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.
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.
Term Part of Speech Definition Encyclopedic Entry ammonia Noun
a gas (NH3) important to food production.
an invisible line around which an object spins.
Encyclopedic Entry: axis day Noun
time it takes for a planet to complete a single rotation around its axis.
our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.
Encyclopedic Entry: Earth 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.
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.
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.
smallest planet in the solar system, and closest to the sun.
chemical compound that is the basic ingredient of natural gas.
eighth planet from the sun in our solar system.
path of one object around a more massive object.
large, spherical celestial body that regularly rotates around a star.
Encyclopedic Entry: planet rotate Verb
to turn around a center point or axis.
sixth planet from 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.
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.
large, gaseous planet in the solar system, seventh from the sun.
planet in the solar system, second from the sun.