So, 12 planets eh?

[Scytale]

I for one could deal with Ceres being labeled a planet, but classifying all the big plutinos as planets is a tad too much. On the other hand, placing a lower limit of a Moon mass or Ceres mass would've been empirical (so would be placing an upper limit of 13 Jupiters, because it doesn't account for metallicity). I think the IAU should've waited to get consistent results on what KBOs / plutinos are, what's their exact place in the Solar System history, and then passed a decision. New Horizons will have done that in a decade or so.

[Malenfant]

Brown Dwarf: (13-73 jupiter masses, can orbit star or be solo, starts fusing in core but can't sustain it)

Looks like this might need refining...

http://news.bbc.co.uk/2/hi/science/nature/5260008.stm

Lowest mass limit for a viable star seems to be 0.083 Solar masses, which is about 83 Jupiter Masses, not 73. So anything below that but above 13 Jupiter masses is a BD, anything more than 83 Jupiter masses is a star.

[bdm]

... but IMHO NOT 2003 EL61, because it's NOT round

2003 EL61 would be in hydrostatic equilibrium even if it does look like someone sat on it. The criterion does not mean "spherical". Jupiter and Saturn are noticeably oblate too, would they not be planets just because they're not round enough?

[Don. Edwards]

Spherical is spherical. It doesn't meet they are perfectly round. No planet is perfectly round. Even with Jupiter's and Saturns oblativness they stil look round or spherical.


Don. Edwards

[ajtribick]

It's fortunate there aren't any really large objects in the Trojan points of the 8 planets everyone agrees on... At least this definition seems to be based on physical properties of the objects.

As regards 2003 EL61, can hydrostatic equilibrium result in a prolate rather than oblate shape?

[Captain-insane]

How do these definition's sound.

Meteoroid: 1 meter-1599 meters

Asteroid: 1-750 miles (1.6-1200KM) in diameter.

Planetoid: 751-2499 miles (1201.6KM-3999KM).

Planet: 2500 miles (4000 KM) in diameter to 13 times the mass of Jupiter.

Brown Dwarf: 13 Jupiter mass to 83 Jupiter mass.

Star: 83+ Jupiter mass.

So IF we were to with these definition's, we have 8 planet's, 11 if Ganymede, Callisto and Titan were independent from there parent planet's.

[Don. Edwards]

Captain-insane,

I would take the size of asteroids down to about 400 miles or so. The simple reason is that this would put Ceres back into the Asteroid catagory. Ceres is a very different objet than the rest of the Asteroids. So as I said, if the body has enough mass to make to make it spherical than it should be considered a planetoid. If a body is iregular in shape its and asteroid. Vesta and Juno are big but they are not spherical like Ceres is. Ceres is 480 miles in diameter. that is almost double the size of Saturns moon Enceladus and its spherical.

So see we still can't go strickly by size, we have to take into consideration the shape and what the body is made up of.

Don. Edwards

[Hamiltonian]

As regards 2003 EL61, can hydrostatic equilibrium result in a prolate rather than oblate shape?

Google on "Jacobi ellipsoid".

[MKruer]

Um just base it upon mass, which will make the size irrelevant, and account for gravity, and is more consistant

[Brendan]

I didn't like the part about the barycenter. What about the Earth's Moon that has a greater acceleration toward the Sun than to the Earth? Maybe they used the barycenter instead because the smaller object's mass is involved in finding where the barycenter is, but is not be used when comparing its accelerations toward the Sun and the larger object. The bad thing about this is that it uses the mass of the larger object too. If we put Charon into orbit around Earth, it would likely not be a planet anymore. It would need to be really far away, but maybe it would be outside Earth's Hill sphere and then wander off again and be a planet again, at least until its ice sublimes in the brighter sunlight.

What about Triton that Neptune captured? I prefer to use the properties of the objects themselves, rather than where they are in the solar system because the orbits can change like this. So I would call objects like Ganymede and Titan planets that happen to be strongly perturbed by other planets in a manner that would have them be called moons too. After the strongly perturbed planets, the Earth's Moon would be in between, then there would be weaker sorts of relationships like Trojans, like Theia could have been. Then there would be the smaller effects like between Venus and Mercury.

That is seperating out the perturbing effects out of the object's identity by calling the moon-primary relationship a very strong type of effect. But such a relationship could have effects on the smaller object like with Io and Europa and with Triton. But they don't change things like the mass of the objects much, just a little thrown off by any volcanic activity.

So with this idea, Triton was a planet that was independent, but then entered a strong perturbing relationship with Neptune that we call a moon-primary relationship that caused lots of resurfacing activity.

Cases like Pluto and Charon could be called double planet relationships (maybe with another type of double planet relationship for the Moon being accelerated more to the Sun) instead of moon-primary relationships without having the barycenter stuff exclude much larger objects that just have different sorts of relationships with other planets.

That is creating different types of relationships, including different types of double planet relationships without worrying about them affecting planet status. Then we won't have them calling Charon a planet and lumping worlds like Titan into the same category as all of those moonlets.

We have multistar systems, why not have multiplanetary systems like Jupiter and 4 little planets in the strong perturbing relationship we call the moon-primary relationship?

I got an idea. A double planet could be made in Celesita that has the barycenter be at the surface of the more massive object. Use a model for the more massive object to have mountains and valleys and make it rotate faster than the lower mass object revolves around it. That could make the barycenter enter and exit the more massive object many times as it rotates.

[Cormoran]

I'm watching this discussion with interest (now that I can view the site )

One would assume that the commitee making the decision was and is having these same discussions.

While I have no deep overall opinion on the subject, I think the IAU seem to be thinking a little too heliocentrically? The definition of a Pluton seems a little specific. I'd prefer a simpler definition: A non-luminous object that directly orbits a star with a surface gravity greater than .1 gee (?)

(Will the Inquisition grab Consty and poke him with soft cushions? Will Mkruer be made to sit in the comfy chair? Will I be asked if I want ' Cake or death'? The spirit of Giordano Bruno is watching, guys...)

As Selden said, It appears that no matter the eventual decision, somebody somewhere is going to disagree with it.

[Malenfant]

Even "non-luminous" is vague though. In what wavelengths?

Larger gas giants (even Jupiter) emit infrared radiation. The superjovians (2-12 juipter masses) will be warmer and emit more IR too, and of course Brown Dwarfs are definitely luminous in IR to some extent, even if they're old.

[ajtribick]

In addition, young gas giants would presumably radiate at visible wavelengths.

[bdm]

How do these definition's sound.

Meteoroid: 1 meter-1599 meters

Asteroid: 1-750 miles (1.6-1200KM) in diameter.

Planetoid: 751-2499 miles (1201.6KM-3999KM).

Planet: 2500 miles (4000 KM) in diameter to 13 times the mass of Jupiter.

Brown Dwarf: 13 Jupiter mass to 83 Jupiter mass.

Star: 83+ Jupiter mass.

So IF we were to with these definition's, we have 8 planet's, 11 if Ganymede, Callisto and Titan were independent from there parent planet's.

These definitions cannot work because for many bodies the size is uncertain. It's impossible to get an accurate reading of radius from an unresolved dot of light. Stellar occultation can help to measure the radius, but that is uncertain as well if the body has an atmosphere.

[MKruer]

If i didn?€™t repeat myself, I think the mass of an object and a few other choice wording would be the best, and I would even consider making some of the moons around the other planets, planetary moons, meaning that the moons are large enough to be called planets, or classified as such. Mass would also be easier to ascertain as well.

[ogg]

You have to realise that being a planet or a moon is not and never has been an essential property of the things that are planets and moons: Ganymede for example is bigger than Mercury, so if it were orbiting the sun instead of Jupiter there'd be no contest about it being a planet (unless we'd want to try demoting Mercury as well). The point is that 'planet' or 'moon' are orbital offices or roles that large-enough bodies play within a planetary system.

The new definition just fines up the criteria for 'large-enough' (hydrostatic equilibrium) and for the orbital role (orbiting a star + the barycentre rule)

Reason why I think the hydrostatic collapse criteria is a good one for 'large-enough':

it's a natural transition point from low mass to high mass, and scientifically interesting in itself. None of the proposed size or mass cut-offs have any scientific interest at all, they're just arbitrary points on a measuring line.

Reason why I think the barycentre criteria is a good one for distinguishing between planet-moon and planet-planet (ie double-planets):

Imagine a fictional Charon and Pluto that had nearly the same mass. If you want to say that the biggest of the two is the planet and the other one the moon, then by definition there can never be such a thing as a double planet system. And it'd be possible for them to swap moon-planet roles just by being hit by a big enough third object. Add enough mass to Charon and it becomes the planet, take away some mass again and Pluto is the planet again. This again just seems arbitrary. It's less arbitrary to look at where the barycentre is exactly, and relate it to the planets themselves, rather than the mid-point on some imaginary line that runs between them (you could use 1/3 the distance between them as a yardstick instead, but again that's just an uninteresting arbitrary point).
Another point: If the smaller of the two is still robust enough to be swinging the larger one around completely (ie the barycentre is outside of it), then surely that's good enough?

Anyway. The point is that this is just a technical definition. "planet" just used to mean 'wanderer' or something like that, and AFAIK this included the moon and the sun! The term has been constantly pushed along by new discoveries (Ceres was originally called a planet on it's discovery and only afterwards demoted), but now the discoveries with KBOs and exoplanets are coming in so thick and fast that a decision actually had to be made. I think it's a good one. Astronomers need preferrably to have a single definition to work with, to make sure they're not talking past each other. If you want resist their new conception of 'planet', then fine. It's a technical concept, and it'll be interesting to see if the public at large pick it up or just keep referring to "the nine planets". Just be aware that you're not talking about what the scientists are talking about anymore. Personally I think we should adopt it as well, or else it'll be one more widening of the rift between scientists and the rest of us. Why not just trust the experts on terminological matters? It's not like anything really depends on it.

Nuff ranting...

[Captain-insane]

Captain-insane,

I would take the size of asteroids down to about 400 miles or so. The simple reason is that this would put Ceres back into the Asteroid catagory. Ceres is a very different objet than the rest of the Asteroids. So as I said, if the body has enough mass to make to make it spherical than it should be considered a planetoid. If a body is iregular in shape its and asteroid. Vesta and Juno are big but they are not spherical like Ceres is. Ceres is 480 miles in diameter. that is almost double the size of Saturns moon Enceladus and its spherical.

So see we still can't go strickly by size, we have to take into consideration the shape and what the body is made up of.

Don. Edwards

Don't mean to point this out but according to Wiki, Ceres is 609.375 Miles (975 KM) in diameter, http://en.wikipedia.org/wiki/1_Ceres just thought i'd point that out.

[Don. Edwards]

Wow, then they upped its size. It has been 480 miles in diameter for as long as I can remember. With the new size, I think it surely doesn't belong in the asteroid camp anymore.


Don. Edwards

[AlexChan]

If they choose new defination, most of new planets will appear in Kulper Belt.....It is too crowded...

[ANDREA]

If they choose new defination, most of new planets will appear in Kulper Belt.....It is too crowded...

AlexChan, would you be so kind to show us your .ssc file with all the 24 "planets", please?
Thanks a lot, this will avoid me to duplicate your work
Bye

Andrea