In August 2006, the International Astronomical Union (IAU) declared that Pluto was no longer a planet. But why is Pluto not a planet anymore?
History of Pluto
On 18 February 1930, an astronomer named Clyde Tombaugh discovered Pluto and scientists soon declared it as the 9th planet of our solar system. In 1978, another astronomer named James Walter Christy discovered Pluto’s largest moon Charon. As previously mentioned, the planet was demoted to the status of a ‘dwarf planet’ in 2006.
What is a dwarf planet?
A dwarf planet is a celestial body which is similar to regular planets, but lacks certain criteria that are required for it to be recognised as a planet.
What criteria defines a planet?
According to resolution 5A of the International Astronomical Union (IAU), in order to declare a celestial body as a planet, it must meet three criteria:
- It orbits the Sun.
- It has enough gravity to overcome external forces and as a result, achieve a round shape.
- It has cleared its neighbouring region of other objects like debris and other celestial bodies.
Resolution 5A also established two new categories of celestial bodies that orbit the Sun, namely ‘dwarf planets’ and ‘small solar system bodies’.
Why is Pluto not a planet?
Pluto meets the first two criteria of resolution 5A, but fails to meet the third one. This means that it has not cleared the majority of debris like asteroids and other space rocks in its orbit.
Another factor that affects Pluto’s credibility as a planet is its largest moon Charon. Charon is almost half the size of Pluto. For comparison, the Earth’s moon is only a quarter the size of our planet. The sheer size and gravitational force of Charon causes Pluto to wobble in its orbit.
How dark is Pluto?
As Pluto is very far from the Sun, it is extremely dark. To understand how dark Pluto is, scientists have come up with the concept of ‘Pluto time’. It allows us to realise how dark Pluto really is. This concept suggests that if we look at how dark Earth is during dusk, it would be equivalent to how dark Pluto is during noon.
Controversy and debate between scientists
The demotion of Pluto as a planet sparked public controversy. Some scientists and astronomers debated that the IAU’s definitions were not useful, while others supported the decision.
Some argued that big planets like Earth and Jupiter have also not cleared their orbits of debris, while others suggested that if Pluto was to be accepted as a planet, they would have to accept every other celestial bodies in the solar system as planets as well.
For now, it seems like Pluto will remain a dwarf planet for the foreseeable future.
Ms Kalinga