A beginners' guide to exoplanets

What are exoplanets?

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.

The smallest exoplanets are a few times larger than the Earth, and many are several times larger than Jupiter. The fact that most of these planets are so much larger than Earth is thought to be because more massive planets are currently easier to detect than small ones.

How are they detected?

Until recently it has not been possible to see exoplanets directly 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.

Because of the Doppler Effect 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.


As a star moves towards an observer, the intervals between consecutive light waves decrease, and the light is detected at a higher frequency (or shorter wavelength). As it moves away the intervals increase and the light is detected at a lower frequency (or longer wavelength). The amount by which this changes depends on how quickly the star moves, which in turn depends on the mass of the planet. Picture: European Southern Observatory


 Another method of detection is the transit method. When a planet passes in front of a star, the planet will block a tiny amount of light. Telescopes can detect this slight dimming and can work out the radius of the planet by how much light is blocked and the orbit by how frequently the planet passes in front of the star. One drawback is that a star needs to have the right orbit so it passes in front of the star.

 Picture: NASA








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. 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 that lie in habitable zones orbit red dwarves, stars with a mass less than half that of the Sun.

The planned European Extremely Large Telescope – a revolutionary telescope that will be the biggest in the world with a mirror 42 m in diameter – and the James Webb Space Telescope will look for Earth-like planets, with both NASA and the European Space Agency considering further missions.


The European Extremely Large Telescope will be sited in either Chile or the Canary Islands and is expected to be completed in 2016. It will give clearer views of space than even the Hubble Space Telescope. Picture: European Southern Observatory





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