Hi all,

I was reading an article interesting article on Sedna @ www.gps.caltech.edu/~mbrown/sedna/ which brought up a subject which I have heard discussed before " What exactly is a planet?". With the recent discoveries of new "objects" such as Quaoar (how do you pronounce that?...) and 2004 DW (Couldn't they come up with a better name than that?....) and now Sedna (now thats alot better!...), the line between asteroid and planet has become blurred. No where in all my searching, have I been able to come up with a solid definition of a Planet..... there just isn't one! There are those who would even say that Pluto should be demoted from it's planetary status!

I'd be very interested to hear your views on what makes a planet, a planet. I've also added a poll (my first, so I hope I've done it correctly...) on whether or not Pluto holds on to the "Planet" status. I say "Yes!" but then, I'm sentimental. What do you say?

Hey! The poll worked!

My apologies to Terry, I had read the piece you wrote on the "From Terry..." string which asks (and answers) the same questions that I ask above :oops: . So in the interest of being neighbourly, and (showing that I didn't just ignore your remarks, I will quote the whole piece here:

Hi all,

1. The Solar System is now 2 billion miles bigger in diameter than we thought! A new planet, discovered by Michael Brown & his team at Caltech, provisionally named Sedna (the Inuit goddess of the sea) orbits about 2 billion miles further out than Pluto. Is it a planet, I hear you ask? Well, it's only slightly smaller than Pluto, and quite a bit bigger than the other Trans-Plutonian objects in the Edgeworth-Kuiper Belt (EKBO's|). The official announcement will be tomorrow.
In 2002 Brown's team discovered "Quaoar", about 800 miles across, and orbiting 1 billion miles beyond Pluto. (Pluto's mean distance from the Sun is 3.6 billion miles). Then last month they reported the discoverey of another body, provisionally named 2004DW, which is 10% larger, and slightly further away. These are the biggest of the 800 or son EKBOs, most of which are are about 60-100 miles across.
Sedna is thought to be about 1,200 miles in diameter, compared with Pluto's 1520 mile diameter. It's the biggest object discovered in the Solar system since Pluto 74 years ago; it's 50% bigger than Quaoar.
Some astronomers want to re-classify Pluto from being a planet to being a large EKBO, but the majority voted to retain its planetary status.
So is Sedna a planet or not? I always thought that the totally arbitrary figure of 1000 miles would be a reasonable dividing line between a planet & either an asteroid or an EKBO. It's a nice round figure, which I'm partial to, even if it is in the old imperial units! So I'll regard it as a planet until there's an official designation otherwise!

And sometimes tradition is as good a guide in these matters as anything - after all, we still refer to 'Planetary Nebulae', although they have nothing to do with planets! And we still retain the original constellations, with their weird boundaries (we even have one, Serpens, which is divided in two by another constellation, Ophiuchus!). So if we can put up with those anomalies, surely we can at least keep Pluto as a planet, even if Sedna is eventually classified as an EKBO!

2. IAA member Peter Paice's excellent new solar images are now featured on today's Spaceweather.com website.

BT, don't forget the IAA meeting, Stranmillis College, 7.30 on Tues evening, and the lecture in St Patrick's Trian, Armagh, on Wed night - see last email for details.

Clear Skies,

Terry Moseley

I humbly beg your forgiveness..... :oops: :oops:

My apologies to Terry, I had read the piece you wrote on the "From Terry..." string which asks (and answers) the same questions that I ask above

Sorry, I mean't "hadn't read".

And can someone at Cosmos please sit down with Terry and see why he cant log into/post on this board, so I can stop crossposting (or rather forgetting to crosspost)

I think pluto should hold it regardless of what definition we ever come up with. It would be a bit silly to demote it at this stage, especially since it has a moon.

As for a definition... any object upon which we can land an astronaut and have him/her jump of the ground and return to the plaets surface before tea time is a planet.

That has a practical basis. If they arnt big enough for gravity to have a useful effect then its not worth thinking about setting up camp there. so ts not a planet. Oh and it has to orbit the Sun and not be a moon of something else either!

Cheers,
~Al

Hey Al, I'd like to see an astronaut land on the surface of Jupiter, or Venus, and then pop back into the lander for tea!

I'd leave Pluto as a planet, simply for historical value, but I'd also set it as the lower limit. Quououaarrrr and Sedna should just be large EKB objects.

I'll sit down with Terry at Cosmos and try get him sorted out for posting to the boards.

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.

Al, you definition is kinda nice except that is de-classifies all the gas Giants from Planet status becaues you ain't gonna land on any of them and go for a stroll!

Bart.

I must admit I'd prefer Sedna to be a planet, and I agree when Terry says 'planet' should have a diameter of 1000 miles as a lower limit, which I also think has a nice 'round' feel to it

...of course having a planet for each of our digits is hardly a scietific reason behind having Sedna as a planet, but it is a fun one

Have Fun,
Jim.

If nasa weren't to use the diameter as a means of deciding its a planet, what other ways do they have of deciding

gravity ? how spherical is the body ???

I'd also like it to be called a planet, when will they decide ?

A term I have'nt seen for a few years now is "minor planet"
I rmemeber see the bigger asteroids been refered to in this way.

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.

I like this method and have voted accordingly

voyager wrote:

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.

would this not mean that you'd have to reclassify some of the largest asteriods and titan and the galilean moons to planets? should a planet not have at least some sort of atmosphere?, or at least the mass to retain one.
stephen.

voyager wrote:

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.

would this not mean that you'd have to reclassify some of the largest asteriods and titan and the galilean moons to planets? should a planet not have at least some sort of atmosphere?, or at least the mass to retain one.
stephen.

By that logic stephen wouldn't mercury need to be reclassified as well

ladies and gents,
www.space.com/scienceastronomy/sedna_earth_040316.html
there could even be another earth sized planet out there.
stephen.

finnjim2001

stepryan wrote:
voyager wrote:
Quote:

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.


would this not mean that you'd have to reclassify some of the largest asteriods and titan and the galilean moons to planets? should a planet not have at least some sort of atmosphere?, or at least the mass to retain one.
stephen.


By that logic stephen wouldn't mercury need to be reclassified as well

finnjim2001
not necessarily i said that it could have the mass to retain to retain it would not necessarily have one. besides that i think i read somewhere that it does have a trace atmosphere but it is very thin.
stephen.

voyager wrote:

As for what makes a planet ... that is a really difficult question! My definitions would be a large body in an almost circular orbit around a star. Hence I would re-classify Pluto as an EKBO and reduce us to 8 planets.

would this not mean that you'd have to reclassify some of the largest asteriods and titan and the galilean moons to planets? should a planet not have at least some sort of atmosphere?, or at least the mass to retain one.
stephen.

OK, allow me to rephrase that some what better:

A planet is a large circular body in a near circular orbit around the Sun.

That rules out the asteroids and any moons and makes Pluto and EKBO.

The real planets all orbit in the same plane, in the same direction and in similar near circular orbits. This to me implies a common origin and hence these objects should bea class to them selves.

Pluto and the other EKBOs all have inclines and highly eliptical orbits, they too seem to share a common source but it does not seem to be the same as that of the planets and hence these objects should not be called planets IMHO.

voyager wrote:

OK, allow me to rephrase that some what better:

A planet is a large circular body in a near circular orbit around the Sun.

That rules out the asteroids and any moons and makes Pluto and EKBO.

The real planets all orbit in the same plane, in the same direction and in similar near circular orbits. This to me implies a common origin and hence these objects should bea class to them selves.

Pluto and the other EKBOs all have inclines and highly eliptical orbits, they too seem to share a common source but it does not seem to be the same as that of the planets and hence these objects should not be called planets IMHO.

just to keep the arguement going. by what rule says that a planet has to travel :-
(A) in a circular orbit
(b) around the sun
(c) in the same direction as all the other planets

if you look at the solar system venus axis rotates backwards and the pole of uranus is at 90 degrees from where it should be with respect to the ecliptic so it faces us. this with our own planets inclination should show us that this narrow definition is simply silly. we have yet to survey any of the other solar systems around any other star other than our own in any detail. who says that the universe has to obey the rules of our system. there could be other systems where planets rotate at different inclinations to each other. this could be the norm rather than the exception. planets could orbit planets, i.e. who said you could not have an earth size planet with a moon the size of mars. it would probably not last that long unless the mars object was very light but it could happen. all models of the origin of the solar system are just that models. no one can prove it happened. no one has a time machine that can go back there. they only way that we can get an idea is to explore other systems particularly ones that are forming and that is not likely for a good while. a nice and neat solar system with all the planets behaving in the one way as stated above would be the exception rather than the rule giving the chaos of the event and the likelyhood of it occuring. diversity seems to be the key to nature not clones. it was not that long ago that the earth was the center of the solar system, comets were imperfect objects in the atmosphere and the planets were heavenly gods. while ekbos are far out and may not be circular to dismiss them as debris is silly as seeing we did not have any proof that they existed until recently so we know nothing about them, for most astronomers the did not exist.
stephen.
p.s.:- the earth is slightly oblate and not pefectly circular .

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.

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.

[img size=320] :twisted: Just to stir things up a little, There are a number of definitions orbiting the science communitity which I personnally find to be flawed. Here are a couple of examples: 1.) From the discovers of sedna: "We define a planet to be any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit. For example, many asteroids cross the orbit of the earth. Yet the earth is more massive than all of those put together. Thus, the earth is a planet. Ceres, the largest asteroid, is not greater in mass than the sum of the masses of the remaining asteroids. Hence, not a planet." If this where the case, then if an earth sized object was discovered in the same orbit as Jupiter, it would not be classed a planet. - Nope, this can't be right! 2.) From Gibor Basri of the University of California, Berkeley: "A planet is a spherical object never capable of core fusion, which is formed in orbit around an object in which core fusion occurs at some time." This definition gives no reference at all to mass, - are we then to class all the Ceres, Pallas, Vesta and all the other asteroids as planets? This debate will probably rage on forever and a day. I think that the only way this can be settled is if the definition of a planet is split into a number of catagories, for example (and these are ONLY SUGGESTIONS, I'll leave the actual definitions to people with bigger brains than me!) A possible catagory could be (a) class 1 planets - planets (above a certain mass or size to be defined) that orbit around a star. (b) class 2 planets - (of the same sized definition) that orbit around a primary planet (basically moons, this gets over the fact that these also orbit the sun and may be larger than some primary planets). Class 3 planets - free floaters, planets that do not appear to orbit anything. I think that the core fusion element must also be considered or stars will also be thrown into the mix and really bugger thing up. or maybe we're just making it all too complicated......... [/img] :!: :!: :!:

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

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

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! :wink:
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

Ok, this is the way I see it.
Planet = body with a diameter greater then 1000km/miles which orbits a star/s, implying that it orbits nothing else.
2 exceptions

1, double/multiple planet, where both/all bodies have a diameter greater then 1000km AND their center of gravity is located outside of either mass, (they obviously orbit each other and that pair/set orbit the star).

2, rogue planet, where a body greater then 1000km is wondering in intersteller/intergalactic space with no associate star, (presuming it is not a star itself).

Shape/brightness/observability/direction or composition do not come into it.