Exoplanets
Thousands of planets have been confirmed orbiting other stars — and astronomers are still finding more.
What Is an Exoplanet?
An exoplanet is any planet beyond our own solar system, orbiting a star other than the Sun. Most are far too distant and too faint to see directly next to the glare of their host star, so astronomers infer their existence using indirect methods — watching for the subtle effects a planet has on the star it orbits.
Since the first confirmed detection in 1992, astronomers have confirmed thousands of exoplanets, with many more candidates awaiting confirmation. They come in a huge range of sizes and compositions — some larger than Jupiter, some smaller than Earth, and some orbiting in the "habitable zone" where liquid water could exist on a rocky surface.
Explore NASA's Exoplanet SiteHow We Find Them
Exoplanets are found using several complementary techniques, each suited to different kinds of planets and stars.
Transit Method
Watching a star's brightness for the tiny, periodic dip caused by a planet passing in front of it. This is how NASA's Kepler and TESS missions have found most known exoplanets.
Radial Velocity
Measuring the slight wobble in a star's motion caused by the gravitational tug of an orbiting planet, detected as a shift in the star's spectral lines.
Direct Imaging
Capturing an actual image of a planet by blocking out the overwhelming glare of its host star — challenging, but increasingly possible for large planets far from their star.
Gravitational Microlensing
Detecting a brief brightening of a distant star's light as its gravity — and that of any orbiting planet — bends and focuses light from a background star passing behind it.
Kinds of Exoplanets
Exoplanets discovered so far fall into a few broad categories, many with no direct counterpart in our own solar system.
Gas Giants
Jupiter- and Saturn-size worlds of hydrogen and helium, including "hot Jupiters" orbiting scorchingly close to their stars.
Neptunian
Neptune- and Uranus-size planets with hydrogen- and helium-rich atmospheres surrounding icy or rocky cores.
Super-Earths
Rocky or gaseous worlds more massive than Earth but lighter than Neptune, with no equivalent in our own solar system.
Terrestrial
Earth-size, rocky worlds — the type of planet most likely to be habitable, and a key target in the search for life beyond Earth.
Missions Leading the Search
NASA's Kepler space telescope revealed that planets are common throughout the galaxy, discovering thousands of candidates during its mission. Its successor, the Transiting Exoplanet Survey Satellite (TESS), is surveying the whole sky for planets around nearby, bright stars. TheJames Webb Space Telescope is now studying the atmospheres of known exoplanets in unprecedented detail, searching for water, methane, and other molecules that could hint at habitability — or even life.
Exoplanet Hunting from Your Backyard
You don't need a space telescope to be part of exoplanet science. Amateur astronomers with modest telescopes and CCD or CMOS cameras can record transit light curves of known exoplanet-hosting stars, contributing real data to projects like theNASA Exoplanet Watchcitizen science program. PAS members interested in exoplanet observing are welcome to connect with fellow members at our public star parties and meetings.
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