Methods for Detecting Exoplanets and the Reasons We Have Yet to Discover a Second Earth

Over the past three decades, astronomers have discovered more than 6,000 planets orbiting other stars, yet almost none have been observed directly. Their existence is inferred from barely perceptible signs, such as a brief dimming or slight wobble in their host stars. Nevertheless, none of these planets truly resembles Earth. This is due less to the planets themselves than to the methods used to search for them.

In October 1995, at an astronomy conference in Florence, Swiss astronomers Michel Mayor and Didier Queloz announced the discovery of a planet orbiting 51 Pegasi, a star approximately 50 light-years from Earth. The gas giant, with a mass of at least half that of Jupiter, orbits so close to its star that its year lasts just over four days. By comparison, Mercury takes 88 days to orbit the Sun.