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

[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 = alb?©do

C => Diam??tre estim?© (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---

[ajtribick]

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.

I'm thinking more of extrasolar planets here... (minimum) mass can be determined easily, radius is much harder to determine - in fact at present we can only get the radii of planets which transit their stars, which is an incredibly small fraction.

In addition, I'm not sure about the way you are going about saying if a planet is smaller than X then it is a terrestrial, bigger than Y a gas giant. Surely the definitions of terrestrial and gas giant are based on more than the radius of the planet! Instead of coming up with these arbitrary dividing lines by pulling numbers out of a hat as you seem to be doing, maybe it would be more useful in coming up with defining physical characteristics of all these classes, rather than these limits based on a single parameter which probably don't apply in a large number of cases.

Most classification systems are quite interesting from a more psychological standpoint anyway - most of the time there are loads of different classes for various stages of an Earthlike planet's evolution, then at least one class for each of the other terrestrial planets in the solar system, then we try and stick Jupiter and Saturn in the same class, same for Uranus and Neptune, and a few random classes tacked on, e.g. panthalassics and carbon planets (as if carbon planets won't have the diversity shown by the silicate planets).

Presumably a classification by a hypothetical Jovian lifeform would have loads of different classes for various stages of Jupiter's evolution, then at least one class for each of the other gas giant, probably then the silicate planets would have one or two classes between them (provided they haven't been overlooked by the Jovians' definition of planet), and then there would be a few random classes tacked on.

This suggests to me that any classification system is probably useless beyond the absolute broadest classes - since we want to have a class for each terrestrial planet, it could be that every planet is going to be worthy of its own separate class in whatever scheme we come up with, in which case we might as well not bother with coming up with the system in the first place.

Just (some of) my thoughts on the whole classification system thing.

[Spaceman Spiff]

I second chaos syndrome!

... at least radius/diameter classification is nice and simple ...

... and can vary by a factor of more than 2 for a given mass by virtue of composition, so is the least relevant ...

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

Gak! Cough! Bode's law is not arbitrary? "Hmm, let me see, if I take the distances of the known planets from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn and Uranus, and assume that they fit to a law D = a + nb, then I find by least squares fit that a and b are actually real numbers. Gosh."*

One could wear a T-shirt with pride to planetary science conventions saying: "Neptune isn't a planet because it doesn't obey Bode's law, but don't you dare demote Pluto just 'cos it's small!!!"

And what 'Bode's law' fits to Upsilon Andormedae, or 55 Cancri?

Give up on the size thing now Michael, while you're young!

But I'm glad you noticed orbits are easier to use than radius to determine planethood...

Spiff.

* Sarcastic of me I know, sorry. I've got a run of it this month.

[Scorpiove]

This suggests to me that any classification system is probably useless beyond the absolute broadest classes - since we want to have a class for each terrestrial planet, it could be that every planet is going to be worthy of its own separate class in whatever scheme we come up with, in which case we might as well not bother with coming up with the system in the first place.

Just (some of) my thoughts on the whole classification system thing.

I agree I think that whatever objects we find orbiting a star are going to vary somewhat or a lot from objects orbiting another star. Even differing from other objects in the same system.

[d.m.falk]

Yes, but I believe a mass-condensation/composition threshhold is constant, and something we can rely on, both for intrasolar and extrasolar systems.

d.m.f.

[Michael Kilderry]

Ok, I've got a new idea that you might agree with more.

How about we keep 8 of our current planets, demote Pluto, and then let in Vesta (since it seems to posess certain qualities of a terrestrial planet), and Sedna.

I think Sedna deserves to be classified as a planet, at least temporarily, like Pluto and Ceres were. At the moment Sedna is pretty much alone, but we may find other Inner Oort Cloud objects very similar in the future. At that point we can demote Sedna and call it a 'minor planet', as it has proven itself not to be a planet.

But for the time being it deserves the label 'planet' too, for consistency as we gave Pluto and Ceres (and Pallas and Vesta and Juno?) the same benefit of the doubt.

Either that or we could have a system where the biggest object of a large group of minor planets is considered a major planet. This would give us at least ten planets. (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto and possibly Sedna.)

[d.m.falk]

Some thoughts:

One of the arguments for planethood is that it does not share its space with other co-orbitals.

If that was the case, only Mercury, of the inner planets, should be called a planet! Venus, the Earth and Mars all share their orbits with co-orbitals and other trans-orbital asteroids.

It currently appears that the so-called "Kuiper Belt" is less dense than the inner planets, and none of the worlds past Pluto are all that close to each other.

Even our gas giants share their orbits with Trojans and Trojan-type asteroids, so we cannot say their orbits are truly solkitary (discounting moons).

We have a very cluttered Solar system, but it seems there are alot of people are adamant about only accepting a very minimal number of planetary bodies as "planets", almost afraid that they will dilute the uniqueness of the planets they know. Most of the arguments for planetary status are very arbitrary, and very unscientific- And most involve some way of demoting Pluto, regardless of its condition, size, mass, whatever.

d.m.f.

[ajtribick]

One of the arguments for planethood is that it does not share its space with other co-orbitals.

The "does not share orbit space" argument is pretty bad I will agree. However I thought that definition was not an exclusion of all other objects in the same region, but that a planet should have a mass greater than the total mass of all other objects in the same region, which gives our solar system 8 planets.

Unfortunately all it takes is a system like Mu Arae to show the weakness of that definition - the less massive of the two outer gas giants may not be a planet in this scheme (note that the configuration of the planets as depicted in Celestia is unstable - try running it in Gravity Simulator). Also there is the possibility of planetary-mass objects in Trojan points - a system consisting of the Sun, Jupiter and a Jupiter twin in either of Jupiter's Trojan points is stable! The whole definition just starts getting really messy.

Also we have to consider remote observations of systems many light-years away - will we be able to tell whether a planet is a member of a swarm or not?

Either that or we could have a system where the biggest object of a large group of minor planets is considered a major planet. This would give us at least ten planets. (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto and possibly Sedna.)

Strike Pluto off that list, and replace it with 2003 UB313. You see, the problem with that idea is that it assumes that all the objects in the solar system have been discovered, or established planets can suddenly be demoted under this system without changing their physical properties, which I feel is less than ideal. The argument regarding extrasolar systems also applies here.

I feel any definition of the term "planet" should be applicable to extrasolar systems as well as our own. This reduces the available parameters to base the definition on to mass and orbit, and we must consider that it is very unlikely we have discovered all of the planets in a given system! There may easily be undiscovered Neptune-mass objects, or even Saturn-mass objects at large distances in some of the well-known extrasolar systems, let alone Earth-mass and below.

[Planet X]

Unfortunately, the "officials" will strictly be using size as the determinate, and that may prove to be even more controversial than we currently think. As we now know, Spitzer failed to detect 2003 UB313 due to a technical glitch. What if Spitzer actually sees it on the next attempt (scheduled for Thursday)? This would mean that 2003 UB313 is at least the size of the moon, and possibly as large as 5000km. Now, what if 2003 UB313 actually turns out to be 5000km in diameter? Well, I'll tell ya what would happen. Not only would the planetary status of Pluto and (probably) 2003 UB313 face the demotion ax, but so would the planetary status of Mercury! Think about that! Later!

J P

[d.m.falk]

This (I reiterate) effectively leaves spheroidal mass-condensation threshholds as the only scientificly practical determiner.

And a few people have postulated the stability of multiple-planet orbits, such as on this Java-laden site: http://burtleburtle.net/bob/physics/kempler.html

(More applets on the site here: http://burtleburtle.net/bob/physics/index.html)

d.m.f.

[Michael Kilderry]

Strike Pluto off that list, and replace it with 2003 UB313.

Oops... I completely forgot about 2003 UB313, I wouldn't want to demote Pluto though.

I think the best way to determine if something is a planet or not might be whether it is spherical in shape or not, but even this has problems.

What if we discover an object 1km in diameter that is very spherical in shape, just by chance? And where do we draw the line between spherical and irregularly shaped objects? What about the oblate Jupiter?

I'm beginning to think the universe wasn't made for planets to be classified depending on characteristics, there's always something that breaks the rule.