Definition of a planet

[ThunderChild (Chris)]

Hey all,

2003 UB313 (aka the 'tenth planet') certainly put alot of people into a spin. The question regarding whether it should be classified as a planet has even been discussed in the main stream media. The last one I heard was yesterday on ABC radio from some professional astronomers talking about the upcoming IAU meeting where they hope to come out with a definition.
I guess we never had a strict definition before since no-one expected to find any more (?).

One possibility raised which I had not heard of before was the idea of stripping planethood status from Pluto all together. It seems there are three options :
(a) Create a definition which includes Pluto (and would therefore probably include 2003 UB313 which is larger than Pluto).
(b) Create a definition from which Pluto is technically excluded, but keep Pluto included out of respect for historical/cultural baggage.
(c) As (b), create a definition which excludes Pluto and go back to 8 planets.

I must confess that removing Pluto as a planet seems like a very hard thing to do, given that it is deeply embedded into the social conscience (even if there are good scientific reasons for doing so).
I also understand that this can even be an emotional topic for some.

Anyway, that got me wondering what opinions would be from the good people here at IceInSpace? Since there seems to be division among professionals, I imagine there would be a fair range of thoughts from the people here - and I'm interested to hear them.

What you do think?

[Robby]

I guess we have to move forward, not backwards.. So I reckon that the new object should be called a planet for sure. After all history marks progress with the ever increasing number of planets.. 1,2,5,7,8,9 etc... So we are advancing further so it's really a no brainer that this advancement heralds yet another planet. We can't go back, that would juet be silly...

if (question == 0xff)
hamlet++;
else
pluto--;

Cheers

[acropolite (Phil)]

I ticked other. IMO If a body orbits the sun it should be classifed as a planet or minor planet. Maybe the criteria could exclude those bodies that are obviously not spherical and small (asteroids), if it's spherical, of reasonable size and or has orbiting moons it should be classed as a planet or perhaps planetoid.

[Thiink]

Although this is non-technical, generalised descript, here is a definition of a planet from dictionary.com:

"A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto."

Personally I think we should keep Pluto as a planet, and also classify UB313 as a planet as well. But then again what do I know?

[janoskiss (Steve H)]

I was thinking along the same lines as Phil. If it is large enough to be naturally close to spherical shape (definition still required here) and orbits the Sun directly (i.e., it is not a moon of a larger body) then it has earned the right to be called a planet.

Here is another scenario: What if two bodies orbit each other as well as the Sun but neither is significantly smaller than the other to be called the other one's moon? Are they both planets? Are they both moons? Or say one is a bit smaller but a bit more massive at the same time. Which one is the planet which one is the moon?

[ThunderChild (Chris)]

Originally Posted by janoskiss

I was thinking along the same lines as Phil. If it is large enough to be naturally close to spherical shape (definition still required here) and orbits the Sun directly (i.e., it is not a moon of a larger body) then it has earned the right to be called a planet.

In that case, I'm interested to know why you voted for 'other' instead of including all suitably attributed Kuiper belt objects (ie do not exclude them), which is what it seems like you're saying.

I was thinking that if an object is large enough and orbits Sol, then it's a planet. I hesistantly propose that it has to be more or less spherical too - but I hesitate because having whopping huge asteroids classifies as such, while smaller ones are called 'planets' just because their shapes are rounder seems a little picky in some ways.

Originally Posted by janoskiss

Here is another scenario: What if two bodies orbit each other as well as the Sun but neither is significantly smaller than the other to be called the other one's moon? Are they both planets? Are they both moons? Or say one is a bit smaller but a bit more massive at the same time. Which one is the planet which one is the moon?

I'd say their both still planets. Are binary stars still called stars?
So, binary planets?

[Jonathan]

I think that 2003 UB313 should be known as the 10th Planet. I guess there does need to be a better definition of what size and shape defines a planet rather than a minor planet or asteroid.

Removing Pluto from planet status seems a bit silly to me. Every astronomy book I own and have read defines it as being a planet and I can't see why it shouldn't be. I really fail to see much credit in the argument for removing it from being a planet. I think the small number of people that don't want it to be kown as a planet seem to make a lot of noise and get very emotional about it and that blows the debate out of proportion, but that's fine, everyone's entitled their own opinion.

[janoskiss (Steve H)]

Originally Posted by ThunderChild

In that case, I'm interested to know why you voted for 'other' instead of including all suitably attributed Kuiper belt objects (ie do not exclude them), which is what it seems like you're saying.

I just think that the definition should not include any reference to the Kuiper belt. Whatever the definition it should be universal. It should apply to objects in the KB as well as closer in and further out. And it should apply to planets orbiting other stars too.

Originally Posted by Jonathan

I think that 2003 UB313 should be known as the 10th Planet. ...

Yes, but we need a name something a bit more palatable than 2003 UB313. There's got to be a few Roman gods still left after Pluto.

[Jonathan]

Originally Posted by janoskiss

Yes, but we need a name something a bit more palatable than 2003 UB313. There's got to be a few Roman gods still left after Pluto.

Of course! I didn't mean name it "The Tenth Planet or 2003 UB313" , I meant it should be recognised as the 10th planet in our solar system. I wish they would hurry up and name it.

[janoskiss (Steve H)]

I knew what you meant Jonathan.
Planet X

[circumpolar (Matt)]

I think it should be called a planet regardless of it's possition in our solar system. As they say "if it walks and talks like a duck......."I mean, it seems to fit our idea of what a planet should be. As for a new definition on what a planet should be, I'm all for it.

[ThunderChild (Chris)]

Originally Posted by janoskiss

Yes, but we need a name something a bit more palatable than 2003 UB313. There's got to be a few Roman gods still left after Pluto.

Actually (from what I understand), the answer to that is no. Well, except for some obscure ones. It seems that all the good names have already been given to various asteroids.

Source : refer to "What is the real name going to be?" at
http://www.gps.caltech.edu/~mbrown/planetlila/

One exception is 'Vulcan' - this was reserved for a hypothetical planet closer to the sun than Mercury (Vulcan, god of fire etc).
Somehow seems less appropriate in the coldest parts of space...

[janoskiss (Steve H)]

How about planet Iceinspace then?

[AGarvin]

Hi all,

I'm still sitting on the fence with this. I'm wondering if maybe orbit eccentricity and angle of orbit to the plane of the solar system should also be considered ?

Tough one.

[ving (David)]

... or planet hollywood?

how about this, a planet:

*is roughly spherical
*is in a orbit around sol that roughly matches that of the other 9(10) known planets
*has some sort of atmosphere
*has its own orbiting body (moon).

now i am not too sure about the last one as in saying it must have a moon i am oly comparing it to earth... it could very well be that having a moon is not a prerequisite.


one could just ask a professional tho. wheres Silvie?

[wavelandscott (Scott)]

I reckon it is in as a planet or Pluto is out...

I think it should be in...I don't want to unlearn Pluto as a planet...

[Volans]

G'day All,

To my mind, Pluto should be kept as a planet for historical reasons. This may seem to be a senitmental approach but I point to a number of "historical oddities" in astronomy. We talk about the Quadrantid meteor shower but where is the constellation of Quadrans Muralis from whence it got its name? My own nickname in this forum is a good example; Volans used to be called Piscis Volans which made sense as the "Flying Fish" but now it is just Volans or "Flying"!

If we talk about pure science then Pluto should be stripped of its planethood status. Its orbit is just too whacked from the plane of the ecliptic and its compisiton is out of order with all the other planets. I too heard the ABC radio report and one of the people interviewed talked about groups of objects and this line of reasoning made sense, to me at least.

You have the Terrestrials, the Asteriod Belt, the Gas Giants and then the Kupier Belt. Pluto just happens to be the closest object within the Kuiper Belt. So should 2003 UB313 be called a planet? To my mind, no. The definition of a planet is incredibly tough to reason out and I for one do not envy those who are on the IAU committee.

Spheriod in nature...hmmm...there is a difference of nearly 10,000km between Jupiter's equatorial girth and its polar girth (143,000km vs 133,700km). What of Ceres with dimensons of 975 x 909 km? Percentage wise, both objects share very similar oblateness.

Moons... a no brainer really as neither Mercury nor Venus have moons yet are still planets. Ganymede's mean diameter is 5262.4km whereas Mercury has a diameter of 4879.4km!

Atmosphere...Mercury again. But then there is Titan, a moon with an atmosphere and as spherical as other planets.

The other thing is that of evolution, no, not the Darwinian type but planetary evolution. The first 8 planets have all evolved, changed over time but KBO's (to the best of our knowledge) have not altered. It is this last concept which makes me believe that 2003 UB313 should not be classified as a planet.

In short, keep Pluto as a planet for historical reasons but everything else should remain a KBO.

And a name? Pluto's wife Persephone I think would be a good candidate. If a minor planet already has that name then simply change it! Nobody remembers the names of the lessor minor planets anyway.

Peter.

PS..speaking of KBO's, we all know of Sedna. Where else is Sedna in the Solar System?

[iceman (Mike)]

Great thread! And great reply Volans, what you've said makes sense.

And I agree that while Pluto isn't really a planet, it's probably too late to change it now... or is it? I mean we've only known about Pluto for what, 70 or 80 years?

If we say it's not a planet now, it'll probably only take 10 or 20 years before people stop calling it one.

[Argonavis (William)]

An interesting article which canvasses all these issues on how to define a planet can be found at the Astronomical Society of the Pacific's web site at:

http://128.241.173.3/education/publi...netdefine.html

This was written before the discovery of 2003 UB313, but the issues are still the same.

[Argonavis (William)]

Originally Posted by Volans

Spheriod in nature...hmmm...there is a difference of nearly 10,000km between Jupiter's equatorial girth and its polar girth (143,000km vs 133,700km). What of Ceres with dimensons of 975 x 909 km? Percentage wise, both objects share very similar oblateness.

Peter - would an oblate spheroid squeek through?

btw excellent points.

and where else is Sedna?