Exoplanets: The Universe beyond imagination

Beyond our solar system—called exoplanets—existed only in theory and science fiction. It seemed nearly impossible to detect planets light-years away, since the relatively tiny worlds would appear billions of times fainter than their parent stars. But in the last two decades astronomers have successfully developed indirect detection methods, most of which rely on measuring the effects of orbiting planets on far-off stars.

In 1992 astronomers reported the first planet-size masses around a dead star, the pulsar PSR1257+12, which sits 2,000 light-years away. Three years later came news of the first known exoplanet, a Jupiter-like gas giant orbiting its star closer than Mercury circles our sun. That world was detected around the sunlike star 51 Pegasi, a mere 50 light-years from Earth.

The rate of exoplanet discovery has since climbed rapidly thanks to the development of three main detection techniques, which all involve both ground- and space-based observatories. As of January 2012, the tally of confirmed alien worlds has skyrocketed to just over 720. However, the hunt is still on for a truly Earthlike planet, one with the right size, temperature, and composition to host liquid water—and maybe life.

The various techniques to find planets

The most successful planet-hunting technique to date has been radial velocity, also called the Doppler wobble, with more than 400 newfound planets to its credit. Astronomers using this method look for a star's “wobble”—telltale shifts in a star's light spectrum—caused by the gravitational tugs of orbiting planets. The more massive a planet and the tighter its orbit, the greater its effect on the host star. As a result, the majority of discoveries made with this method have been so-called hot Jupiters, because of their large masses and close proximities to their parent stars.