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.

approximate
Adjective

generally or near an exact figure.

astronomer
Noun

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

Noun

layers of gases surrounding a planet or other celestial body.

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.

Noun

natural satellite of a planet.

Noun

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

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.