WHY DID PLUTO STOP BEING A PLANET?

  At present day, Pluto (or 134340 Pluto) is a dwarf planet in the solar system located next to the orbit of Neptune. Its name comes from the Roman mythological god Pluto (Hades according to Greek mythology, god of the underworld). It was discovered on February 18, 1930 by the American astronomer Clyde William Tombaugh (1906-1997) from the Lowell Observatory in Flagstaff, Arizona, and was considered the ninth and smallest planet in the solar system by the International Astronomical Union and by public opinion since then until 2006, although its membership in the group of planets in the solar system was always the subject of controversy among astronomers; It is much smaller than the rest of the planets of the Solar System, even smaller than the Moon, and despite being a rocky planet (like Mercury, Venus, Earth and Mars), it is a neighbor of the gas (or Jovian) planets. For this reason, some believe that Pluto originated in another part of space and was trapped by the gravity of the Sun. During that period (1930-2006), our solar system officially had nine planets, and so we rooted it in our flow of knowledge.

Clyde William Tombaugh (1906-1997)

However, in 2006, Pluto officially ceased to be a planet, although a small group of scientists and certain sectors of public opinion refuse to accept a solar system of eight planets, but, why did Pluto cease to be considered a planet like the remaining eight?

  Since 1992 numerous objects have been discovered in the same Pluto region of the solar system, the region called Kuiper belt. This led to Pluto´s planet status being controversial and many questioning whether or not to be considered with that denomination. Because objects whose sizes were getting closer to Pluto's were being discovered frequently, it was argued that it should be reclassified as one of the objects in the Kuiper belt. All the planets of the Solar System orbit the Sun in a relatively flat (or angled) manner, however, Pluto's orbit has an angle of 17 degrees. In addition, its trajectory is exceptionally erratic and crosses the orbit of Neptune. To make it worse, its moon Charon is about half the size of Pluto, so some astronomers consider it a binary system instead of a planet and its satellite.

  The American astronomer and physicist Michael E. Brown, is credited as being the "man who killed Pluto", and even has a book and many articles about it; his scientific team at the astronomical observatory at the California Institute of Technology (Caltech) is responsible for the discovery of dozens of transneptunian objects (celestial bodies that are beyond the orbit of Neptune), but it was the discovery of Eris, a dwarf planet more massive than Pluto, which led the latter to its "degradation," according to Brown, there are many objects like Eris or Pluto in our solar system, and if we continue to catalog them as planets, we would have a solar system with tens or even hundreds of planets, with all the difficulties that it would entail.

Michael E. Brown

  On August 24, 2006, the International Astronomical Union (UAI) approved the Resolution 5A, which, among other things, established the official definition of planet, and the three primary conditions that a celestial body has to fill to be considered as planet, these are:

• a): The object must be in orbit around the sun.
• b): The object must be massive enough so that its gravity has made it spherical.
• c): The object must have its orbit clean (and must not interfere with the orbit of another planet)

 Although Pluto fills the first two conditions, it fails the last one because its orbit crosses Neptune´s, besides, its mass is only 0.07 times the mass of the other objects in its orbit. Therefore, Pluto cannot be considered as a planet. According to the aforementioned resolution 5A, the bodies that only fill the first two conditions will be called dwarf planets, and thus it was officially established as of September 13, 2006.

  However, not all astronomers agree with this classification, since the term "dwarf planet" can be confusing (considering that it is not really a planet) and that it was only a select group of experts who participated in the vote and not the totality of criteria was reflected in the astronomical community. The aforementioned Michael Brown said that: 

"through all this crazy circus procedure, they stumbled in some way with the correct answer. It was about time. Science, in the end, corrects itself, even when strong emotions are involved."

The popular reception of the decision of the UAI was varied, and although many accepted it, there were those who tried to cancel it with requests in the network in which they urged the UAI to consider the restoration of the category of planet, and even, by derisory that seems, there were demonstrations in the streets demanding the reinstatement of the category of planet for Pluto, and it was reported that some of the scientists involved in the decision received threats from some people, although they did not go beyond being just threats.

  One of the scientists who most emphatically rejected resolution 5A is Alan Stern, lead investigator of the New Horizons mission, and who publicly ridiculed the UAI resolution stating that "the definition stinks for technical reasons." But despite the controversy and the diversity of criteria, the official resolution 5A has prevailed and our solar system consists of eight planets, each with its distinctive and fascinating characteristics.

Alan Stern

  The New Horizons mission, an unmanned space mission of the US space agency (NASA) and launched on January 19, 2006, and whose objective was to explore Pluto, its satellites and asteroids from the Kuiper belt, went on to 12,500 km of Pluto's surface at the time of maximum approximation on July 14, 2015 at a relative speed of 49,600 km/h. It passed 28,800 km from Charon. The probe was the first object built by man to fly over Pluto and its nearby objects and took the first and spectacular photographs of the surface of the dwarf planet, as well as being able to photograph its five moons (Charon, Hydra, Nix, Kerberos and Styx) given its closeness.

New Horizons space probe

  As a curious fact, on board the probe, in addition to the technical devices, there is a capsule containing the ashes of Pluto's discoverer, Clyde William Tombaugh.