What's an exoplanet?

An exoplanet is a planet that orbits a star outside of the Solar System. As of Spring 2018, nearly 4000 exoplanets have been discovered, along with over 4000 additional exoplanet candidates.

There are several methods for detecting exoplanets, but radial velocity, planetary transits, and direct imaging have been among the most popular and productive techniques for exoplanet discovery and characterization. In the radial velocity method, the presence of the planet is inferred from tiny wobbles in the frequency of light (the Doppler effect) coming from the host star. This method can be used to estimate the mass of an orbiting planet. In the transit method, a planet is detected when it transits, or passes in front of the disk of its host star, causing a tiny dip in the amount of starlight we observe. This method can be used to estimate the radius of an orbiting planet, and can also be used to provide clues about the composition of the planet's atmosphere. Thousands of exoplanet candidates have been detected by the Kepler spacecraft using the transit method. In recent years, increasing numbers of planets have been found by direct imaging in which light from the planet can be seen directly and separately from its host star. This usually requires a coronograph that blocks out the starlight while leaving the planet visible. As imaging and detection techniques continue to improve, more and more planets are expected to be studied using this method.

The directly-imaged planetary system around the star HR 8799. This system is located 750 trillion miles from Earth in the constellation of Pegasus. Image Credit: NRC-HIA, Christian Marois, and the W.M. Keck Observatory