Photo courtesy NASA/STScI
Credit: Greg Bacon

Artist's concept of planet-orbiting star 79 Ceti

Are we alone in the universe, or does life exist elsewhere? We have begun to search for extraterrestrial intelligence (SETI) by looking for radio transmissions from other civilizations. An important aspect of SETI is deciding where to look for these radio transmissions -- after all, the universe is an awfully big place. To refine the SETI searches, it is helpful to know which stars have planets orbiting them.

Until 1991, our sun was the only star known to have planets around it. This changed when astronomer Alex Wolszczan discovered two planets orbiting a pulsar in the constellation Virgo. Since his discovery, there have been over 50 planets found in orbit around other stars -- these orbiting bodies are called extrasolar planets.

In this edition of HowStuffWorks, we'll find out what planets are, how they are formed, how we search for extrasolar planets (and what we've found so far) and about the new methods being developed for planet hunting.