This method was used to detect a planet orbiting Gliese 876.

Radial velocity

Instead of mapping the position of the star against the background, it is also possible to detect the minute Doppler shift in the star's spectrum as it is tugged by the planet. This method is responsible for the majority of extrasolar planet detection.

Transit method

When a planet passes in front of its star, it blocks some of the light of the star. By measuring this periodic momentary dimming of the light, it is possible to identify a planet that is properly aligned. This method does not work if the planet itself does not eclipse the star from our perspective.

Direct imaging

In the situation where a star is not extremely bright, it is possible to detect a planet orbiting it. Such was the case with the brown dwarf 2M1207 which had a detectable companion that was named 2M1207b.

Current count

As of 2007, the number of extrasolar planets detected stands at more than 200.