2.2 Transits of exoplanets
It is also possible to see transits of exoplanets. If our view of a star happens to cross the orbital path of one of the star’s planets, a transit will occur. Just as Mercury and Venus block some of the Sun’s light, in the same way an exoplanet can block some of the light from its host star.
You have already seen that a small planet, such as Mercury, blocks less light than a larger planet, such as Venus. This is true for exoplanets too. So, it is possible to learn how big an exoplanet is from measuring how much starlight is blocked during its transit.
Figure 6 (in the previous section) showed the information we would need to work out the size of Venus from its transit. However, even our next nearest star is very far away compared with distances between planets in the Solar System. This means that for exoplanets, the distance between Earth and the exoplanet is almost identical to the distance between Earth and its star. This actually makes it easier to work out the size of an exoplanet from its transit than it is to work out the size of Venus! In fact, all you need to know to work out the size of a transiting exoplanet is the size of its star.