Pluto lies far from the Sun, being the ninth largest body to orbit the Sun. From its discovery in 1930 to 2006 it was the ninth planet in the Solar System. Pluto was removed from its position as a planet in 2006 by the International Astronomical Union (IAU). It is now classified as a dwarf planet.
In the late 1800s, astronomers began to predict the existence of Pluto based on the deviations of other outer planets’ orbits. Pluto was finally discovered on 18 February 1930 by Clyde Tombaugh at the Lowell Observatory. However, its earliest known photograph was taken in 1909 without being discovered.
It lies between 29.7 and 49.3 astronomical units (AU) from the Sun. One AU is the average distance between Earth and the Sun. It orbits the Sun once in 247.94 years. It takes a little over on Earth years to rotate once on its axis. It has five known moons. It has a diameter of about 2376 km and it weighs 17% of the Earth’s mass.
But why is Pluto not a planet anymore? In 2006, International Astronomical Union reclassified the definition of a planet. For a celestial body in the Solar System to be classified as a planet: it has to orbit around the Sun, should have enough mass to assume a nearly spherical shape and its orbit should not overlap with other planets’ orbits.
Even though Pluto satisfies the first two conditions, it fails to satisfy the third condition. However, it is classified as a Dwarf Planet. The decision by IAU to demote it to a dwarf planet was faced with mixed reactions.
Telescopic observations of Pluto are difficult due to its relatively small size and great distance from the Sun. The first clear pictures of the surface of Pluto came only in 2015 when New Horizons spacecraft visited the dwarf planet in 2015. New Horizons was launched 19 January 2006 and made its closest flyby of Pluto at 11:49:57 UTC on 14 July 2015.