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.

 

Earth and the other seven planets that circle the star we call the sun 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). 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, composed of hydrogen, helium, water vapor, ammonia, and methane.

 

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.

ammonia
Noun

a gas (NH3) important to food production.

Noun

an invisible line around which an object spins.

day
Noun

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

distant
Adjective

far away.

Noun

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

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.

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.

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.

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.

Neptune
Noun

eighth planet from the sun in our solar system.

orbit
Noun

path of one object around a more massive object.

Noun

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

rotate
Verb

to turn around a center point or axis.

Saturn
Noun

sixth planet from 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.

Noun

star at the center of our solar system.

terrestrial planet
Noun

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

Uranus
Noun

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

Venus
Noun

planet in the solar system, second from the sun.