exoplanet
n. A planet outside of Earth's solar system.

Example Citations:
Astronomers said Wednesday that they had found a miniature version of our own solar system 5,000 light-years across the galaxy — the first planetary system that really looks like our own, with outer giant planets and room for smaller inner planets. ...

Sara Seager, a theorist at the Massachusetts Institute of Technology who was not part of the team, said that 'right now in exoplanets we are on an inexorable path to finding other Earths.' Dr. Seager praised the discovery as 'a big step in finding out if our planetary system is alone.'

Since 1995, around 250 planets outside the solar system, or exoplanets, have been discovered.
—Dennis Overbye, "Smaller Version of the Solar System Is Discovered," The New York Times, February 15, 2008

Researching the prospects for life beyond our solar system is moving to the next level. Exoplanet hunters are getting instruments that promise to spot Earth-like planets around alien stars. In some cases, they may even yield crude estimates of how life-friendly such a planet may be. ...

Planets are detected by how their gravitational tug makes their parent star wobble or by catching them when they pass in front of the star. The star system's gravity also can act like a lens focusing distorted images of objects behind them. Planets can be detected by their contribution to this lensing effect.

Most of the planets found are large and orbit close to their stars. Now the hunters' goal is to find the full range of exoplanet types, including Earth-like bodies.
—Robert C. Cowen, "Hunt for alien life to expand its scope," Christian Science Monitor, February 21, 2008

Earliest Citation:
For centuries, backyard skywatchers and trained astronomers have wondered whether the Milky Way galaxy — the one we call home — contains other planets like Earth and, if so, whether they sustain life.

It makes sense. With the billions of stars traveling through our galaxy, there must be some that have hospitable planets orbiting about them, just as the earth circles "our" star — the sun.

Astronomers believe there are almost certainly hundreds of large planets in other solar systems — known as "exoplanets" ("exo" means outer).
—"The Search for another Earth," The Patriot Ledger, March 29, 1994

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