• 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.

  • Term Part of Speech Definition Encyclopedic Entry
    ammonia Noun

    a gas (NH3) important to food production.

    axis Noun

    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.

    distant Adjective

    far away.

    Earth Noun

    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.

    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.

    planet Noun

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

    Encyclopedic Entry: planet
    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.

    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.