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.
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.
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."
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."
Term Part of Speech Definition Encyclopedic Entry accumulate Verb
to gather or collect.
a gas (NH3) important to food production.
person who studies space and the universe beyond Earth's atmosphere.
layers of gases surrounding a planet or other celestial body.
Encyclopedic Entry: atmosphere atom Noun
the basic unit of an element, composed of three major parts: electrons, protons, and neutrons.
an invisible line around which an object spins.
Encyclopedic Entry: axis billion Noun
time it takes for a planet to complete a single rotation around its axis.
having parts or molecules that are packed closely together.
a group of 12.
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.
our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.
Encyclopedic Entry: Earth earth Noun
soil or dirt.
capacity to do work.
at some point in the future.
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.
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.
slowly, or at a measured pace.
gravitational pull Noun
physical attraction between two massive objects.
physical force by which objects attract, or pull toward, each other.
a light, colorless gas with the chemical symbol He.
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.
largest planet in the solar system, the fifth planet from the Sun.
language of ancient Rome and the Roman Empire.
fourth planet from the sun, between Earth and Jupiter.
material that makes up a substance.
smallest planet in the solar system, and closest to the sun.
chemical compound that is the basic ingredient of natural gas.
Earth's only natural satellite.
natural satellite of a planet.
Encyclopedic Entry: moon natural satellite Noun
moon, or a celestial body that orbits another celestial body (its primary).
cloud of gas and dust in space.
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.
to move in a circular pattern around a more massive object.
Encyclopedic Entry: orbit planet Noun
large, spherical celestial body that regularly rotates around a star.
Encyclopedic Entry: planet Pluto Noun
dwarf planet in our solar system.
energy, emitted as waves or particles, radiating outward from a source.
to rebound or return light from a surface.
to orbit or spin around something.
ring system Noun
thin circles of dust, gas, and rocks that orbit a planet.
to turn around a center point or axis.
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.
rounded and three-dimensional.
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.
star at the center of our solar system.
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.
all known matter, energy, and space.
large, gaseous planet in the solar system, seventh from the sun.
planet in the solar system, second from the sun.
to tilt back and forth.