A local example of this phenomenon was the transit of Venus across the face of the Sun in June 2004 and July 2012. The amount by which the star dims depends on its size and on the size of the planet. This method has emerged as one of the prevailing techniques to search for exoplanets. As an exoplanet transits in front of its parent star, the observed brightness of the star drops by a very small amount. This method has been by far the most productive method of discovering exoplanets.Īnother popular and effecitve method of detecting exoplanets is the Transit Method which measures the faint dip in brightness of a star when a planet transits the star (passes in front of it as observed from earth). This is similar to the Doppler effect we observe in sound waves when a fire-engine passes us and the pitch of its siren changes! The Anglo-Australian Planet Search was a long-term program that searched for giant planets around more than 240 nearby solar type stars and as of 2010, discovered more than 30 exoplanets using the Doppler method. If the motion of the star is towards the observer, then the received wavelengths are shorter than those emitted by the source, and longer if the motion is away from the observer. Variations in the star’s radial velocity can be deduced from displacements in the star’s spectral lines due to the Doppler effect. This method requires the measurement of the velocity of a star’s centre of mass. One method that is very productive is the Radial Velocity method. However it’s expected that astrometric accuracy from ground-based telescopes will improve and become more useful. As the motion of the star is so small, this method has not yet been very productive in detecting exoplanets. Finding Earth-mass planets by astrometry requires extreme (sub-microarcsecond or 1 millionth of an arcsecond!!) precision. If the star has an orbiting planet or planets, then the gravitational influence of the objects will cause the star to move in a tiny circular or elliptical orbit around the common centre of mass.

I will give an overview of each technique:Īstrometry is a technique that requires astronomers to precisely measure a star’s position in the sky, and then make more observations of the stars movement over time. Each method of observation has its pros and cons, is used in different circumstances, and produces different results. There are a number of methods used to detect exoplanets including astrometry, the transit method, radial velocity, gravitational microlensing, pulsar timing, eclipsing binaries, circumstellar disks and coronagraphy. These are certainly exciting times for astronomers, but just how do astronomers search for exoplanets? At this stage it appears that KOI-172.02 is an Earth-like planet candidate orbiting a star similar to our own sun. It almost seems like old news when scientists announce the discovery of a planet orbiting another star in our galaxy!Īs at Janua total of 859 such planets have been identified (details can be found here). It seems like another day goes by and there’s another discovery of more exoplanets! On January 7, just one week into 2013, astronomers from the Kepler Mission Space Observatory announced the discovery of the latest exoplanet, the creatively named KOI-172.02.

Astrometry vs. doppler method