Ever since its discovery in the 1930s, scientists and astronomers have been divided about whether Pluto should be counted among planets.
While many of us were distraught at Pluto’s demotion to a dwarf planet, members of the International Astronomical Union (IAU) had been debating this issue for several decades before the verdict was finally reached in 2006.
For the curious few who are still wondering why Pluto is no longer a planet, here are five reasons that the IAU took into account before taking the landmark decision that forever changed our science textbooks.
Size
Pluto is very small compared to the eight planets in the solar system. With a diameter of 1,477 miles (2,377 km), it is about half the size of the smallest planet in the solar system, Mercury, which boasts a much larger diameter of over 3,030 miles (4,880 km).
Color image of Pluto as captured by NASA’s New Horizons.
Furthermore, the discovery of comparatively larger celestial objects within our solar system, like Ceres and Eres, has made people wonder if we need to be more inclusive in our planetary list.
Misunderstood
One of the strongest rationales in the debate against Pluto has been its ambivalence. Since its discovery, it has perplexed many and split astronomers into groups for and against, including the celestial body in our solar family.
According to Mike Brown at the California Institute of Technology, Pluto was never a planet. It was misunderstood and never had a footing with the other planets.
Orbit
Unlike other planets in our solar system, whose orbits are in a flat plane around the sun, Pluto’s orbit is a bit erratic and tilted. It takes 248 years for Pluto to finish one revolution around the Sun. At times, Pluto is closer to the Sun than the last designated planet, Neptune, and spends most of its time further away from the sun.
Clearing its orbit
Pluto’s erratic orbit not only crosses paths with Neptune but also with several other objects in the Kuiper belt, a disc of icy bodies that is similar to the asteroid belt but much wider and up to 200 times more massive.
One of the main criteria the IAU sets for a planet is to clear its orbit of other celestial debris, which Pluto has not so far. Interestingly, planets like Earth and Jupiter have also not cleared their orbits of asteroids, but Pluto has other things going against it as well.
Twin planet
One of Pluto’s moons, Charon, is almost half the size of the exoplanet and orbits it extremely close to it. It takes Pluto 153 Earth hours to complete one rotation around its axis and about the same time for its moon to complete one revolution around Pluto.
NASA’s New Horizons captured this high-resolution enhanced color view of Charon just before the spacecraft made its closest approach to Pluto on July 14, 2015.
If one were living on Pluto, Charon would never rise or set and perpetually be seen in the same spot in the sky in a tidal lock.
Astronomers believe that Pluto and Charon are, therefore, not planets and satellites but singular entities, recommending their removal from the list of planets.
If you enjoyed reading this article, you will love our newsletter. Sign up for free!
We also have some creative science merchandise, like this Born to Explore fridge magnet.
Born to Explore Wooden Fridge Magnet
To see more, visit our Shop.
