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ENCYCLOPEDIC ENTRY

ENCYCLOPEDIC ENTRY

planet

planet

A planet is a large object that orbits a star

Grades

5 - 12+

Subjects

Astronomy, Earth Science, Geography, Physical Geography, Physics

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Morgan Stanley

A planet is a large object that orbits a star. To be a planet, an object must be massive enough for gravity to have squeezed it into a spherical, or round, shape,. It must also be large enough for gravity to have swept up any rocky or icy objects from its path, or orbit, around the star.

Scientists believe planets begin to form when a dense cloud of dust and gas, called a nebula, spins around a newly formed star. Gradually, gravity causes the bits of matter in the nebula to clump together. Slowly, these clumps accumulate and grow. Eventually, these clumps become planets.

Earth is one of eight planets that circle the star we call the sun. Together, the sun, the planets, and smaller objects such as moons make up our solar system.

The four planets closest to the sunMercury, Venus, Earth, and Mars—are called terrestrial planets. These planets are solid and rocky like Earth (terra means “earth” in Latin). Earth is the largest of the four terrestrial planets, and Mercury is the smallest. All are surrounded by a layer of gas, or atmosphere. Their atmospheres vary in density from Mercury’s extremely thin atmosphere to Venus’, which is thick with clouds of sulfuric acid.

The four planets that are more distant from the sunJupiter, Saturn, Uranus, and Neptune—are called gas giants. Gas giants are huge compared with Earth, and they do not have solid surfaces. They are big balls of gas. Jupiter and Saturn are composed mostly of hydrogen and helium. Uranus and Neptune have greater proportions of water vapor, ammonia, and methane. Each of the four gas giants also has a ring system. A planet’s rings are made of ice, dust, and small rocks. Saturn’s ring system is the largest.

Every planet except Mercury and Venus has at least one natural satellite, or moon. A planet’s moon orbits it as it revolves around the sun. Jupiter, Saturn, and Uranus each have dozens of moons.

In addition to orbiting a star, planets also rotate, or spin, around an axis. An axis is an invisible line that runs through the center of a planet. One complete rotation is called a day. A day on Earth is about 24 hours. A day on Jupiter takes only 9.8 hours. Venus has the longest day of any planet in our solar system. It takes 243 Earth days for Venus to make a complete turn on its axis.

Unlike stars, planets do not experience nuclear fusion, the process of combining tiny particles called atoms to release energy. Nuclear fusion creates radiation (heat and light) and makes stars glow. Because planets do not have nuclear fusion, they do not produce their own light. Instead, they shine with light reflected from a star. When we see planets in the night sky, such as Venus, the so-called "Evening Star," we're seeing reflected sunlight.

Exoplanets

Because there are trillions of stars in the universe, there are very likely billions of planets. But until the early 1990s, the only known planets were in our solar system. Since then, however, scientists have discovered more than 400 planets orbiting other stars. These are called extrasolar planets, or exoplanets.

Exoplanets appear to be fairly small from our viewpoint on Earth. Telescopes usually cannot observe exoplanets directly, so astronomers have had to come up with methods to detect them. One method astronomers use is to look for a slight wobble in a star’s movement. This wobble is the result of the gravitational pull of a nearby planet. Most of the exoplanets discovered so far are gas giants.

Fast Fact

Pluto, the Nearest Dwarf Planet
Pluto is a small, icy object about 2,302 kilometers (1,430 miles) across that orbits the sun beyond Neptune. Discovered in 1930, it was long considered the ninth planet in our solar system. But in 2006, the International Astronomical Union revised its definition of a planet. Under the new definition, a planet must be massive enough that gravitational forces have cleared its solar orbit of other objects. Because astronomers have discovered other bodies in the neighborhood of Pluto, Pluto did not meet the revised definition of a planet. It is now considered a "dwarf planet."

Fast Fact

Wanderers
in the Night Sky
Ancient astronomers viewed the sky with nothing more powerful than their own eyes. These astronomers noticed a few lights in the sky that looked different from stars. Stars stay in fixed patterns in relation to one another. But these strange lights moved around the sky. The Greeks called them planetes, or "wanderers." This is the source of the English word "planet."

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Writers
Kim Rutledge, Kim Rutledge
Melissa McDaniel, Melissa McDaniel
Santani Teng, Santani Teng
Hilary Hall, Hilary Hall
Tara Ramroop, Tara Ramroop
Erin Sprout, Erin Sprout
Jeff Hunt, Jeff Hunt
Diane Boudreau, Diane Boudreau
Hilary Costa, Hilary Costa
Illustrators
Mary Crooks, National Geographic Society,
Tim Gunther, Tim Gunther
Editors
Jeannie Evers, Emdash Editing, Emdash Editing
Kara West, Kara West
Educator Reviewer
Nancy Wynne,
Producer
National Geographic Society, National Geographic Society
other
Last Updated

May 20, 2022

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