[X] My Assistant

[Jyril]

Like I feared, that -0.4 value was a false alarm, it's back at 0.1:

K05EA4J 15.3  0.15 K058I  50.368 152.552 353.444  8.943 0.04266  3.16664  3 X
K05EC4D 16.2  0.15 K058I 315.090 244.333  35.895  5.214 0.15325  2.56078  3 X
K05F09Y  0.1  0.15 K058I 146.099 296.739  79.519  28.999 0.15458  45.74014  6 X
K05G34P 16.3  0.15 K058I  8.979  97.234 142.622  12.137 0.24768  2.62499  4 X
K05J63S 15.8  0.15 K058I 355.253 102.910 178.954  25.752 0.27856  3.22140  3 X

I've read that 2005 FY9 could be 50%-75% Pluto's size.

[OWW]

In my opinion the whole planet/KBO/asteroid debate is the result of people still thinking like 18th century astronomers. Planets circle around the sun. Moons circle around the planets. Comets have tails and elliptical orbits around the sun. Predictable like a giant clock.

But we know now nature is not organized. There are only smaller and larger clumps of matter and their orbits are sometimes chaotic.

Therefore, the following questions can never be clearly answered.
When does an asteroid become a comet? When does a moon become a planet? When does a ring particle become a moon? When does a KBO become a planet? Is an object in a Lagrange point a moon or a planet?

The solar sytem how I see it:
- The Sun: star ( fusion )
- The rest: 'worlds' ( from dust spec to Jupiter )
- Gas giants: asteroids ( star like worlds, no solid surface )

[abalone]

What peculiar little creatures we humans are that we find it necessary to have everything assigned to its correct pigeon hole. I cant see the slightest relevance to this decision as to whether we call it a planet or not. " It orbits, therefore it is" and "A rose by any other name would smell as sweet"

[dilo]

Based on the ongoing discussion, I made following flow diagram in order to adress main objection rised up to now.
 PIANETI.DOC ( 19K ) Number of downloads: 496

This general method should correctly discriminate between a natural satellite (a planet if central body is a star or a moon for an object orbiting a planet) and a planetoid/”satelloid” isolated or belonging to a wide population like asteroids belt or KB.
Obviously, this is only a sketch and numerical constraint on radius and semi-major orbit can be revised... Anyway, I deribely avoided to use absolute measures (like size in Km) in order to have a general method that can apply also to planet/moons systems. Obviously, these pure numbers are choosen in order to include all solar system planets; inclusion of Pluto and other major KBO could be temporary, because it depends on future discovery of other objects with similar orbit. This is the normal historical process higlighted by Alan...

[Alan Stern]

Dilo--I wonder, what will you do when Mars-sized and larger objects are found in the
Oort Cloud on highly elliptccal orbits that range over the same distance from the Sun?
Classify ejected Mars- and Earth-sized bodies as planetoids? And what about in
extra-solar planetary systems in formation where one routinely expects very large
bodies (Mercury to Uranus sizes) on crossing orbits? Can only the single largest
one be a planet? If an object is small compared to its neighbors but ejected to
a place where it is large compared to its new neighbors, does it get promoted
to planethood just by virtue of lits change in location? If the reverse happens,
do you propose the object be demoted, as the flowchart algorithm suggests
one would have to do?

Localtion and relative-size arguments always have these problems. Planets are planets
because of what kind of body they are. I remain convinced it is better to pick a good
criteria-- like being rounded by gravity-- and let the number of bodies be whatever
Nature delivered.

-Alan

[MiniTES]

Localtion and relative-size arguments always have these problems. Planets are planets
because of what kind of body they are. I remain convinced it is better to pick a good
criteria-- like being rounded by gravity-- and let the number of bodies be whatever
Nature delivered.

Alan: What about Ceres and Vesta? Do you think that we should call them planets by virtue of their shape? Personally I agree with your scheme but I don't think it would be logical to call Ceres and Vesta planets instead of asteroids.

[DEChengst]

Localtion and relative-size arguments always have these problems. Planets are planets because of what kind of body they are. I remain convinced it is better to pick a good criteria-- like being rounded by gravity-- and let the number of bodies be whatever Nature delivered.

Wouldn't that criteria be pretty much the same as classifying objects larger than a certain size as planets ? Afterall, to have an object pull itself into a sphere you need a certain amount of gravity. The larger the object the larger the mass and thus the force of gravity. Ofcourse if you have two objects with the same size but different density the denser object will have the bigger gravity. My guess would be that a less dense object would need less gravity to have itself pull into a spherical shape. So in the end "objects larger than x will be spherical" might still be a approximation.

BTW: With all those big KBOs being discovered, how many New Horizon missions would you like to do ?

[alan]

This is how I would divide things

If it orbits a star it is a planet
If it large enough the remove or control other objects in its vicinity it is a major planet
If it is not it is a minor planet

Pluto is not large enough to remove objects near it so it is a minor planet
A Mars sized object in the Oort cloud would probably eject anything it encountered so would be a major planet

[Mongo]

I think that the problem we have is that the current classification may be too coarse-grained to be all that useful at grouping objects. There will be some classification system -- the objects themselves may be unaffected by what we call them, but their perception by most humans will be affected by their classification.

What are the primary physical break-points among various sizes of objects?

1) Deturium fusion at some point in the object's history would define the upper limit of planet-hood. This is usually put at about 13 Jupiter masses.

2) An obvious break-point is whether the object was almost entirely formed from planetisimals, or whether significant direct accretion of gas also occured. Not that all 'gas giants' are the same. Jupiter and Saturn are basically solar composition, while Uranus and Neptune are depleted in H and He compared to the sun. Other planetary systems are different again. One recently announced planet is about Saturn-mass, but otherwise apparently resembles a hot Neptune.

3) Another possible dividing line is whether the object has a significant secondary atmosphere, although this has a lot to do with the level of heating from its primary -- the colder the object, the easier to retain an atmosphere, unless it's too cold.

4) Sufficient self-heating to cause significant reprocessing of the surface might be a good indicator, but it would be difficult to determine from a great distance.

5) Probably the lowest dividing line of interest is whether the object self-gravitates into a spherical shape, although again, other factors enter into this, such as composition (silicates vs ices) and temperature. Another issue would be: how spherical is spherical enough? Objects at the lower limit might be only roughly spherical, so what degree of departure from the an ideal sphere (allowing for rotation) is acceptable?

My own list of known 'major planets' would include:

CLASS 1 PLANET: (significant direct acretion of gas, no deturium fusion) Jupiter, Saturn, Neptune, Uranus

CLASS 2 PLANET: (significant secondary atmosphere, no significant direct accretion of gas) Earth, Venus, Mars, Pluto

CLASS 3 PLANET: (self-gravitates into sphere, no significant secondary atmosphere) Mercury, 2003UB313, 2005FY9, 2003EL61, Sedna, Orcus, Quaoar, Ixion, Ceres, 2002TX300, 2002AW197, 2002UX25, Varuna, 2002MS4, 2003AZ84, 2004GV9, Vesta ...

While we're at it, here is a list of moons as well:

CLASS 2 MOON: Titan, Triton

CLASS 3 MOON: Ganymede, Callisto, Io, Luna, Europa, Titania, Oberon, Rhea, Charon, Iapetus, Ariel, Umbriel, Dione, Tethys, Enceladus, Mimas, Miranda

Bill

[dvandorn]

If real estate doesn't matter and you classify planet vs. non-planet based solely on size and mass (i.e., is it big enough to have pulled itself into a ball), then what about a majority of the moons of the gas giants? Remember, real estate cannot be a factor in the equation.

Under the classification system that everything massive or large enough to pull itself into a sphere (more or less) is a planet, we have to admit Ceres, Vesta, the Galilean moons of Jupiter, many other Jovian moons, a lot of Saturnian moons, a lot of Uranian and Neptunian moons, plus possibly thousands of KBOs, all into the League of Planets.

Oh, and while it's not usually called a planet, our own Moon would have to be considered a planet, too.

If you're going to make an exception for bodies that orbit other planets to call them moons, then you're allowing real estate to enter the equation, and once you do that, you're simply applying arbitrary dividing lines...

-the other Doug

[Alan Stern]

I do think Ceres is a planet: a dwarf one, but no less a planet than a 100 Mj object
is a star.

I wrestled with this whole question a long time ago, and I think the fundamental
difference between a planet a a big rock is that a planet does something collectively--
i./e., it shapes itself into a sphere by graivty overcoming maerial strength. Anything
too small to do this is acting like a rock. Anything beig enough to do this, but
not so big that it does fusion, is a planet.

And as I point out in public talks sometimes, though many, amny adults claiim
to have never thought about this, any kid knows it: jst ask a kid to draw
a planet; they always draw a circle, never an egg or a box or a pyramid or
even a hamburger. Kids know planets are things that are essentially spherical;
of course most don't know why this is so, but they recognize the observational
telltale, that's for sure.

-Alan

[Alan Stern]

[quote=alan,Jul 31 2005, 06:35 PM]
This is how I would divide things

If it orbits a star it is a planet
If it large enough the remove or control other objects in its vicinity it is a major planet
If it is not it is a minor planet



...Careful, stars orbit stars.

As to Pluto etc. being "minor" planets, I agree in spririt but prefer
the term "dwarf" planet for two reasons. Firstly, this is a well accepted
terminology in astrophysics (i.e., there are no minor stars or galaxies,
but there sure are dwarf stars and dwarf galaxies). And secondly,
some scientists say they feel that the term "minor" is pejorative in that
one who works on "minor" object is not doing something as important as one
might. I personally don't feel this way, but I am sensitive to the many
people who do.

[Alan Stern]

I think that the problem we have is that the current classification may be too coarse-grained to be all that useful at grouping objects.  There will be some classification system -- the objects themselves may be unaffected by what we call them, but their perception by most humans will be affected by their classification.

What are the primary physical break-points among various sizes of objects?

1) Deturium fusion at some point in the object's history would define the upper limit of planet-hood.  This is usually put at about 13 Jupiter masses.

2) An obvious break-point is whether the object was almost entirely formed from planetisimals, or whether significant direct accretion of gas also occured.  Not that all 'gas giants' are the same.  Jupiter and Saturn are basically solar composition, while Uranus and Neptune are depleted in H and He compared to the sun.  Other planetary systems are different again.  One recently announced planet is about Saturn-mass, but otherwise apparently resembles a hot Neptune.

3) Another possible dividing line is whether the object has a significant secondary atmosphere, although this has a lot to do with the level of heating from its primary -- the colder the object, the easier to retain an atmosphere, unless it's too cold.

4) Sufficient self-heating to cause significant reprocessing of the surface might be a good indicator, but it would be difficult to determine from a great distance.

5) Probably the lowest dividing line of interest is whether the object self-gravitates into a spherical shape, although again, other factors enter into this, such as composition (silicates vs ices) and temperature.  Another issue would be: how spherical is spherical enough?  Objects at the lower limit might be only roughly spherical, so what degree of departure from the an ideal sphere (allowing for rotation) is acceptable?

My own list of known 'major planets' would include:

CLASS 1 PLANET: (significant direct acretion of gas, no deturium fusion) Jupiter, Saturn, Neptune, Uranus

CLASS 2 PLANET: (significant secondary atmosphere, no significant direct accretion of gas) Earth, Venus, Mars, Pluto

CLASS 3 PLANET: (self-gravitates into sphere, no significant secondary atmosphere) Mercury, 2003UB313, 2005FY9, 2003EL61, Sedna, Orcus, Quaoar, Ixion, Ceres, 2002TX300, 2002AW197, 2002UX25, Varuna, 2002MS4, 2003AZ84, 2004GV9, Vesta ...

While we're at it, here is a list of moons as well:

CLASS 2 MOON: Titan, Triton


This sounds pretty good!  The one comment I'll make here is that we have been
carefful  not to insist that an object BE spherical, only that it be massive enough
to become spherical. This avoids issues like the one you brought up (who
spherical does it have to be?), as well as issues related to rotational
flattening (ala Jupiter and Earth).



CLASS 3 MOON: Ganymede, Callisto, Io, Luna, Europa, Titania, Oberon, Rhea, Charon, Iapetus, Ariel, Umbriel, Dione, Tethys, Enceladus, Mimas, Miranda

Bill

[dvandorn]

So, is Titan a planet? Is Ganymede? Is Triton?

Or does real estate count, after all?

-the other Doug

[Alan Stern]

If real estate doesn't matter and you classify planet vs. non-planet based solely on size and mass (i.e., is it big enough to have pulled itself into a ball), then what about a majority of the moons of the gas giants?  Remember, real estate cannot be a factor in the equation.

Under the classification system that everything massive or large enough to pull itself into a sphere (more or less) is a planet, we have to admit Ceres, Vesta, the Galilean moons of Jupiter, many other Jovian moons, a lot of Saturnian moons, a lot of Uranian and Neptunian moons, plus possibly thousands of KBOs, all into the League of Planets.

Oh, and while it's not usually called a planet, our own Moon would have to be considered a planet, too.

If you're going to make an exception for bodies that orbit other planets to call them moons, then you're allowing real estate to enter the equation, and once you do that, you're simply applying arbitrary dividing lines...

-the other Doug

We dealt with the satellites issue in a long IAU article which I am happy to share if
anyone wants it; just email me and I'll send it.

Here's what we said: if an object fitting the planetary definition orbits another planet
then it is called a satellite of the planet and a "planetary body", but not a planet.