How would you feel if one day, a group of people who are experts in the field of biology realise that you don’t have all the characteristics to be a part of human species (sapiens). Instead, you get demoted to Neanderthalensis. How would you feel? Betrayed? Outraged? Dejected? I’m pretty sure that’s how Pluto would’ve felt (if a chunk of rock and ice had feelings) on August 24, 2006 — the day Pluto was downgraded to a dwarf planet.
Pluto was discovered by Clyde Tombaugh in 1930 and was declared to be the ninth planet from the Sun. It was the first and the largest object discovered in the Kuiper belt. Pluto has a very unique and inclined orbit. Its distance from the Sun is between 4.4 and 7.3 billion km, meaning Pluto is periodically closer to the Sun than Neptune. Initially, among the many names considered for the planet were Minerva, Cronus and even Constance. However, Venetia Burney, an eleven-year-old English schoolgirl, came up with the name Pluto after the God of Underworld (ironically, Pluto was not exactly a popular God. He even had to find and kidnap his own bride!). With the discovery of a dwarf planet Eris in 2005, the International Astronomical Union (IAU — an international association of professional astronomers) defined the term “planet” formally in 2006 and consequently reclassified Pluto as a dwarf planet.
So what are the characteristics of a planet and where did poor old Pluto fall short on? According to the IAU, a planet should:
- Orbit around the Sun.
- Have sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
- Have a “cleared the neighbourhood” around its orbit.
Pluto meets only two of these criteria, losing out on the third. In all the billions of years it has existed out there, it has not managed to clear its neighbourhood. “Cleared the neighbourhood” means that there shouldn’t be other bodies of comparable size other than its satellites in the vicinity, i.e. they must be gravitationally dominant. Pluto, unfortunately, shares its orbital neighbourhood with Kuiper belt objects such as the plutinos. So not having a clean backyard and front lawn can get you into a lot of trouble!
There are five officially recognised dwarf planets in our solar system, including Pluto. The other four are Ceres, Haumea, Makemake and Eris. Except for Ceres, which is located in the asteroid belt, the other dwarf planets are found in the outer solar system. Even among the dwarf planets, Pluto is the largest one (although it has less mass compared to Eris) and has five known satellites: Charon, Styx, Nix, Kerberos, and Hydra. Of the five moons, Charon is the largest and nearest to Pluto. Charon is more than half the size of Pluto. This is sufficient to cause the barycenter of the Pluto–Charon system to be outside Pluto, i.e. the two bodies orbit each other around a common centre of gravity located between the two. This has led many astronomers to consider referring to Pluto and Charon as a double planet. The other four moons orbit the Pluto-Charon system, making them circumbinary satellites.
Even though Pluto is the largest of the dwarf planets and has more moons than half the planets in our solar system, it was still kicked out of our family of planets, just because it wasn’t massive or dominating enough. Due to our callous and insensitive behaviour, we even managed to break its heart! (It was probably broken by gradual changes in geological and thermal composition, but I disagree).
Nevertheless, Pluto, we still love you!
