Exoplanet (noun, “EX-o-plan-eht”)
A planet orbiting a star outside of our solar system. Planets are celestial objects that orbit stars. They are big enough to have cleared other objects from their path as they whirl around their sun. Planets also are dense enough that gravity will have squashed them into roundish balls. But while we call the big, round objects orbiting our sun planets, exoplanets are those that orbit other stars.
In a sentence
A British teenager may be the youngest person ever to have discovered an exoplanet.
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celestial object Anynaturally formed objects of substantial size in space. Examples include comets, asteroids, planets, moons, stars and galaxies.
exoplanet A planet that orbits a star outside the solar system. Also called an extrasolar planet.
exomoon A moon that orbits an exoplanet.
light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now consists of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.