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Why Pluto Is Not (Anymore) a Planet: History and Reasons for the Controversial Astronomical Decision

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

We are all fond of Plutothis cold, distant world that we learned to love in school books as the final frontier of the Solar System, and which inspired mass culture by giving its name to Mickey Mouse’s dog, Pluto, or to the radioactive element Plutonium.

It is therefore not surprising that there was a wave of disappointment all over the world when theInternational Astronomical Union (IAU) decided, the August 24, 2006Of downgrade Pluto from the ninth planet of the Solar System to the status of dwarf planet.

In fact, at the Prague conference, following the discovery of other objects similar to Pluto in the external Solar System, astronomers decided to establish more rigorously the necessary criteria for a celestial object to be defined as a planetThe criteria include theorbiting the Sun, being spherical in shape, and having cleared its orbit of other bodies.

With these new criteria, the Solar System went from 9 to only 8 major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune). The new category of dwarf planetswhich today includes 5 objects, or Pluto, Ceres, Haumea, Makemake and Eris.

Pluto’s Peculiarities

Pluto was discovered in 1930 from the American astronomer Clyde W. Tombaugh while he was searching for objects beyond the orbit of Neptune that could explain its anomalous orbital behavior. From the beginning, however, Pluto seemed to be a peculiar objectdifferent from the other 8 planets in the Solar System. In fact, it has a mass (0.2% of the Earth) and dimensions (2400 km in diameter) that are smaller than several natural satellites in the Solar System, first and foremost Jupiter’s satellites, but also the Moon itself.

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Not to scale image of Pluto’s highly inclined orbit relative to the rest of the Solar System objects. Credit: NASA

Even more peculiar is thePluto’s orbit. It is in fact very inclined with respect to the ecliptic plane, the imaginary plane on which the Earth-Sun orbit lies. In addition to being inclined, Pluto’s elliptical orbit has another peculiarity, namely its high eccentricity. It is such that it brings Pluto, at aphelion (the point of the orbit furthest from the Sun), to a distance of 49 astronomical units (150 million km) from the Sun, while, at perihelion (the point of the orbit closest to the Sun), it pushes itself into the orbit of Neptune, at “only” 30 astronomical units from the Sun.

The discovery of other objects similar to Pluto

The discovery of other planets between 2004 and 2005 further complicated Pluto’s position as the ninth planet in the Solar System. three objects beyond the orbit of Neptune which shared peculiarities in mass and orbit similar to those of Pluto. The three new objects were renamed by astronomers with the names of Haumea, Makemake and Erisall three having perihelion beyond the orbit of Neptune and a mass similar to that of Pluto (or slightly greater in the case of Eris).

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Collage of real images and artistic representations of the 5 dwarf planets of the Solar System: Ceres, Pluto, Haumea, Makemake and Eris. Credit: NASA

The discovery of these objects, the hypothesized presence of others similar beyond the orbit of Neptune, and the existence of a similar icy body like Ceres in the main asteroid beltconvinced the astronomical community to call a conference to establish more rigorously the criteria that a celestial object must satisfy in order to be defined as a planet. The IAU astronomers then met at Prague to deliberate on the definition of planet.

The new definition of planet

After several days of heated discussions, the IAU on 24 August 2006 reworked the definition of planetThere are three criteria that must be met for a celestial object to be called a planet:

  1. a planet is one that orbits its host star, just as Earth and Jupiter orbit the Sun. This does not include objects orbiting other major planets;
  2. a planet is such if its mass, and therefore its gravitational force, is large enough to overcome the rigid-body forces and make it almost spherical in shape;
  3. a planet is such if it is able to “clean” its orbit, that is, it must have a significant influence on the orbital stability of other objects in its vicinity, and the latter must have a mass that is thousands to millions of times smaller than that of the planet.

Pluto’s Downgrading to a Dwarf Planet

Pluto satisfies the first two requirements of the definition of planet, but, unfortunately, does not satisfy the third. Its mass is in fact only 0.07 times greater than that of other objects in its orbital zone, while, for example, the Earth has a mass that is 1.7 million times greater than that of other objects in its orbital zone.

The IAU then established that the Solar System was composed of only 8 major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), introducing the new category of dwarf planet. It is defined as an object in orbit around the Sun that is large enough to assume a almost spherical shapebut was unable to clear its orbit of debris. Furthermore, dwarf planets are generally smaller than Mercury and may also orbit in an area that contains many other objects. The new category of dwarf planet currently includes only 5 items, which in order of distance from the Sun are: Ceres, Pluto, Haumea, Makemake and Eris.

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Image of Pluto taken by NASA’s New Horizons spacecraft in 2015 from just 35,000 km above the surface. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

Pluto falls into the category of dwarf planets because it resides within a zone that contains other objects that could cross its orbital path, known as trans-Neptunian region. Given its importance, especially historical, at a subsequent conference in June 2008, IAU astronomers decided that Pluto should be recognized as an important prototype of a new class of trans-Neptunian objects, namely plutoids. These are objects that have a semi-major axis of their orbit larger than that of Neptune and have sufficient mass to be nearly spherical in shape.

Criticisms of the new definition of Pluto

Pluto’s downgrading had a impact both on public opinion and on the astronomical community, attracting harsh criticism on both sides. A curious example is the case of the State of Illinoisthe birthplace of Pluto’s discoverer, Clyde W. Tombaugh. Congress in fact decided in March 2009 to vote on a law that would re-establish Pluto’s status as a planet, in honor of the state of its discoverer.

Strong criticism came from the‘inside the astronomical community same, in particular from Alan Sternthe Principal Investigator of the mission New Horizons of NASA, which in 2015 carried out the close flyby Of Pluto and its moon Charon giving us the splendid images visible in the video above, after a journey lasting more than 9 years.

Alan Stern in fact criticized the IAU’s decision, arguing that the vote had involved less than 5% of the more than 10,000 astronomers registered with the IAU in the world. In addition, following the fly-by of the New Horizons probe, it was discovered that Pluto is a much more dynamic world than imagined, with large mountains, craters and signs of flow of chemical elements in liquid form that have massively changed the geology of the place.

Precisely on the basis of these characteristics, Stern argues that Pluto should be considered a planet because it is a dynamic place, not a static place where the only changes are due to micrometeorite impacts. In addition, Pluto has five moons known (Charon, Styx, Night, Cerberus and Hydra)while two objects belonging to the category of planets, such as Mercury and Venus, possess none.