Solar System forum Planets, Sub-planets, dwarf planets, planetoids. How many actually?

Student Academic Forum on Astronomy

University of Oxford


lim ju boo


Quite frankly since I was a child I knew there were just eights planets, namely the inner ones Mercury, Venus, Earth, and Mars, and the outer giant ones being Jupiter, Saturn, Uranus and Neptune. That was all I knew as a child.

Later they included the most distant and smallest one Pluto discovered by Clyde Tombaugh in 1930 to make it nine. Then when I left school, I read many of them have moons including Charon Pluto’s moon

Then came many rocky objects large and small called the asteroids orbiting between Mars and Jupiter, within a belt called the asteroid belt.

After that I learnt  about the comets, the most famous one called Halley’s Comet originating from the Oort’s Clouds far out from the Solar System,  and other objects in the region of Pluto called the Kuiper belt, as well as many objects some even farther away than Pluto. With all these discoveries they downgraded Pluto and now say Pluto is not a planet but a dwarf planet

Then came other objects called plutinos which they say are trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. I wonder what that mean?

Now I am wondering which heavenly bodies circulating around the Sun are planets, dwarf planets, sub-planets, planetoids?  Astronomers sometimes reclassify them.  Do they reclassify them according to sizes just like Ceres has been reclassified after the discoveries of more asteroids and other dwarf planets!

Frankly now I have not much idea how many can be called planets, dwarf planets, sub-planets, large asteroids, comets, meteoroids, etc.  I don’t even know how many of these large and small bodies are there actually orbiting the Sun. It looks to me there may be in their thousands just like the asteroids in the asteroid belt. I really don’t know. Maybe Dr. Grant Miller can tell us.  But I still like to stick to my 9 planets in the Solar System, the rest I just like to call them dwarf planets.

I think we  have to leave this problem to the astronomers to tell us exactly how many planets, sub-planets, dwarf planet, planetoids are there circulating around our Sun, and give us  reasons for their classification, whether  they are according to their sizes, mass, composition such as gaseous, icy, rocky…or whatever

Even the number of moons and their names orbiting around most of the planets is quite a mouthful for me to digest, let alone remember their names, composition, properties and characteristics

I only just need to be aware of all these discoveries in this simple course on astronomy which is already quite a bit of new information and quite enjoyable for me to read and discover

Thank you for enlightening me.