In 2006, the International Astronomical Union (IAU) established definitions for the designation of a planet. By this point in history the general idea had been to call large bodies in space planets i.e. a meteor is too small but Venus is fairly large so it can be a planet. The larger end was fairly easy to approximate; you had rocky planets, larger gas giants and the Sun. The smaller end required some debate. The definition that was worked out was the following:
1. Must orbit the Sun
2. Massive enough to attain hydrostatic equilibrium
3. Has no other large body sharing its orbit – cleared out the neighborhood
Hydrostatic equilibrium occurs when the gravity of a body is strong enough to stabilize its shape in a uniform manner and clearing your neighborhood means that a celestial body needs to be the biggest in its relative area of space. Dwarf planets are defined as qualifying for the first two conditions but fail at the third. They can’t be satellites (moons) and haven’t cleared their neighborhood i.e. larger bodies orbit them, much larger than normal moons. The reason Pluto was listed as a dwarf planet actually came from a new discovery. In 2005, the dwarf planet Eris was discovered; however, it is larger than Pluto (this was later disproven however Eris seems to still be more massive). Thus, it should now be the 10th planet. This prompted the IAU to define what a planet should be.
The main problem Pluto had was its moon, or moons. It actually has five, but one in particular was a bit large. Its moon Charon is smaller than Pluto but it is about 12% the size of the dwarf planet; a bit large for a moon. It also means that Charon does not orbit Pluto like our Moon; instead, Pluto orbits a point in space which Charon does as well because Pluto isn’t big enough to force Charon to orbit it. The space around Pluto is also occupied by many rocky bodies and, by mass; Pluto occupies very little space in comparison to everything near it. Thus, it can be a dwarf planet but not a real one. An article published in The Astronomical Journal goes into detail regarding Pluto’s inability to clear its neighborhood due to its low mass. The main measurement they calculated was Π (capital form of pi), the planet discriminant, which is the relationship between the mass of a planet and its orbit-clearing mass or ability to push away a majority of the mass surrounding it. It has to be at or above 1 in order to qualify as a planet, with Pluto well below that limit. Aside from this, there is actually another, more unique problem. Pluto is too small.
If we compare Pluto to our moon you begin to see a problem arise. By comparison, Pluto is 6 times less massive, 3 times less voluminous and less than half the area of the Moon. So, is the Moon a planet? Can a planet in any star system be smaller than a moon? It raises a serious question about how we define planets. If a planet doesn’t dominate its nearby space and has to share the space it is a good indication that it either isn’t large enough to be a planet or represents a different category of planets. If Pluto ever drifted too close to Earth it could easily become a Moon itself.
With all that said, I know there are people sad about Pluto’s demotion. Plenty of scientists debate the topic and what our classification system should look like, which is open to interpretation. People who miss Pluto tend to cling to the old charts and songs talking about Pluto but, to be fair, did any of these people care about Pluto before it lost its status? It is a reaction to the world changing but science is a discipline concerned with understanding the universe, not preserving old ideas.
Mass: 7.342 × 10^22 kg
Volume: 2.195 × 10^10 km3
Surface Area: 3.793 × 10^7 km2
Mass: 1.30 × 10^22 kg
Volume: 7.05 × 10^9 km^3
Surface Area: 1.779 × 10^7 km^2
IAU Planet Definition
New Horizons Probe Reveals That Pluto is Bigger Than Expected
A Quantitative Criterion For Defining Planets, The Astronomical Journal
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