I like Pluto. When I was a kid it was a planet, the ninth planet in fact.
Now it's not.
What gives and why does the International Astronomical Union (IAU) hate Pluto so much?
My Definition Of A Planet
No one cares about my opinion, I know that, but for this post I just want to provide what I personally think a planet is.
I believe that if an object satisfies all of the following three simple conditions then it is a planet:
- It is a body that orbits the Sun.
- It compresses itself into a spherical shape (or near-spherical shape) under the pressure of its own weight. In more technical language: it has sufficient mass to assume hydrostatic equilibrium (a nearly spherical shape).
- It has never produced nuclear fusion reactions.
That's it.
A body that is in orbit around another body that is itself orbiting the Sun is not a planet even if it is spherical, it's a moon. For instance Titan is round and it is in orbit around Saturn. This means that Titan is a moon.
This definition is simple, elegant and easy to figure out. It also means that Pluto is a planet.
This kicker though it means that these other objects would also have to be called planets:
This would means the solar system might have up to 14 planets. I am okay with that.
IAU's Definition Of A Planet
The IAU have their own definition of a planet which goes as:
- A body that orbits the Sun.
- It compresses itself into a spherical shape (or near-spherical shape) under the pressure of its own weight.
- It has "cleared the neighbourhood" around its orbit.
You will notice the third requirement which is to clear the neighbourhood around its orbit.
This would clearly eliminate Ceres because Ceres is in the asteroid belt which includes many bodies of substantial size such as Vesta, Pallas, Hygiea and many, many more.
This also eliminates Pluto because since it lurks in the neighbourhood of Neptune it clearly has not cleared the neighbourhood of its orbit.
In fact all of Pluto, Eris, Haumea, Quaoar and Makemake etc lie in the Kuiper belt and also have not cleared their orbits of competitors.
My Criticisms on The IAU Definition
I have problems with the IAU definition for a few reasons.
The third criterion is not a parameter that can be determined crisply and clearly.
For instance, Pluto has not cleared its orbit of Neptune, but the same can be said of Neptune since Pluto is also lurking in the area.
The Earth has this large thing near it called "The Moon" (end sarcastic tone) which is relatively large and spherical. The Earth therefore does not seem to have cleared its orbit of objects as well.
Mars lies very close to the populous asteroid belt. We must therefore also eliminate Mars.
Even the gas giants have interlopers in their orbits as well as Trojan asteroids that occupy their orbits so it would seem even these colossi have not managed to clear their neighbourhoods.
This zone clearing requirement is awkward and clunky and feels like something only a soulless bureaucrat could invent.
Closing Words
There is apparently some professional resistance building in the astronomical community to get the status of Pluto redefined.
If they succeed I would consider them to be heroes, nerd heroes, but heroes nonetheless.
Thank you for reading my post.
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Well at a point o heard pluto went of course and is far gone... I don't know how true this is but i would like to know the truth.
Okay, i agree! Pluto is a planet nonetheless.
But why on earth (or in the solar system) would someone make it a criteria for the planets to "clear their neighborhood?" i never knew there were solar system cleaners 😁
I totally agree with you, Pluto will always remain a planet to me, it orbits the sun and has a spherical shape. Any other ambiguous requirement, is just a farce.
Pluto is still 9th planet for sun to me. Coz when I was student my teacher told me. So it will always remain 9th for me.