Methods of detecting exoplanets
Any planet has an extremely faint light source compared to its parent star. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the extrasolar planets reported as of April 2014, have been observed directly, with even fewer being resolved from their host star.
Instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. At the present time, several different indirect methods have yielded success.
Established detection methods
The following methods have proved successful for discovering a new planet or detecting already discovered planet at least once:
Radial velocity
A star with a planet will move in its own small orbit in response to the planet's gravity. This leads to variations in the speed with which the star moves toward or away from Earth, i.e. the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. The radial-velocity method measures these variations in order to confirm the presence of the planet.
Transit
Transit may refer to:
Computing
Science and engineering
Transportation
Transit (astronomy)
The term transit or astronomical transit has three meanings in astronomy:
The rest of this article refers to the first kind of transit.
Theodolite
A theodolite /θiːˈɒdəlaɪt/ is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are used mainly for surveying applications, and have been adapted for specialized purposes in fields like meteorology and rocket launch technology. A modern theodolite consists of a movable telescope mounted within two perpendicular axes—the horizontal or trunnion axis, and the vertical axis. When the telescope is pointed at a target object, the angle of each of these axes can be measured with great precision, typically to seconds of arc.
Theodolites may be either transit or non-transit. Transit theodolites (or just "transits") are those in which the telescope can be inverted in the vertical plane, whereas the rotation in the same plane is restricted to a semi-circle for non-transit theodolites. Some types of transit theodolites do not allow the measurement of vertical angles.
The builder's level is sometimes mistaken for a transit theodolite, but it measures neither horizontal nor vertical angles. It uses a spirit level to set a telescope level to define a line of sight along a horizontal plane.
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