In Depth: Exoplanets
An exoplanet, or extrasolar planet, is a planet outside of our solar system that usually orbits another star in our galaxy.
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.
NASA/JPL-Caltech
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.
Although exoplanets are far – even the closest known exoplanet to Earth, Proxima Centauri b, is still about 4 light-years away – scientists have discovered creative ways to spot these seemingly tiny objects.
How Do We Find Exoplanets?
There are five methods scientists commonly use to discover exoplanets.
The two main techniques are the transit and radial velocity methods.
When a planet passes directly between an observer and the star it orbits, it blocks some of that starlight. For a brief period of time, that star’s light actually gets dimmer. It's a tiny change, but it's enough to clue astronomers in to the presence of an exoplanet around a distant star. This is known as the transit method.
