Detecting Exoplanets

Since the first confirmed detection of a planet orbiting a star other than the Sun in 1995, astronomers have identified thousands of these exoplanets using several distinct detection methods. The transit method, currently the most productive, relies on measuring the slight dimming of a star's light as a planet passes in front of it from Earth's perspective; the size of the dip reveals the planet's approximate diameter. The radial velocity method instead detects the subtle wobble a planet's gravity induces in its host star, revealing the planet's mass rather than its size. Because these two methods measure different properties, astronomers who obtain both types of data for the same planet can calculate its density and thereby make inferences about its composition, distinguishing, for instance, a rocky world from a gas-dominated one. A third approach, direct imaging, captures actual light from the planet itself, but it remains technically difficult and is currently limited to large planets orbiting far from their stars, where the glare of the host star is less overwhelming. Each method carries its own biases, meaning the population of known exoplanets does not yet represent an unbiased sample of what likely exists throughout the galaxy.