Definition of a planet

[Greg Bryant]

Originally Posted by 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.

Hi AGarvin,

I don't think we can include orbital eccentricity as a factor in determining planetary status. Yes, Pluto has an orbit that is very elongated, and it's long stood out as being strange. But many extrasolar planets have similarly very elongated orbits, and those bodies are Jupiter-size.

Regards,

[Volans]

Originally Posted by Argonavis

Peter - would an oblate spheroid squeek through?

Bill,

Methinks we are getting into the realms of semantics here! By nature of the word, a spheroid is oblate. If it was a true sphere it would not be oblate. Large bodies that have a relatively fast axial rotation must become oblate and hence spheriodal. It would be interesting to find out the axial rotational rate of 2003 UB313. The spherical nature of an object does, to my way of thinking, impact on its status of planethood but you cannot take that factor by itself as per my previous post.

Originally Posted by Argonavis

btw excellent points.

Thank you!

Originally Posted by Argonavis

and where else is Sedna?

On Venus - Sedna Planitia - all features on Venus are named after women except for one place...no I'm not going to tell you...

[mickoking]

Definition of a planet? while not very scientific my approach is that a planet is an object that orbits the Sun (directly) and is larger than Pluto.

[AstroBug]

We really can't strip Pluto of it's status after 70 odd years can we? . As a fellow Kuiper belt object 2003 UB313 (which also has a moon!) may have to join the list. A great thread, and there was also a story in yesterday's Melbourne Age where they stated "...the number of planets in the solar system is almost certain to be changed for the first time since 1930". It will be interesting to see the outcome.

[ThunderChild (Chris)]

Originally Posted by Volans

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.

Yes - that name would have been a natural choice, but it's taken (along with almost everything else suitable).
And no, it appears the powers that be will never, ever, ever change a name once given.

If only we'd shown more thought when naming asteroids. Who cares about them having good names?

Chris

[AGarvin]

Yes, Pluto has an orbit that is very elongated, and it's long stood out as being strange. But many extrasolar planets have similarly very elongated orbits, and those bodies are Jupiter-size.

Quite true Greg. There are a lot of things to consider, composition is another. Many are distinguishing between between planet and KBO, but why can't a planet be in the belt? I do feel that there must be a minimum size though.

It will be interesting to see the IAUs take on this.

[circumpolar (Matt)]

Originally Posted by wavelandscott

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...

It seems most people who study or are interested in science are used to name changes, or have at least learnt to put up with it.
eg. Botony and Horticulture
I am always having to re-learn plant names and it is annoying.

The three main reasons for name changes are
1. Earliest published name has precedence
2. Taxinomicly(scientificly) incorrect
3. Mis-idendified

I guess once the definition is settled, the later two reasons are applicable.
Time to shuffle the memory again.

[Robert_T]

Hi All- this is really a problem of scientific classification and quite typical of other branches of science wherever an initial classification scheme (such as what's a planet, and what's a moon etc) is constructed on a very limited sample of objects (eg just what we could see in our solar system) which is later greatly expanded through new discoveries (eg extra-solar and new solar system discoveries as technology develops).

This sort of thing has been rife in archaeology (my field) for example in stone tool studies. Early classifications based on samles from a few sites lumped objects into categories such as "flake", "blade", "handaxe" and "core" assuming these were meaningful in some ways, but as new work progressed and new studies and discoveries have been made the divisions between such categories have blurred - there being more of a continuum of form that we've arbitrarily broken into specific "types".

The same is likely true for planets and moons - as more and more a re dicsovered the divisions in forms, behaviours etc will shrink more and more and there will be overlap and anomolies. What's important then is not the specific tag or type attributed, but those aspects of form and behaviour which tell us something about the universe and how it came to be - that's the essence of science and is far more meaningful than what we name different objects.

Names like "planet" are convenient and I'm not suggesting abandoning them, but that we focus more on what makes each distinctive rather than pigeon-holing. Pluto is clearly unusual (at least so far as we know) for it's eccentric orbit, number of satellites and ration of satellite size as a proportion of the parent body.

Hooray for the eccentric, planety, kuiper-belt sweeping, moon collecting Pluto

cheers,

[circumpolar (Matt)]

We earthlings do love to pigeon-hole things.
I wish them SETI folk would hurry up and download the universal dictionary from outer space to set us straight.

[[1ponders] (Paul)]

Great discussion guys. What are the feelings about using a naming system based on size? Planet (minimum size = mars), planetoid (minimum size = 500km) and planetesimal (down to about 150 km), smaller than that asteroid. Comets stay as they are.

Or alternatively, based on distance from the sun and size, major planets = mercury out to pluto, minor planets out past pluto

Just some thoughts

[circumpolar (Matt)]

You also have to consider that some moons are as large as planets too. A combination of size, orbit and partnership with other bodies(moons) would have to be taken into account as well.

[[1ponders] (Paul)]

You also have to consider that some moons are as large as planets too.

That's why I thought about making Mars the smallest planet . That way Mercury, Jovian moons, Titan, Triton, Pluto, Sedna, Quoar etc come into the planetoids category. Some of them would still carry the colloquial term moon, but technically be classed as planetoids

[circumpolar (Matt)]

Originally Posted by [1ponders]

That's why I thought about making Mars the smallest planet . That way Mercury, Jovian moons, Titan, Triton, Pluto, Sedna, Quoar etc come into the planetoids category. Some of them would still carry the colloquial term moon, but technically be classed as planetoids

Sounds like a good place to start.
Planetoid - the planet you have when your not having a planet.
But what happens if we find two 'larger than mars' size objects that are almost the same size as each other orbiting in partnership. Now we also have to define what is a moon.

[[1ponders] (Paul)]

Originally Posted by matty

But what happens if we find two 'larger than mars' size objects that are almost the same size as each other orbiting in partnership. Now we also have to define what is a moon.

Dual planetary system

[Robert_T]

Originally Posted by [1ponders]

Great discussion guys. What are the feelings about using a naming system based on size? Planet (minimum size = mars), planetoid (minimum size = 500km) and planetesimal (down to about 150 km), smaller than that asteroid. Comets stay as they are.


Or alternatively, based on distance from the sun and size, major planets = mercury out to pluto, minor planets out past pluto

Just some thoughts

All reasonable approaches Paul, but as I've outlined below this sort of classification by drawing boundaries along continuum variables such as distance or diameter is entirely arbitrary - meaning you could easily draw boundaries at different points and be equally justified, so it will be difficult to get agreement. If we are to classify we should seek qualitative rather than quantitative variables that distinguish bodies with fundamentally different evolutionary histories - easier said than done. A good example is the method of distinguishing stars from non-stars based on internal fission of hydrogen into helium. This provides a convenient qualitative point of difference rather than simply saying stars with >0.05 solar masses are stars. It's also scientifically meaningful in that the history and future of the "body" on one side on this divide will experience a very different history and future to one on the other side.

cheers,