Astronomers began looking for a ninth planet in the 1840s after noticing that Uranus orbit was being affected by an object other than Neptune. Clyde Tombaugh, a 23-year-old astronomer at the Lowell Observatory in Flagstaff, Arizona, was tasked with finding this Planet X. He had to take a series of shots of the night sky in pairs and then review them to see if anything had moved. After almost a year of this, Tombaugh saw something on images taken on January 23 and January 29 that appeared to be moving. He had discovered Pluto, which was later supported by additional photographic evidence.
Venetia Burney, an 11-year-old girl from Oxford, England, proposed the name after much debate about it. She was interested in ancient mythology and Pluto, the Roman god of the underworld. Former Bodleian Library librarian Venetia's grandpa passed along the name to an astronomy professor in Oxford, who subsequently passed it along to US colleagues. The name Pluto was chosen by a unanimous vote of the Lowell Observatory members.
It is 2,380 km diameter, smaller than our moon, and orbits the Sun at 5.8 billion kilometers in the Kuiper Belt. On Pluto, a year is equal to 248 Earth years, and a day is equal to 153 Earth hours, or around six Earth days. Pluto hasn't made a full orbit of the Sun since it was discovered since it is so far away. It lacks a ring system, but it does have five moons that are named after mythological characters connected to the underworld: Styx (discovered in 2012), Nix (discovered in 2005), Hydra (discovered in 2005), Kerberos (discovered in 2011), and Charon (discovered in 1978). Charon, the largest moon, is so enormous that it orbits Pluto like a twin planet. Pluto’s surface temperature is about -228̊ C to -238̊ C, which is much too low to support life. The largest known Kuiper Belt object is Pluto, which, like the other objects in the belt, is believed to be a remnant planetesimal—a piece of the protoplanetary gas and dust disc that failed to develop into a full-fledged planet.
Up until 2006, there were nine planets in our solar system, with Pluto being the one that was furthest distant from the Sun.
On January 19, 2006, an Atlas V rocket operated by NASA lifted out from Cape Canaveral Air Force Station with the New Horizons mission. The spacecraft started getting close to Pluto on January 15 of 2015, and on July 14 of that year, when it soared 12,500 kilometers over the surface, it became the first spacecraft to study the dwarf planet. Before moving on to other Kuiper Belt Objects, it sent back the last of its recorded data from Pluto on October 25, 2016. When it captured images of Pluto's whole northern hemisphere and equatorial regions down to around 30° south, Pluto became incredibly clear. Additionally, it measured and observed Pluto and its moons in detail.
Pluto served as the ninth planet for 76 years. Its status as the smallest planet in the solar system with a moon half its size was unimportant. Its skewed, oval-shaped orbit didn't bother anyone. Pluto was strange, but it was our strange.
Pluto was downgraded from being the ninth planet from the Sun in 2006 to one of five "dwarf planets" by the International Astronomical Union (IAU). The widespread indignation that followed the adjustment in the solar system's line up was probably not something the IAU had anticipated. People from all around the world objected to the planet's demotion on principle at the time of the announcement (and even more than ten years afterwards), claiming that it changed convention and history rather than considering the scientific justification. What motivated the IAU to downgrade Pluto at that time? Why is Pluto not a planet anymore?
The proposal to demote Pluto, which was the primary event of the 2006 IAU General Assembly, marked a turning point for the entire solar system. The resolution that was adopted formally defined the term "planet," after much debate among the union's members. Planets are astronomical objects massive enough to have been rounded by their gravitational orbit around the Sun and to have chased away nearby planetary objects and debris. What was once a general term used to designate a large object inside the solar system was now defined.
Pluto is presently regarded as a dwarf planet because, while being huge enough to have grown spherical, it lacks the power to dominate its orbit and clear the area around it.
The term "planet" had no practical definition before to the 2006 resolution and was based on a categorization that predated some of the most significant recent astronomical discoveries that have been achieved thanks to technological advancements. It was a positive move toward new light, new knowledge, and shifting conceptions of the cosmos for many Earthlings who felt that Pluto's downgrade represented a break from tradition.
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