Exoplanets Or extra-solar planets have recently been discovered. There are important to find to help fill in the Drake Equation that determines the probability of life existing elsewhere in the universe. What is a 'planet'? A planet is an object that has a mass between that of Pluto and the Deutrium- burning limit (0.015 solar masses) that forms in orbit around a star.

The formation of planetary systems

Finding Exoplanets Finding exoplanets is tough because they are very dim compared to the star that they orbit, and they are very close. Jupiter viewed from 5 pc away will just be separated by 1 arcsecond and is just times as bright! Therefore one away to detect the planet is by the gravitational pull of a planet around the star it orbits. As planets orbit a star they tug it causing the star to wobble. Bigger planets cause bigger wobbles so we can generally only observe the largest planets using this technique. The first exoplanet was observed in 1994 by detecting a wobble in the pulse of a pulsar!

The wobble can be seen directly in some nearby stars by measuring the shift in the stars position in the sky (astronometry) or be detecting a red-shift / blue-shift wobble due to the Doppler effect (radial velocity). Like eclipsing binary star systems exoplanets may be discovered by observing tiny dips in the lightcurve of a star Microlensing can also be used to spot the existence of a planet around another star

Nulling Interferometry We could find much smaller planets, and find out more about their nature if we could directly detect the light from them. One technique to do this is by nulling interferometry which uses an array of telescopes in space separated by millions of miles to improve the spatial resolution and hence separate out the light of the planet from the light of the star. It is known as nulling interferometry because by cancelling out lightwaves from the star it blocks all of the star's light allowing the planets light to be detected. From this we can observe its spectrum and hence see what its atmosphere is made of and possibly detect the presence of water – vital for life to exist.

So far we've found over 100 planets – mostly Jupiter sized or bigger