Why Pluto is no longer a planet

Pluto Dwarf Planet

A 6-year-old girl living in Ireland recently wrote to NASA asking the US agency in unusual request to have Pluto reinstated as a planet in our solar system. Indeed, Pluto, which was discovered in 1930, was until 2006 the ninth planet in the solar system. But why did Pluto cease to be a planet in the first place? What is a dwarf planet? What makes a planet a planet? Let’s take a look!

The New Horizons spacecraft, which aimed to study Pluto and its satellites, took off from our planet in 2006. At the time, Pluto was still considered to be the ninth planet in our Solar System. But a few months later the then most distant “planet” in our system was “downgraded” to a dwarf planet. Yet, the observations of Pluto by the probe did not begin until 2015!

On February 17, 2018, the Washington Post reported an endearing story. Cara Lucy O’Connor, a 6-year-old Irish girl, wrote to NASA asking for Pluto’s return to the solar system planetary club. In her mail, the girl gives precise scientific data including the Kuiper belt — a ring of small spatial bodies located beyond Neptune which is also the home of several dwarf planets.

But why is Pluto no longer considered a planet in the same way as the Earth or Mars? Although it is in orbit around the Sun and has a roughly round shape, the argument put forward by the researchers explains the shortcomings of Pluto. Indeed, its gravitational force is not important enough to either attract and therefore agglomerate other bodies around it, or push them away. Thus, Pluto was downgraded and gave rise to a new class of dwarf planet: the plutoids, a category in which was placed Eris, the most massive dwarf planet known, located beyond the Kuiper belt.

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So what makes a planet a planet?

The word planet comes from the Latin word planetus, which itself comes from the Greek word “planeta” which means “wandering body”, or “moving body”. Although the origin of the term goes back to ancient Greece (the observation of the movement of the planets goes back much earlier), it is interesting to remember that the definition of the term has varied over time: the Earth itself was not considered a planet as such before the 16th century!

As for the exoplanets, we had to wait for the 1990s for the first discoveries, although the idea of planets existing outside our solar system dates back several centuries ago. Note that if the term “planet” in everyday language refers to the idea of planet in general, in a more scientific context, it generally refers to only the planets of our solar system. For the other planets, we use the term of exoplanets. In the context of our articles on celestial objects, we will take again these two terms (of planet and exoplanet) and their respective definitions.

According to the International Astronomical Union (I.A.U), for a celestial body to be considered a planet, it must meet the following criteria:

  1. It must in orbit around a star
  2. It must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
  3. It must be neither a star nor a satellite of a planet
  4.  It must have cleared the neighbourhood around its orbit.

Pluto meets two of the 4 criteria aforementioned. It is in orbit around the Sun, it is neither a star and nor a satellite of a planet but as we mentioned earlier it does not have sufficient mass to attract and agglomerate other bodies around it or push them away.

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What makes a dwarf planet a dwarf planet?

A “dwarf planet” on the other hand is a celestial body that:

  1. is in orbit around the Sun
  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
  3. has not cleared the neighbourhood around its orbit
  4. is not a satellite

Per that definition, Pluto is definitely a dwarf planet. However, not everyone agrees.

The debate over Pluto’s status

Scientists like Kirby Runyon of Johns Hopkins University and five of his colleagues at the NASA New Horizons mission have already proposed to simplify the definition of a planet as “a sub-stellar mass body that have never been through nuclear fusion and which has a gravitational strength sufficient to be round thanks to hydrostatic equilibrium”. In other words: “a round spatial object smaller than a star”.

However, if the supporters of “planet Pluto” were to win a redefinition of what a planet is, it would then be necessary to add 110 planets at once to the solar system: Pluto of course, but also Trans-Neptunian objects and current satellites like the Moon, Europa or Titan. As you can see, things can get complicated.

A planet or not, Pluto will remain on our scientific consciousness for years to come. Moreover, the decommissioning of Pluto had the effect of a bomb in the United States; it was the only planet in the Solar System to have been discovered by an American.

Carl Frantz

Polyglot, humanitarian, Carl was born in Germany but raised in the USA. He writes mostly on tech, science and culture.