Old debate re-opened - What is a planet?

Dave Lillis wrote:

Hi All,
You can see why the IAU are having trouble with this.

QUESTION, would it be a good idea to split the solar system into 2 sections, one area is contained inside a sphere which is inside the kuiper belt, and the second zone would be the kuiper belt itself.

A planet would be defined as a body greated then 1000km within the area before the kuiper belt, ALL bodies in the kuiper belt would be KBOs.
After all, every objects within the asteroid belt is called an asteroid, albeit, there are asteroids all over the solarsystem.

BUT, are are we just mincing words here, what if the Kuiper belt was not called the Kuiper belt and called just another asteroid field, then the easiest thing to do would be to make EVERYTHING with a diameter greater then say 1000/2000 km a planet and all else an asteroid,
which bring us back where we started.

david,
then by your definition the moon, galilean satellites and titan are all planets as they all exceed your definition. plus the are circular go round the sun (as well as around their respective planets) inside the plane of the solar system and in the same direction as the rest of the planets as suggested by bart. this would still mean that pluto is a planet as it exceeds all definitions here and it has you to be proven conclusively that it is an ekbo. . so i am happy to announce that there are 6 more planets in the solar system the moon,galilean satellites and titan .
stephen.

In fairness, it was a given that the bodies I was talking about would be orbiting the sun and only the sun.

dave lillis wrote

stepryan wrote:
Dave Lillis wrote:
Quote:

Hi All,
You can see why the IAU are having trouble with this.

QUESTION, would it be a good idea to split the solar system into 2 sections, one area is contained inside a sphere which is inside the kuiper belt, and the second zone would be the kuiper belt itself.

A planet would be defined as a body greated then 1000km within the area before the kuiper belt, ALL bodies in the kuiper belt would be KBOs.
After all, every objects within the asteroid belt is called an asteroid, albeit, there are asteroids all over the solarsystem.

BUT, are are we just mincing words here, what if the Kuiper belt was not called the Kuiper belt and called just another asteroid field, then the easiest thing to do would be to make EVERYTHING with a diameter greater then say 1000/2000 km a planet and all else an asteroid,
which bring us back where we started.

david,
then by your definition the moon, galilean satellites and titan are all planets as they all exceed your definition. plus the are circular go round the sun (as well as around their respective planets) inside the plane of the solar system and in the same direction as the rest of the planets as suggested by bart. this would still mean that pluto is a planet as it exceeds all definitions here and it has you to be proven conclusively that it is an ekbo. . so i am happy to announce that there are 6 more planets in the solar system the moon,galilean satellites and titan .
stephen.


In fairness, it was a given that the bodies I was talking about would be orbiting the sun and only the sun.

just another take on this.
but who says one planet cannot rotate around another?. bring back aristotle all is forgiven . it depends on what you define as a planet. you can have a binary star why not a planet ?.
stephen.

There's lots of good valid arguments being made here about size, gravity and so on. How about visual brightness? Could a planet be defined as a body in an orbit around the sun, only, with or without attendant satelites, which can be detected with a telescope from earth? Never mind extra-solar planets for the time being.

This visual brightness restriction includes a cap on size. Sedna is too small to be seen visually with a moderately large telescope, therefore it's not a planet. I don't expect to provide a definite answer, nor is this my final definition, just a thought.

C'mon - shoot it down :lol:

There's lots of good valid arguments being made here about size, gravity and so on. How about visual brightness? Could a planet be defined as a body in an orbit around the sun, only, with or without attendant satelites, which can be detected with a telescope from earth? Never mind extra-solar planets for the time being.

This visual brightness restriction includes a cap on size. Sedna is too small to be seen visually with a moderately large telescope, therefore it's not a planet. I don't expect to provide a definite answer, nor is this my final definition, just a thought.

C'mon - shoot it down :lol:

spculleton wrote:

There's lots of good valid arguments being made here about size, gravity and so on. How about visual brightness? Could a planet be defined as a body in an orbit around the sun, only, with or without attendant satelites, which can be detected with a telescope from earth? Never mind extra-solar planets for the time being.

This visual brightness restriction includes a cap on size. Sedna is too small to be seen visually with a moderately large telescope, therefore it's not a planet. I don't expect to provide a definite answer, nor is this my final definition, just a thought.

C'mon - shoot it down

proceeding to shoot down arguement . what size scope would be moderately large?. by this definition vesta etc would become planets, so could chiron. brightness doesn't always relate to size, the moon only reflects 7% of incident light but is very close therefore very bright.
stephen.

Shoot down part deux!
It would also depend on who is looking through the scope: is it my granny with her 2 inch thick glasses or is it Stephen James O'Meara observing from an altitude of 7,000ft on the side of a volcano on Hawaii?Besides, how many of us have ever seen Pluto?

However, to be fair, it's a reasonable attempt at answering a difficult question. Pesonally, I think the size limitation factor is the easiest thing to do. The ordinary punter on the street can also get their head around it. 1000 miles or 1000 km would be reasonable. If there is uncertainty concerning a planet's size and the estimated range falls across this 1000 boundary then take the conservative approach and rule it out until proven otherwise.

Michael