Why is Pluto no longer a planet?
Short answer: Pluto is not classified as a planet anymore because of its size, its shape, its orbit and the discovery of hundreds of other things just like it!
When was Pluto reclassified as a dwarf planet?
When I was a school, we learnt that there were nine planets in the Solar System and Pluto was the ninth. Then in August 2006, the International Astronomical Union (IAU) voted to reclassify Pluto as a dwarf planet. But why did they change its status? Lets start with its discovery first.
How and when was Pluto discovered?
The planet Uranus’ position wasn’t as expected in its orbit leading astronomers to predict another planet’s gravity affecting it, planet X. In 1930, Pluto was discovered by US astronomer Clyde Tombaugh, using the Lowell Observatory in Arizona. At discovery, it was classed as a planet.
But what next?
Pluto was so small and far away it was hard for astronomers to observe it. It became easier as bigger telescopes were built. In 1978, astronomer James Christy discovered Charon, Pluto’s largest moon. Astronomers used this to estimate Pluto’s mass and discovered that it is even smaller than our own Moon.
Pluto also has an unusual orbit, This combined with its small size made astronomers start to wonder is it really a planet?
Then in 1990’s, CCD cameras were invented and allowed astronomers to see further and more clearly into space. Astronomers discovered many Pluto-sized bodies out beyond the orbit of Neptune. These objects are in a region known as the Kuiper Belt, a vast field of debris left over from the formation of the Solar System. The Kuiper Belt Objects are like fossils. Then in 2002, some larger bodies were found: Quaoar, Sedna, Makemake and Eris.
https://www.bbc.co.uk/news/science-environment-33462184
Astronomers began to debate Pluto’s status. If Pluto was a planet, should all the other objects like it be planets too?
No was the decision. In 2006, the IAU defined what makes a planet, and all the other objects became dwarfs.
What is a planet?
A planet is a celestial body that:
- orbits the Sun
- has a nearly round shape
- has cleared the neighbourhood around its orbit.
The last was the breaking point for Pluto. Its small size means Pluto hasn’t cleared its orbit.
Pluto was demoted to a dwarf planet.
Conclusion
- Pluto is a dwarf planet.
- It is NOT a planet anymore, because it is too small, its orbit is full of other stuff and there are hundreds more like it in the Kuiper Belt.
Fun Facts about Pluto
- Was a planet for 76 years!
- Smaller than our Moon.
- Has a “heart”-shaped glacier.
- Has five moons.
- Is really really cold! (around -225 degrees Celcius)
Written by Dr Heather Campbell for Just Good Science Ltd.
References
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