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In the night sky: Orion
In the night sky: Orion

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4.3.2 Planet hunting – identification and observing techniques

As exoplanets are generally too small to be observed directly, the star they are orbiting is observed instead. Very small changes to the star can be seen as an effect of the orbiting planet.

There are two main methods that can detect small changes to a star which are caused by an orbiting planet. These are the radial velocity and transit methods.

Radial velocity method

The radial velocity method measures the very slight wobble of a star which results from the gravitational pull of the orbiting planet. These wobbles cause a slight change in the observed light which can be measured. This was the first method that successfully identified an exoplanet.

Transit method

A diagram showing a planetary transit.
Figure 25 A planetary transit.

The transit method measures very small changes in brightness of the star as a result of the planet passing in front of the star (known as a transit) and blocking out a small amount of light when viewed from Earth. If this slight dimming in brightness is detected at regular intervals, it is likely that a planet is orbiting around the star.

The dimming of a star during transit directly reflects the size ratio between the star and the planet. A small planet transiting a large star will create only a slight dimming, while a large planet transiting a small star will have a more noticeable effect. For example, a Jupiter-sized planet passing in front of a Sun-sized star would cause a 1% dip in light from the star. The transit method only works when the planet passes directly in front of the star.

In the next section, you will hear about the red dwarf star Gliese 581 and find out for yourself about the planets orbiting it.