To date, the most effective way to detect extrasolar planets has been the radial velocity or Doppler shift method. Using this technique, teams of astronomers measure the elemental spectra of individual stars with high precision, and any regular patterns of wobbles that appear in the spectral lines are analyzed as possible gravitational effects of planets in orbit around those stars.

In more specific terms: Starlight analyzed by a spectrograph reveals a pattern of vertical lines corresponding to the wavelengths of light that have been absorbed by the target star's constituent molecules. These so-called "absorption lines" in the stellar spectrum can exhibit variations over time, produced by the movement of the star. The term "radial velocity" signifies the speed at which an object is traveling toward or away from an observer (e.g., one located on or near the Earth) along the line of sight (i.e., radially). Stellar absorption lines shift toward the red end of the spectrum if the radial movement of the star carries it farther away from the Earth, and toward the blue end if the radial movement carries it closer.

A giant planet accompanying a target star on an approximately edge-on orbit will tug the star away from us when it passes behind the star and toward us when it passes in front. Sufficiently sensitive instruments can measure these minuscule variations, which can then be tracked over time and analyzed for regular patterns that would indicate a planetary orbit. See these examples of radial velocity curves for two different exoplanetary host stars, illustrating the wavelike patterns created by regular variations in each star's motion.

Three other methods have also been employed, with varying returns: pulsation timing, gravitational microlensing, and photometric transit observation.

• Pulsation timing can detect only planets in orbit around pulsating white dwarfs or pulsating neutron stars (pulsars), both of which are rare in the Milky Way Galaxy. To date, it has revealed two pulsar systems, the closer of which is 300 parsecs distant (almost 1000 light years), and one gas giant orbiting a white dwarf (GD 66) at a distance of 51 parsecs (166 light years).
  
  
• Gravitational microlensing seeks out chance line-of-sight relationships between extremely massive objects, such as galaxies or stars, and otherwise invisible and undetectable objects, such as planets. The alignment of the two objects creates a virtual lens that magnifies all electromagnetic wavelengths associated with the less massive object. These line-of-sight relationships or "microlensing events" are transient, offering a one-time opportunity for astronomical observations. So far this method has revealed about a half-dozen planets, all quite remote from the Solar neighborhood but spanning the mass range from gas giants to Super Earths.
  
  The most spectacular achievement of the various microlensing search programs, to date, has been the detection of a two-planet system known as OGLE-2006-BLG-109 (Gaudi et al. 2008). Located at an estimated distance of 1500 parsecs (almost 5000 light years), this system comprises a low-mass host star (.50 MSOL, similar to the nearby red dwarf GJ 849) and two gas giants orbiting outside the system's predicted ice line. With orbital separations of 2.3 and 4.6 AU, and masses of 0.71 and 0.27 MJUP, respectively, this pair has inspired comparisons to Jupiter and Saturn.
  
  
• Photometric transit observation seems even more promising, as it has characterized more than 50 extrasolar planets as of September 2008 (Charbonneau et al. 2007, Torres et al. 2008, EPE). In many cases it has contributed detailed measurements of planets that were previously discovered by radial velocity observations. In the transit method, the luminosity of candidate stars is measured over time to detect regular dips in brightness that might be caused by a planet crossing the face of the star. To date, however, it has provided data only on giant planets in very tight orbits. The smallest exoplanet so far observed by this method is GJ 436, a "Hot Neptune" orbiting a nearby red dwarf star (Gillon et al. 2007).
  

Although the radial velocity method has succeeded in locating more than 200 exoplanetary systems, many of which contain two or more planets, it also has significant limitations. It is not yet applicable to stars of spectral class A or the early range of spectral class F, because the spectra of such stars lack clearly defined absorption lines. Nor can this method analyze extremely young stars, for the same reason. Equally problematic is its propensity (shared with the transit method) to detect massive planets in short-period orbits.

The very first exoplanet detected through radial velocity analysis, 51 Pegasi b, illustrates its bias. The planet has a mass about half that of Jupiter, yet it orbits a Sun-like yellow star at an average distance or semimajor axis of only 0.05 astronomical units (AU) – equivalent to one-eighth of the average distance between Mercury and Sol. Accordingly, the planet’s “year” is just over four Earth days long. In 1995, when the discovery was announced, these bizarre statistics sent shock waves through the astronomical community, equivalent perhaps to the consternation produced by the first close-up photos of Mars in the 1960s. Even as the sample of exoplanetary systems has grown, the proportion of these so-called “Hot Jupiters” remains large. At least 30% of the known extrasolar planets are Hot Jupiters or Hot Neptunes orbiting within 0.1 AU of their primaries, while more than 40% of all extrasolar planets have orbital radii smaller than Mercury’s (see statistics at Exoplanets.org and Exoplanet.eu).

Last updated September 2008