What is a Planet?

Debate Forces New Definition.

I would very much like to resurrect the debate here.

I was very disappointed with the demotion of PLUTO.

I understand that astronomers are disagreeing whether Pluto is or is not a planet. I personally believe that it is a planet. I also believe that we do not know enough about Pluto to really say otherwise. I did find some things on the issue and hope this is relevant.


Astronomers agree about two things. To be a planet, and object must orbit a star. If it orbits something else, like another planet, it is a moon instead. The second point is that the object must be big enough that gravity makes it into a sphere. Many asteroids and comets have odd shapes. They are definitely not planets.

I understand that astronomers are disagreeing whether or not Pluto is or is not a planet.

I don't think they are. They were a year and a half ago, but now a definition is in place that states that Pluto is not a planet.


I also believe that we do not know enough about Pluto to really say otherwise.

If you had an object, and did not have enough information to know whether to call it a cup or not, would you call it a cup?


Astronomers agree about two things. To be a planet, and object must orbit a star. If it orbits something else, like another planet, it is a moon instead. The second point is that the object must be big enough that gravity makes it into a sphere. Many asteroids and comets have odd shapes. They are definitely not planets.

The official definition by the International Astronomical Union:


The IAU...resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:

(1) A planet [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Footnotes:

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Hope this helps.

What is a Planet?
Any sphere of a certain size. Pluto is too small for a planet, so falls now in the group of dwarf planets. Anything (much) smaller than Pluto is classed as an asteroid.

Any sphere of a certain size. Pluto is too small for a planet, so falls now in the group of dwarf planets. Anything (much) smaller than Pluto is classed as an asteroid.

There are 2 more conditions for a planet to meet - see my post.

There are 2 more conditions for a planet to meet - see my post.
Incorrect. If you study what I posted, you will see that it was correct.
The way I stated it I did not have to specify the "clearing" of debris etc. I kept to the basics of the question "what is a planet".

Copying text from other websites we all can do, but I do not think that was necessary here.

Incorrect. If you study what I posted, you will see that it was correct.
The way I stated it I did not have to specify the "clearing" of debris etc. I kept to the basics of the question "what is a planet".

Copying text from other websites we all can do, but I do not think that was necessary here.

By your condition, Ganymede and Titan would be considered planets, as they are bigger than Mercury, but they are moons.

Right?

I wonder what ROLCAM thinks?

Thanks!

By your condition, Ganymede and Titan would be considered planets, as they are bigger than Mercury, but they are moons.Right?
Right. I should have added "in orbit of a star, not of a planet". But that was logical anyway, wasn't it? :)

Right. I should have added "in orbit of a star, not of a planet". But that was logical anyway, wasn't it? :)


No, apparently not.

JudyKayTee

Why that unnecessary negative attitude? If Capuchin had posted that, it would have been a different matter.
But he did not do that. You did.
All that is apparent here is that you seem to have a problem, but do not address that in the relevant topic.
A post that is an 'expert' unworthy !