Illustration showing how the transit method can be used to detect extrasolar planets (exoplanets). The transit method relies on the fact that as a planet passes in front of its host star, the star light dims (see graph). If such a dimming is detected at regular intervals and lasts a fixed length of time, then it is likely a planet is orbiting the star. As well as the presence of a planet, the amount of light dimming reflects the size ratio between the star and the planet. And, because the size of the host star can be known with considerable accuracy, the orbiting planet's size (but not its mass) can be estimated. Spectroscopic measurement (such as the radial velocity method), can provide an estimate of a planet's mass, but not its size. Using both methods, combining mass and size, the planet's density can be calculated which is an important step towards assessing the exoplanet's composition.