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What is planet? Plain-English meaning
A planet is a large round body that orbits a star, such as the Sun, and has cleared its orbit of other debris.
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Definition
A planet is a large round body that orbits a star, such as the Sun, and has cleared its orbit of other debris.
Also seen as: planets
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- September 7, 2026
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Plain-English explanation
Planets are big, round objects that travel around a star. They do not make their own light like stars do; instead, they reflect the light from their star. In our solar system, eight planets orbit the Sun, including Earth, Mars, and Jupiter. Planets form from the same clouds of gas and dust that create stars. The gravity of a forming star pulls material together, and some of that material clumps into planets. Planets come in different sizes and compositions, from rocky worlds like Earth to giant gas planets like Jupiter.
Why it matters
Understanding what a planet is helps you make sense of the solar system and the wider universe. When you hear about new discoveries of planets around other stars, you can appreciate that they are formed in similar ways. It also helps you distinguish planets from stars, moons, and other objects like asteroids and comets. Knowing the basic definition of a planet gives you a foundation for learning about astronomy and space exploration.
Concrete example
Earth is a planet. It is a large, round body that orbits the Sun, our star. Earth reflects sunlight, which is why we see it as a bright object in space. It is one of eight planets in our solar system, and it formed from the same cloud of gas and dust that gave birth to the Sun.
Often confused with
People often confuse planets with stars. Stars are huge balls of glowing gas that produce their own light and energy, while planets do not produce light; they only reflect the light from their star. Another common confusion is between planets and moons. Moons orbit planets, while planets orbit stars. For example, our Moon orbits Earth, not the Sun directly.
Short definition: A planet is a large round body that orbits a star, such as the Sun, and has cleared its orbit of other debris.
Plain-English explanation
Planets are big, round objects that travel around a star. They do not make their own light like stars do; instead, they reflect the light from their star. In our solar system, eight planets orbit the Sun, including Earth, Mars, and Jupiter. Planets form from the same clouds of gas and dust that create stars. The gravity of a forming star pulls material together, and some of that material clumps into planets. Planets come in different sizes and compositions, from rocky worlds like Earth to giant gas planets like Jupiter.
Why it matters
Understanding what a planet is helps you make sense of the solar system and the wider universe. When you hear about new discoveries of planets around other stars, you can appreciate that they are formed in similar ways. It also helps you distinguish planets from stars, moons, and other objects like asteroids and comets. Knowing the basic definition of a planet gives you a foundation for learning about astronomy and space exploration.
Concrete example
Earth is a planet. It is a large, round body that orbits the Sun, our star. Earth reflects sunlight, which is why we see it as a bright object in space. It is one of eight planets in our solar system, and it formed from the same cloud of gas and dust that gave birth to the Sun.
Common confusion
People often confuse planets with stars. Stars are huge balls of glowing gas that produce their own light and energy, while planets do not produce light; they only reflect the light from their star. Another common confusion is between planets and moons. Moons orbit planets, while planets orbit stars. For example, our Moon orbits Earth, not the Sun directly.
Related terms
star, moon, solar system, orbit, dwarf planet
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More context
The word 'planet' comes from the Greek 'planetes,' meaning 'wanderer,' because ancient astronomers noticed these objects moved differently from stars. Planets exist beyond our solar system, called exoplanets, with thousands discovered. The study of planets helps us understand the formation of planetary systems and the potential for life.
Additional background
Planets are not just passive spheres; they are dynamic worlds with geological activity, atmospheres, and sometimes magnetic fields. For example, Earth is the only known planet with liquid surface water and life, but Mars shows evidence of past water. Venus has a thick carbon dioxide atmosphere with a runaway greenhouse effect, while Jupiter's Great Red Spot is a storm larger than Earth. The study of planets—planetary science—combines geology, atmospheric science, and astrophysics. Understanding planets in our solar system helps us interpret observations of exoplanets, which are often too distant to see directly. The formation of planets is thought to occur in protoplanetary disks around young stars, where dust and gas coalesce under gravity. The diversity of planets, from hot Jupiters to super-Earths, challenges our models and expands our definition of what a planet can be.
Sources and further reading
- Solar SystemThe solar system includes Earth and its Moon, the planets, and many smaller bodies around the Sun.
- StarsStars form from clouds of gas and dust, and planets can form around them; stars also group into galaxies and massive stars can collapse into black holes.
- NASA Learning ResourcesNASA explains space and astronomy topics such as planets, stars, galaxies, black holes, eclipses, telescopes, astronauts, orbits, comets, asteroids, meteors, satellites, and space stations.