Planet classification

here it comes, here it goes. Today I will write about planet classification since I want to start to write about dwarf planets. This post will be little similar to Stellar classification but you will see that for planets there are not those classes for size differences. The most important thing is of course to know what is planet and what is not, which I will explain.

Ok in the year of 2006 International Astronomical Union anounced that Pluto is not a planet. It`s been almost ten years but I still know people that can not get over it. I know this picture is sad (and not to scale).

For now the definition go as follows:

Planet has to be orbiting Sun
Have to be generally spherical
Has to have enough strong gravity to clear its orbit.

So the thing is that Pluto can only check first two criteria but not the third so such a object is called a dwarf planet.
Also you can see that planet is only object that is orbiting Sun which as it seems is not any exoplanet. This means that those 2000 planets that we found are just a huge objects, but for the official definition they are not planets either. And.. because they are so far away, it can not be known yet if they are spherical or not.

You can see that those definitions are not very good but luckily one guy on some conference proposed new definition which is not yet agreed to be new one but anyway International Astronomical Union will have to make a better one which will be probably very similar to this:

Planet has to be orbiting at least one star or the remains of one. (So yeah, stuff that is orbiting white dwarf is still a planet if it meets the other criteria. Read about multiple star systems)
Planet has to have a clear path to itself. (Sorry Pluto.)
Planet has to have mass lower than the mass of Jupiter. (This is good so we dont accidentaly name brown dwarfs as planets.)

Now we know what are planets. Lets move what are dwarf planets. Those are the objects that are not able to satisfy the third rule, their orbit around Sun is not clean. Dwarf planets are: CeresCeres, the only dwarf planet in the asteroid belt imaged by Dawn, PlutoPluto seen by New Horizons on 13 July 2015, HaumeaHaumea with its two moons, as seen by Keck, MakemakeMakemake imaged by the Hubble Telescope in 2006, ErisEris and its moon seen from Hubble and Sedna for example.Sedna seen through HubbleThere are some other candidates also.

There is type of objects that are called minor planets. Those can be at the same time dwarf planets, like Ceres. The number of minor planets is increasing by very large amount every month. Only few of them are named, large part is numbered and there is  rest for which we dont even know exact orbit. Together there are almost 700,000 minor planets.

I did not find exact definition but minor planets should be those that are orbiting Sun, that is about it. They dont have to have any particular size, shape and their path can be trafic jam of asteroids. Actually yes, the asteroids are minor planets and also all the trojans and so on.
Above you can see all the types of objects that can appear in our Solar System except Sun.
Thats about it, I will definitely make post about or more about asteroids, comets and of course I will be continuing with moons.


Pictures are from wikipedia pages: IAU definition of planets, Minor planets and Dwarf planets and the first one is from this page: