Extra-solar planets (or exoplanets) are planets orbiting stars other than the Sun. The first of these was detected in 1995 and we now know of the existence of more than 400 exoplanets, most of them within a 300 light-year radius of the Sun
The smallest of these are a few times larger than the Earth, and many are several times larger than Jupiter. This is likely to be a selection effect, however – more massive planets are currently easier to detect than small ones.
How are they detected?
Until recently it has not been possible to directly see planets as they are at great distances and usually outshined by their parent star, so other ways of detecting them are used. The most successful methods have been to measure the dip in a star’s brightness as a planet moves in front of it, or by detecting the “wobble” in a star’s position caused by a planet’s gravitational pull – we receive light from the star at a longer wavelength as it moves slightly away from the Earth and at a shorter wavelength as it moves towards us; the amount by which the wavelength changes can be used to calculate the mass of the planet.
Could they support life?
It is thought that life requires liquid water, so conditions have to be just right – not too hot, and not too cold; hence the distance from a star at which a planet could be the right temperature is known as the “Goldilocks Zone”, or more prosaically the habitable zone. This varies with the size of a star – for bigger and hotter stars it’s further away. The best known candidates for planets in their stars’ habitable zones orbit red dwarves. The European Very Large Telescope in Chile and the James Webb Space Telescope will look for Earth-like planets, with both NASA and the European Space Agency planning further missions.