I Suggest a revised definition of "planet"...

[Planet X]

I wonder if someone would argue for imperial units (1,000 miles) with radius rather than diameter somehow, then both Pluto and 2003 UB313 wouldn't make planet status.

Spiff.

Well, maybe not. On Mike Brown's site, he has different values for 2003 UB313's size based on certain albedo figures. The lowest albedo figure of 38% gives a diameter of 3550 km, or 2235 miles. If this proves true, then 2003 UB313 would attain planet status, even if a 1000 mile radius were to be eventually implimented. At any rate, I have a far simpler way to tell planets apart from asteroids/KBO's. Just so everyone knows right now, I DO NOT view asteroids and minor planets as the same thing. So, here goes our solar system's breakdown as I see it:

Giant Planet: 49000-143000 km in diameter: Jupiter, Saturn, Uranus, and Neptune

Terrestrial Planet: 6000-12800 km in diameter: Venus, Earth, and Mars

Subterrestrial Planet: 2000-6000 km in diameter: Mercury, Pluto, and 2003 UB313 (so far)

Asteroid/KBO: fist sized-2000 km in diameter: (All asteroids and KBO's)

Of course, this does not take into account subclasses and other classes of planets NOT in our solar system. See Michael Kilderry's post above for that stuff. Later!

J P

Note: edited on 08/07/05 to change the name "Minor Planet" to "Subterrestrial Planet."

[symaski62]

http://science.nasa.gov/headlines/y2005 ... lanetx.htm

http://www.spaceweather.com/swpod2005/0 ... allen1.jpg
http://www.spaceweather.com/swpod2005/0 ... immons.jpg
http://mak-newt.com/visitors/MPEC2005-O ... index.html

[Planet X]

Hey, I should also point out that Brian Marsden of the Minor Planet Center (MPC) thinks that only bodies Mars sized or larger should be called planets. This means that if he had his way, our solar system would be down to just 7 planets! What a prick! Later!

J P

[Michael Kilderry]

So then, there's nothing special about 1,000,000 of our modern metres. Yet, choosing 1,000km as nice and round while at the same time letting Pluto into the (major) planet club, but rejecting Ceres, is to me 'sizist'! You favour one but not the other based upon an arbitrary size limit.

Well it's all just arbitrary in the end, isn't it? The term 'planet' is just a label for some of the many objects in our universe.

The point of the 1,000 km diameter limit is to keep things nice and simple and make it easier for everyone, rather than having some complicated system of what gets classified as a planet or not, like internal structure, which may take a while to investigate properly for certain objects like those of the Kuiper Belt.

I think favouring Pluto over Ceres is logical, at least for historical reasons. Pluto was known as a planet for a long time before all these TNO's stepped into our telescopes, Ceres on the other hand, has not been considered a planet for a very long time.

And if we were to consider Ceres a planet, then it would be arbitrary not to consider all the TNO's at least as large as this asteroid into the circle of planets as well, and then it would be arbitrary not to consider anything that's just a little bit smaller, as why should Ceres be the cut off mark for planet sizes? What's so special about it? This is why it's good to keep things simple with the 1000 km cut-off mark, after all, no planetary classification system is going to be perfect and to everyone's taste.

And also, neither Pluto or Ceres have been studied up close. Pluto could be quite planet like and Ceres could be rather asteroidal in appearance. We may as well be arbitrary at classifying planets at this point in time because we don't know enough to set a proper limit.

But don't get me wrong, I have nothing against asteroid Ceres, but I do against Varuna, every time I tried to search for info on KBO's, Varuna was all that ever came up, what shameless overpromotion!

And the 1000km size limit just barely stops Varuna from being classified as a major planet, that's the REAL reason I chose the 1000 km cut off mark. [/joking]

Hey, I should also point out that Brian Marsden of the Minor Planet Center (MPC) thinks that only bodies Mars sized or larger should be called planets. This means that if he had his way, our solar system would be down to just 7 planets! What a prick! Later!

Brian Marsden (anyone notice he's got the word "Mars" in his name?) must be thinking along the lines of nothing being classified as a planet unless it is at least half the size of the Earth. This would make Mars-sized the cut-off mark for planethood.

I think it's funny how it's got to the point where not only Pluto's planetary status is being debated, but so is Mercury's! Mars, you're next in line....

- Michael

[Spaceman Spiff]

Just so everyone knows right now, I DO NOT view asteroids and minor planets as the same thing. So, here goes our solar system's breakdown as I see it:

Giant Planet: 49000-143000 km in diameter: Jupiter, Saturn, Uranus, and Neptune

Terrestrial Planet: 6000-12800 km in diameter: Venus, Earth, and Mars

Minor Planet: 2000-6000 km in diameter: Mercury, Pluto, and 2003 UB313 (so far)

Asteroid/KBO: fist sized-2000 km in diameter: (All asteroids and KBO's)

Hey, I should also point out that Brian Marsden of the Minor Planet Center (MPC) thinks that only bodies Mars sized or larger should be called planets. This means that if he had his way, our solar system would be down to just 7 planets!

So you propose that Mercury is considered a minor planet, but are unhappy with Brian Marsden after you claim he thinks and is wrong to think Mercury and Pluto are too small to be considered major planets? Hmm... You do realise that the term minor planet does have a particular meaning in planetary science? That 'Asteroids' and 'Kuiper Belt Objects' are two specific types of minor planets?

If you move Mercury, but not Pluto, into 'terrestrial planets', and label the rest 'minor planets', I'd go along with that.

Spiff.

[Spaceman Spiff]

Well it's all just arbitrary in the end, isn't it?

Not if you use statistical approaches on scattergrams of orbit elements and object masses... Do that, and it's clear Pluto is in a different population from terrestrial planets and giant planets...

I think favouring Pluto over Ceres is logical, at least for historical reasons. Pluto was known as a planet for a long time before all these TNO's stepped into our telescopes, Ceres on the other hand, has not been considered a planet for a very long time.

Er... think carefully about that history. Ceres was considered a planet for several years until three more asteroids were found orbiting between Mars and Jupiter like itself. Pluto is considered a planet because of the anticipation generated by Lowell's predicted planet. For decades, although Pluto was much dimmer than expected, it was argued it might still have a radius of up to 10,000km. Now that Pluto's been found too small to be Lowell's planet, and many comparable bodies have been found with similar orbits to Pluto, it seems only logical that Pluto follows the same fate as Ceres, and it's happening on a similar timescale too...

I think it's funny how it's got to the point where not only Pluto's planetary status is being debated, but so is Mercury's! Mars, you're next in line....

Charge up the Death Star, commander, I think it's about time we demonstrated to these rebel planetoids the full planet-determining capability of this battle station ...

Spiff.

[Planet X]

So you propose that Mercury is considered a minor planet, but are unhappy with Brian Marsden after you claim he thinks and is wrong to think Mercury and Pluto are too small to be considered major planets? Hmm... You do realise that the term minor planet does have a particular meaning in planetary science? That 'Asteroids' and 'Kuiper Belt Objects' are two specific types of minor planets?

If you move Mercury, but not Pluto, into 'terrestrial planets', and label the rest 'minor planets', I'd go along with that.

Spiff.

I was suggesting changing the meaning of the term "Minor Planet" a bit, so that it no longer means the same thing as asteroid, but covers only bodies in the 2000-6000 km diameter range. To me, the Inner Belt and Kuiper Belt Objects represent two specific types of "Asteroids," not minor planets. To me, bodies under 2000 km in diameter are just to small to be called planets of ANY kind.

Brian Marsden (anyone notice he's got the word "Mars" in his name?) must be thinking along the lines of nothing being classified as a planet unless it is at least half the size of the Earth. This would make Mars-sized the cut-off mark for planethood.

I think it's funny how it's got to the point where not only Pluto's planetary status is being debated, but so is Mercury's!

- Michael

Now, the reason I'm so ticked with Brian Marsden is that he doesn't consider bodies smaller than Mars to be planets of ANY kind. I agree that sub-Mars sized bodies in the 2000-6000 km diameter range shouldn't be called Major Planets, but I disagree with the notion of them getting totally stripped of planetary status. At least my system would enable Mercury, Pluto, and 2003 UB313 (should be named Styx instead of Xena) to retain planet status. Marsden's idea would (wrongfully) eliminate those 3 worlds from the list of planets, resulting in them being viewed as overgrown asteroids, which they are clearly not. To me, Mercury, Pluto, and 2003 UB313 are minor planets, but PLANETS (NOT ASTEROIDS) just the same!

Our solar system: 7 Major Planets + 3 Minor Planets = 10 planets total! Later!

J P

[symaski62]

The formula to estimate the size of an asteroid is:

C=1329*(10^(-A/5))/(B)^0,5

FRENCH

A => H = Magnitude Absolue

B => G = albédo

C => Diamètre estimé (en km)


English

A => H = Absolute Magnitude

B => G = albedo

C => Estimated diameter (in km)

how to make?

H = -1,1 || G = 0.15 => " 2003 UB313 "

5694,81147 Km =1329*(10^(-(-1,1)/5))/(0.15)^0,5

baah

[Planet X]

Also, applying a system like this wouldn't be too different from the way astronomers handle the vast moon systems of Jupiter, Saturn, Uranus, and Neptune. Astronomers call them "Major Moons" and "Minor Moons." So, why not do the same thing with the planets? In fact, here's the way I would break it down:

Major Planet: Any body that is over 6000 km in diameter that is not massive enough to exhibit star-like conditions, i.e. is not a brown dwarf
Worlds of this type typically have atmospheres. These worlds typically have moon systems. The smallest worlds of this group tend to have thinner atmospheres and small moons (though the Earth/Luna pair is an exception). The largest worlds have extremely thick atmospheres and rather large moons. Most major planets tend to have orbits of low eccentricity, though there are some exceptions (especially with extrasolar planets).

Subterrestrial Planet: Any body that is 2000-6000 km in diameter that has characteristics that clearly distinguishes it from asteroids
The largest worlds resemble smaller (Mars to Earth-sized) Major Planets. They have the capability to have atmospheres, though these atmospheres tend to be thin. Under some circumstances, atmospheres on such worlds may not exist. The orbits of Minor planets tend to vary in eccentricity. Moons of Minor Planets vary in size from very tiny up to half the size of the planet itself.

Asteroid: Any body 2000km in diameter down to dust sized chunks that lacks most, if not all, planetary characteristics
The largest asteroids are round, but do not have multiple surface layers or even distinguishable cores. They rarely, if ever, have any real traces of atmospheres. Most asteroids have no satelites, but there are rare exceptions. In fact, some asteroids have binary companions up to at least 3/4 their size. Unlike both Major Planets and Minor Planets (based on my definitions above), asteroids of all kinds tend to orbit in huge belts spanning large distances.

There you have it, my own personal definitions of the types of bodies in our solar system. Of course, each of the classes mentioned above can be broken down into groups and even subgroups (some of which undoubtedly exist in other star systems), but I don't think that's necessary right now. Later!

J P

Note: edited on 08/07/05 to change the name "Minor Planet" to "Subterrestrial Planet."

[Michael Kilderry]

We could also classify planets by whether they follow Bode's law or not. This would mean Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Pluto, and Sedna (follows bode's law at perihelion) are planets. However, Neptune is no longer as it doesn't follow the rule, making it a giant gas KBO.

Following this system, we would have five small inner planets (Mercury through to Ceres)

Three gas planets (Jupiter, Saturn, Uranus)

And two small outer planets (Pluto and Sedna)

- Michael

[Scorpiove]

Minor Planet: Any body that is 2000-6000 km in diameter that has characteristics that clearly distinguishes it from asteroids
The largest worlds resemble smaller (Mars to Earth-sized) Major Planets. They have the capability to have atmospheres, though these atmospheres tend to be thin. Under some circumstances, atmospheres on such worlds may not exist. The orbits of Minor planets tend to vary in eccentricity. Moons of Minor Planets vary in size from very tiny up to half the size of the planet itself.

Problem is that you could get very thick atmospheres on smaller bodies (titan) if they are cold enough and far away enough from the sun.

[Planet X]

Problem is that you could get very thick atmospheres on smaller bodies (titan) if they are cold enough and far away enough from the sun.

True. I should have worded that part a bit differently, as there's bound to be exceptions to the rule. Anyway, after some further thinking, I now think it would be better to call the 2000-6000 km in diameter objects Subterrestrial Planets, instead of "Minor" Planets.

Also, I think the best nickname for Asteroids/KBO's would be "Minor Bodies," or even "Minor Worlds." Soon, I'll get it all straight . Later!

J P

[ajtribick]

Why use radius (or diameter) for a classification system? It's probably one of the least useful characteristics to go for - mass is probably better, but even so there are still grey areas.

What next, planetary cladistics?

[Spaceman Spiff]

Quite so!

We could also classify planets by whether they follow Bode's law or not.

If I can just dissociate myself from that when I proposed sorting planets from the chaff by orbit statistics first...

Spiff.

[Michael Kilderry]

Why use radius (or diameter) for a classification system? It's probably one of the least useful characteristics to go for - mass is probably better, but even so there are still grey areas.

There are going to be grey areas in every planet defining system, at least radius/diameter classification is nice and simple and doesn't take as much time as other factors to determine. Mass is a problem because it is not well determined for many objects, like the Trans-Neptunians.

What next, planetary cladistics?

That could actually be a good idea.

If I can just dissociate myself from that when I proposed sorting planets from the chaff by orbit statistics first...

Why? Classifying planets by Bode's law is even easier since it takes less time than determining diameters and size. It's not even arbitrary!

---Michael---