Artist's impression of a possible exoplanet.
(Image Courtesy of PPARC)

The search for exoplanets is one of the most exciting fields in astronomy and will perhaps one day answer the question of whether we are alone in the universe. Although searching for alien worlds dates back to ancient times the techniques needed to detect them have only recently been developed with the first exoplanet being discovered in 1992. We now know of over 250 extrasolar planets although only a small number of these are know to transit their star. These transiting planets are very valuable to scientists as their size and composition can be determined. It wont be until the space based search missions begin operation, however, that we will be able to detect 'Earth' like worlds which reside inside the 'Habitable zone' where liquid water can exist. It is on these worlds that we believe life may exist. The space missions include NASA's Space Interferometry Mission, Terrestrial Planet Finder and Kepler , ESA's Darwin, Gaia and (Eddington) and France's COROT.

Exoplanets are very difficult to detect because they don't emit any light of their own and are completely obscured by their extremely bright parent stars - normal telescope observation techniques cannot be used. This image shows Gliese 229B which is a Brown Dwarf  (a failed star slightly larger than a planet) which illustrates the problem and also demonstrates the power of space based observatories. In order to find exoplanets we use a variety of techniques to detect the effect they have on their stellar system. Below is a brief summary of the most common methods for finding extrasolar-planets:
 

Pulsar timing
Pulsars are neutron stars for which the magnetic and spin axes are misaligned. As pulsars rotate, flashes of radio waves are emitted like a light-house which reach the Earth at regular intervals. These radio flashes can be detected and timed. The intervals between pulses are so regular they are more accurate than an atomic clock. A planet orbiting this pulsar will cause very slight variations in the timing of these flashes which we can use to detect it. The very first exoplanet orbiting around Pulsar PSR B1257+12 was found using this technique in 1992.
Radial-velocity: (Image)
A planet orbiting a star exerts a small gravitational pull which causes the star to wobble very slightly about the system's centre of mass (barycentre). If the planet is aligned edge-on to the Earth we can observe this wobble as a 'Doppler' shift in the emitted light. As the star is pulled away from us its spectrum is shifted towards the red end and as it is pulled towards us it is shifted to the blue end. The gravitational pull from the planet is minute and so very accurate spectroscopic measurements are required. This is the most common type of detection technique and is responsible for the most planets to date. By measuring the radial velocity of a star it is possible to determine the exoplanet's orbital period but only a minimum mass (as the system's inclination is not known). It is also not possible to determine the size of these planets.
Astrometry: (Image)
This technique uses extremely precise measurements of stars' positions to detect the tiny shifts caused by orbiting planets. It is most effective for planets orbiting face-on where the positional motion is greatest - however, the measurements are very difficult to obtain.
Gravitational lensing:
According to Einstein's theory of relativity, massive foreground objects can bend the light from background objects by their gravitational pull. This bending of light causes a 'lensing' effect which magnifies the distant, background objects allowing the light-curves (and planetary transits) of distant systems to be observed. The OGLE (Optical Gravitational Lensing Experiment) group very successfully make use of this technique and have detected several planetary systems - see their website for more details.
Photometry: (Image)
This is the 'Transit' technique as used by SuperWASP. When a planet passes in front of its parent star edge-on, a decrease in the star's brightness can be detected. Periodic decreases in brightness can indicate the presence of a planet and measurements of the light-curve and spectral type of the star can indicate the size and orbital period of the planet. See the How it works section for more information. When combined with the Radial-velocity technique a large number of parameters can be accurately determined including the mass which can be used to infer the composition of the planet. The Spitzer space telescope has recently been used to study the surface temperatures and atmospheric composition of gas-giant planets, helping us to understand their formation and evolution.

An up to date list of discovered exoplanets can be found here (Exoplanets Encyclopedia).

 

Life on other worlds?

A large number of the exoplanets found so far are known as 'Hot Jupiters' - gas giant planets similar in size to Jupiter that orbit extremely close to their parent star. They are easy to detect because of their large size and short orbital period. However, they are unsuitable for hosting life. It is thought the most likely location for life to exist is on small 'Earth-like' rocky planets within a region of the stellar system known as the 'Habitable zone' where temperatures are suitable for liquid water to form.

Unfortunately these planets are extrememly difficult to detect. Within the next decade a series of space based observatories will be launched that will have sufficiently high resolution to detect possible life bearing 'Earth-like' worlds. Once found we can use the next generation of ground and space based telescopes to determine if they might hold life. The CoRoT space telescope is already in orbit and is predicted to detect a small number of terrestrials planets slightly larger than the Earth. More information of the individual missions can be found from their websites listed above.

Hubble image of possible exoplanet TMR-1C (courtesy of NASA)