Like the Andromeda Galaxy shown here, our Galaxy (the Milky Way) is made up of over 100 billion stars, including the Sun. It is estimated that there are 100-200 billion galaxies in the Universe, generally with a comparable number of stars to our own Galaxy. This means that there are many places where we might look for exoplanets, but where do we start? Telescopes allow astronomers to view galaxies that are billions of light years away, as well as the other planets in the Solar System. However, the closest star system to our Sun, Alpha Centauri, is 4.2 light years away; could an orbiting planet this far, or even farther, ever be studied from Earth?
Astronomers can detect and identify exoplanets up to 22000 light years away by studying subtle features of light emitted from a parent star. Using these features, an exoplanet's properties (such as mass, radius, and atmospheric composition) can be studied. This section outlines the detection methods astronomers use, and some of the discovery highlights that have been made so far.