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 sun—Mercury, 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 sun—Jupiter, 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.

planet
Clouds swirl above Antarctica.

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."

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."

accumulate
Verb

to gather or collect.

ammonia
Noun

a gas (NH3) important to food production.

ancient
Adjective

very old.

astronomer
Noun

person who studies space and the universe beyond Earth's atmosphere.

Noun

layers of gases surrounding a planet or other celestial body.

atom
Noun

the basic unit of an element, composed of three major parts: electrons, protons, and neutrons.

Noun

an invisible line around which an object spins.

billion
Noun

1,000,000,000

day
Noun

time it takes for a planet to complete a single rotation around its axis.

dense
Adjective

having parts or molecules that are packed closely together.

detect
Verb

to notice.

distant
Adjective

far away.

dozen
Noun

a group of 12.

dust
Noun

microscopic particles of rocks or minerals drifting in space. Also called cosmic dust or space dust.

dwarf planet
Noun

celestial body that is nearly spherical but does not meet other definitions for a planet.

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

earth
Noun

soil or dirt.

energy
Noun

capacity to do work.

eventually
Adverb

at some point in the future.

exoplanet
Noun

planet outside the solar system, orbiting a star other than the sun. Also called an extrasolar planet.

extrasolar planet
Noun

planet outside the solar system, orbiting a star other than the sun. Also called an exoplanet.

gas
Noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

gas giant
Noun

one of the four enormous outermost planets in the solar system (Jupiter, Saturn, Neptune, Uranus), composed mostly of gases instead of rock. Also called a Jovian planet.

gradually
Adverb

slowly, or at a measured pace.

gravitational pull
Noun

physical attraction between two massive objects.

gravity
Noun

physical force by which objects attract, or pull toward, each other.

helium
Noun

a light, colorless gas with the chemical symbol He.

hydrogen
Noun

chemical element with the symbol H, whose most common isotope consists of a single electron and a single proton.

International Astronomical Union
Noun

organization promoting and safeguarding the science of astronomy through international cooperation.

Jupiter
Noun

largest planet in the solar system, the fifth planet from the Sun.

Latin
Noun

language of ancient Rome and the Roman Empire.

Mars
Noun

fourth planet from the sun, between Earth and Jupiter.

matter
Noun

material that makes up a substance.

Mercury
Noun

smallest planet in the solar system, and closest to the sun.

methane
Noun

chemical compound that is the basic ingredient of natural gas.

Moon
Noun

Earth's only natural satellite.

Noun

natural satellite of a planet.

natural satellite
Noun

moon, or a celestial body that orbits another celestial body (its primary).

nebula
Noun

cloud of gas and dust in space.

Neptune
Noun

eighth planet from the sun in our solar system.

nuclear fusion
Noun

process where the nuclei of one element, usually hydrogen, fuse with each other to form the nuclei of another element, usually helium.

Verb

to move in a circular pattern around a more massive object.

Noun

large, spherical celestial body that regularly rotates around a star.

Pluto
Noun

dwarf planet in our solar system.

proportion
Noun

amount.

radiation
Noun

energy, emitted as waves or particles, radiating outward from a source.

reflect
Verb

to rebound or return light from a surface.

revolve
Verb

to orbit or spin around something.

ring system
Noun

thin circles of dust, gas, and rocks that orbit a planet.

rotate
Verb

to turn around a center point or axis.

Saturn
Noun

sixth planet from the sun.

solar orbit
Noun

revolution of an object around the sun.

solar system
Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

spherical
Adjective

rounded and three-dimensional.

star
Noun

large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.

sulfuric acid
Noun

toxic chemical made of hydrogen, sulfur, and oxygen.

sun
Noun

star at the center of our solar system.

telescope
Noun

scientific instrument that uses mirrors to view distant objects.

terrestrial planet
Noun

one of the four planets closest to the sun: Mercury, Venus, Earth, or Mars.

trillion
Noun

1,000,000,000,000.

universe
Noun

all known matter, energy, and space.

Uranus
Noun

large, gaseous planet in the solar system, seventh from the sun.

Venus
Noun

planet in the solar system, second from the sun.

wobble
Verb

to tilt back and forth.