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.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.
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.
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
(Optical Gravitational Lensing Experiment)
group very successfully make use of this technique and have detected several planetary systems -
see their website for more details.
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.
An up to date list of discovered exoplanets can be found