This illustration shows the approximate sizes of the planets relative to each other. Outward from the sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, followed by the dwarf planet Pluto. Jupiter's diameter is about 11 times that of the Earth's and the sun's diameter is about 10 times Jupiter's. Pluto's diameter is slightly less than one-fifth of Earth's. The planets are not shown at the appropriate distance from the sun.

Illustration courtesy Lunar and Planetary Institute

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  • On August 24, 2006, the International Astronomical Union officially defined the term "planet.” A planet orbits a star and is large enough to form itself into a nearly round shape (due to gravity). A planet must also “clear the neighborhood” of space debris in its orbital zone. Our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Astronomers have also identified nearly a thousand planets outside our solar system, called extrasolar planets.

    Celestial bodies that have not “cleared the neighborhood” are called “dwarf planets.” Dwarf planets in our solar system include Pluto, Ceres, Eris, Haumea, and Makemake. Hundreds of more distant objects may still be classified as dwarf planets in the future.

  • Term Part of Speech Definition Encyclopedic Entry
    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    gravity Noun

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

    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
    solar system Noun

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

    star Noun

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