Exoplanets – planets outside our solar system – are everywhere. But why do we study them? What makes them so interesting? At NASA, we're surveying and studying exoplanets to learn all about their weirdness, their variety, and all the fascinating things they can tell us about how planets form and develop.
Most of the exoplanets discovered so far are in a relatively small region of our galaxy, the Milky Way. ("Small" meaning within thousands of light-years of our solar system; one light-year equals 5.88 trillion miles, or 9.46 trillion kilometers.) That is as far as current telescopes have been able to probe. We know from NASA’s Kepler Space Telescope that there are more planets than stars in the galaxy.
In depth: What Is an exoplanet?