Charon (Pluto's Moon) is now a planet?

[Giles]

Well, Mercury is far more like Ceres than it is like Jupiter or Saturn, so I do think it’s that crazy to call Ceres a planet. (It was called a planet when first discovered, but then they started finding som many more objects like it that it seemed odd to call them all planets, so they were called asteroids.)

The Pluto-Charon case is odd, not because Pluto is small, but because Charon is so large in relation to it. (The Earth-Moon system is not much less odd, since the Moon also is very large as a satellite in relation to the planet that it goes around). But promoting Charon to the status of a planet does make Pluto very special, because it will be part of the only known double planet system.

[Jackknifed_Juggernaut]

@Angua

On average, the CoM of the Earth/Moon system (also called the barycentre) is about 4,670 km from the centre of the Earth. The radius of the Earth is ~6,400 km, so the CoM is within the Earth. As the Earth’s centre and the barycentre are different, the Earth wobbles a bit on its axis. linky

Thanks. So if we began dumping our garbage onto the moon, we might have a new planet in a few million years.

[DSYoungEsq]

@Angua

Actually, its not all that silly. Part of the definition of planet proposed by the IAU, is that a planet “is in orbit around a star, and is neither a star nor a satellite of a planet.” Ganymede is blatantly the satellite of a planet, and hence a moon, and not a planet. Ceres, on the other hand, despite being smaller than Ganymede is in orbit directly around the Sun, and under the new scheme would be termed a “dwarf planet” rather than a planet proper.

This response shows you missed my whole point. The fact that you can create a seemingly logical definition scheme under which the objects can be classified doesn’t mean that such a scheme makes true sense. To call Ceres a planet, when an object the size of Ganymede isn’t, is to create a meaningless distinction. Ganymede is a planet in orbit around another planet; Ceres is a small rocky body that just happens to be spherical in orbit around the sun, along with a gazillion other such bodies which just happen to be unlucky enough not to be spherical enough to qualify.

Or if you want another comparison, then Luna and Charon are apt. Yes, Luna and Earth orbit a point which remains inside the surface of Terra. But Luna wouldn’t have to be much bigger, or much further away, for that not to be true. Again, it’s only a matter of arbitrary definition, rather than a true difference in what the bodies in question are.

Admittedly ANY definition of “planet” set by the IAU will be arbitrary, but it seems to make more sense to use one that fits MOST of our existing notions about what the word means (losing Pluto in the process), than to create a whole new definition that includes a whole group of heavenly bodies that most people don’t think of as “planets.” IMHO, anyway.

[wolf_meister]

I was always a Pluto is a planet proponent. One of the best arguments (I used to think), was to say it had a moon. NOW, it has a planet? :dubious:

Why couldn’t they just say Pluto’s diameter or bigger? It’s a planet.

[pulykamell]

@wolf_meister

Why couldn’t they just say Pluto’s diameter or bigger? It’s a planet.

And that was exactly part of the IAU’s shortlist of definitions for a planet voted upon last year.

In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of Sedna in 2003, narrowed their choices to a shortlist of three, using approval voting. The definitions were:

*A planet is any object in orbit around the Sun with a diameter greater than 2000 km. (eleven votes in favour)

*A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. (eight votes in favour)

*A planet is any object in orbit around the Sun that is dominant in its immediate neighborhood. (six votes in favour)

The first would be an essentially cultural/historical definition, recognising Pluto’s historical identity as a planet by setting an arbitrary limit immediately below its diameter. Under this definition, the only known planets in our solar system would be the current nine, plus 2003 UB313.

Citation.

Anyhow, I’ve got no problem with calling Ceres a planet and Ganymede a moon. I’ve grown up with the definition that a planet orbits a star and a moon orbits a planet, so Ganymede’s size really is not an important point for me.

[Angua]

@DSYoungEsq

Ganymede is a planet in orbit around another planet;

For which, culturally, there is a well established term – “moon”. The general populace would be getting riled at us if we ‘promoted’ the moons to minor planets.

The only reason why Pluto and Charon are being termed a double planet is the comparative masses of the two objects:

M[sub]Pluto[/sub]=0.0021M[sub]Earth[/sub]
M[sub]Charon[/sub]=0.0002M[sub]Earth[/sub]=0.1M[sub]Pluto[/sub]

M[sub]The Moon[/sub]=0.01[sub]Earth[/sub]

M[sub]Jupiter[/sub]=320M[sub]Earth[/sub]
M[sub]Ganymede[/sub]=0.02M[sub]Earth[/sub]

Basically the masses of Pluto and Charon are comparable, whereas the masses of The Moon and Earth, and Jupiter and Ganymede really aren’t.

Admittedly ANY definition of “planet” set by the IAU will be arbitrary, but it seems to make more sense to use one that fits MOST of our existing notions about what the word means (losing Pluto in the process), than to create a whole new definition that includes a whole group of heavenly bodies that most people don’t think of as “planets.” IMHO, anyway.

I can see your point, and I have to admit, that whilst I am defending the IAU, that yes, their definition is entirely arbritary, but there is a large element of internal ‘politics’ that goes on too; if you demote Pluto and its ilk as ‘not planets’, you will end up making a lot of influential astronomers very angry. There comes a point where that’s not worth it, and a compromise solution is desirable.

[Noone_Special]

@Angua

…but there is a large element of internal ‘politics’ that goes on too;

OK, same as any profession, I suppose… but

if you demote Pluto and its ilk as ‘not planets’, you will end up making a lot of influential astronomers very angry.

why? Whose career depends on the classification of Pluto, anymore?

Me, I (from my position of ignorance) say Pluto should have been chucked out as a planet way back when, as soon as it was established that its orbit was much more elliptical, and much more inclined relative to the ecliptic, that those of the “classical” planets. Actually, the very fact that, back in the late 1920’s, they were looking for a mercury-sized object way out there in Gas Giant Territory should have been a giveaway – I don’t see whay anybody ever considered Pluto a “planet,” to begin with (Other perhaps than the fact that discovering a Whole New Planet carries a whole lot more prestige than discovering a comet or an asteroid, of course.)

[Angua]

@Noone Special

why? Whose career depends on the classification of Pluto, anymore?

No one’s really but…

(Other perhaps than the fact that discovering a Whole New Planet carries a whole lot more prestige than discovering a comet or an asteroid, of course.)

There’s your answer. The people who have been the strongest proponents of “Pluto is a Planet” are the ones discovering things like 2003 UB313; if its just another asteroid, there’s no glory, you’ve just discovered yet another Kupier Belt object, if on the other hand, its a planet, then woo, you’re the first person to discover a planet for 70 years. That’s one of the major reasons why people are getting het up about it.

Now, as for considering Pluto as a planet: hang the revision for a moment

Pluto was discovered entirely by accident. In the early 20th Century, Percival Lowell observed the orbits of Uranus and Neptune, and thought that he saw some perturbations in the observations; perturbations that would have been caused by a trans-Neptunian planet, whose position Lowell predicted. Lowell presented his announcement at the American Academy in 1915, but at the time no planet was found.

In 1930, observers at Flagstaff Observatory (the observatory that Lowell established) announced that they had managed to find the mysterious 9th planet, and named it Pluto.

However, it was soon realised that there were no perturbations in the orbits of Neptune and Uranus, and that Lowell’s calculations of the orbit were flawed; the discovery of Pluto had been entirely accidental, and it had happened to be in the right place at the right time. Further, in the 1970s when it became possible to accurately determine Pluto’s mass, it was found that it was far too small to cause any perturbations anyway.

Hence, the main reasons why it was thought that Pluto, when it was discovered, was a planet was due firstly to the fact that Lowell had predicted, based on erroneous calculations that there should be a planet there, and secondly, that when it was discovered, getting a handle on its mass was not easy; thus Pluto became the 9th planet. Had it been discovered in the 1970s or later, I think it would have just been classed as a large Kupier Belt object, and we probably wouldn’t be having this discussion!

[crowmanyclouds]

@Noone Special

…Whose career depends on the classification of Pluto, anymore?
…

Well until January 17, 1997, it meant a lot to Clyde Tombaugh’s career!

CMC fnord!

[GingerOfTheNorth]

@Fish

[spelling corrected]

See, the reason you had such bad luck was that Jupiter was rising while Pluto was in the house of Neptune, borrowing a cup of Mercury. While Pluto was there, he borrowed a second cup for his co-orbital partner. That just goes to prove the old saying “Charon share alike.”

Dammit, that’s funnier if you aren’t pronouncing it the way I’ve heard it on the radio news (keren is not the only the proper pronounciation, sheren seems to also be used. I read it as keren share alike and it took me a read or two).

[Fern_Forest]

@DSYoungEsq

But Luna wouldn’t have to be much bigger, or much further away, for that not to be true.

Hmm, how much further? It is moving further away. Has been for billions of years. Would this mean at some point the
CoG would move outside of the Earth’s surface and it would become a planet? When?