Neptune, named for the Greek god of the sea, is not blue due to the presence of water. Neptune's blue clouds are mostly made of methane. Neptune is the only planet in the solar system that cannot be seen without a telescope.

Photograph courtesy NASA
  • On September 23, 1846, French scientists discovered Neptune, the outermost planet in our solar system. Neptune is the only planet in the solar system that cannot be viewed without a telescope.

    Neptune is the only planet discovered by mathematical prediction. Long before its discovery, astronomers hypothesized the existence of Neptune because Uranus seemed to be affected by an unknown source of gravity. Several mathematicians successfully calculated the approximate position of the planet, which helped astronomers know where to look.

    Since its discovery, scientists have learned much about Neptune. In 1989, NASA’s Voyager 2 spacecraft provided images of the planet, which appears bright blue due to the methane in its upper atmosphere. Neptunian rings and moons were also identified.

  • Term Part of Speech Definition Encyclopedic Entry
    approximate Adjective

    generally or near an exact figure.

    astronomer Noun

    person who studies space and the universe beyond Earth's atmosphere.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    calculate Verb

    to reach a conclusion by mathematical or logical methods.

    gravity Noun

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

    hypothesize Verb

    to form a statement or suggestion that explains certain questions about certain facts.

    mathematician Noun

    person who studies the theory and application of quantities, groupings, shapes, and their relationships.

    methane Noun

    chemical compound that is the basic ingredient of natural gas.

    moon Noun

    natural satellite of a planet.

    Encyclopedic Entry: moon
    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.

    telescope Noun

    scientific instrument that uses mirrors to view distant objects.