An Extrasolar planet also called Exoplanet is defined basically as a planet, outside our solar system, which revolves around a star (like our Sun). As of July 31, 2009 there are exactly 357 exoplanets listed in Extrasolar planet Encyclopedia.
How do we find EXOPLANETS?
There are many different methods used to find Extrasolar planets. Basically planets are very faint light sources as compared to their parent stars so finding such planets are extremely difficult and in addition to this the light from the parent star causes a glare that causes problems while detecting exoplanets. For this reason alone very few exoplanets have been found directly. The various indirect methods used to detect exoplanets are as follows:
ASTRONOMY: It consists of accurately measuring a star’s position in the sky and keeping a track of that particular position over a period of time to see how it behaves. Earlier handwritten records were kept but in the later 19th Century new photographic plates are used which improve the credibility and accuracy many times over. In this an exoplanet is found by searching for planets revolving around stars in a small circular and elliptical orbit.
• Radial Velocity: In this method similar to astronomy, the fact that a star with a planet revolving around it will move in its own small orbit in response to the planet’s gravity is of main importance. The next goal now is to detect and measure variations in the speed with which this star is moving toward or away from Earth [Radial Velocity]. This radial velocity can be determined from the displacements of the spectral lines because of Doppler Effect. This method is also called Doppler Spectroscopy and is more productive than astronomy.
• Pulsar Timing: Pulsars are small ultra dense neutron stars which emits radio waves. In this method these pulsars revolving, simultaneously radiating waves are recorded and these timings of radiations are observed and plotted to find any planets attached to it.
• Transit Method: when a planet crosses or transits in front of its parent star’s disc, then the observed visual brightness of the star decreases by a small amount. This amount by which the star dims slightly depends on the size of the star and on the size of the planet. The main drawback of this method is that it requires confirmation by radial velocity method.
• Gravitational Microlensing: This phenomenon occurs when the Earth, a lens star, a distant star are in perfect alignment. This occurs when the gravitational field of a star acts like a lens, magnifying the light coming from a distant background star. Such lensing events are short, lasting for days or weeks, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years.
• Some other methods used prominently are Circumstellar discs, Eclipsing Binary, Orbital phase, Polarimetry etc. There are new scientific developments in the field of discovering Exoplanets.