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[Chicago Astronomer - Astro Joe]

If it's round, it's a planet

Here is a segment from a yahoo group (All astronomy all the time)that was discussing the Pluto debate.

I saw a presentation on the Science Channel about the
debocle. I kind of agree with one of the Astronomers,
but I'd add an addendum to his definition of planet.
Don't remember his name, he's the one who worked with
Dr. Luu and discovered the 1st Kuiper object a few
years ago.

He said if it's round, it's a planet. That is, if
it's big enough for it's gravity to affect the shape,
it's a planet. He felt the definition should be a
simple one.

I'd just add the criteria that it goes around the sun.
That is, in can be round, but if orbiting another
solar object then it is not a planet, but a satellite
of the larger object.

I don't see the big issue about whether Pluto is a
planet. That issue was resolved years ago when the
asteroids were discovered. The first few of those
were considered planets till it was realized they are
generally too small to even be round. Then they were
classified as minor planets, or asteroids.

Seems to me that what's missing is not a planet
definition, but a similar concept on moons of planets.
We have nothing in the name to tell us a moon is big
enough (round) to be a planet, or too small
(irregular). Maybe we need "major moons" (planet
sized) and "minor moons" (asteroid sized).

Then it seems the nomenclature would be clear. A
planet orbits the sun and is round. A minor planet
orbits the sun, and is irregular shaped (too small to
be round). A major moon orbits a planet and is round,
and a minor moon orbits a planet and is too small to
be round. In either case, a moon doesn't directly
orbit the sun, but another object that orbits the sun.

Source: mail01.mail.com/scripts/mail/read.mail?folder=INBOX&order=Newest&mview=a&mstart=1&pbox=2&msg_uid=1155603316&mprev=1155604579&mnext=1155603314

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It reflects my own personal feeling as stated before in another thread here. If it's round, has an atmosphere and orbits the Sun... it's a planet.

[Chicago Astronomer - Astro Joe]

Roundish Objects orbiting the Sun to be considered for planet classification

"If gravity can make it round, it's a planet," Binzel told New Scientist. That is not quite the whole story – planets also have to orbit a star, and not be either stars themselves or satellites of other planets – but the new part of the definition is roundness.

Whereas very small heavenly bodies tend to be irregular rocks, larger ones are crushed by their own gravity into a spherical shape. The committee decided that the threshold of roundness should distinguish planet from non-planet. "We let nature decide," says Binzel.

The threshold size depends a bit on the material, but for ordinary ice or rock it turns out to be well below 1000 kilometres. So the new definition includes Ceres, previously classed as the largest asteroid, which is 950 kilometres across.
Round asteroids

Ceres would become the fifth planet from the Sun and the outermost of the five rocky planets, orbiting between Mars and Jupiter. "It was considered a planet once before, after it was discovered in 1801," says Binzel. "It is again."

Ceres might not be the only asteroid to be promoted. If the definition passes, the IAU will also have to consider the claims of Vesta, Pallas and Hygiea, as all of them are round-ish. The smallest, Hygiea, has a diameter of only about 400 kilometres.

Perhaps an even more surprising planet is Pluto's companion Charon. At more than 1000 kilometres across, it is certainly large enough to have settled into a sphere; and according to the new definition, Charon is not a satellite of Pluto.

Charon is so heavy that the centre of gravity of the two bodies is in space, between the two objects (whereas the Earth and Moon orbit each other about a point deep inside the Earth, making the Moon a true satellite). Pluto and Charon would become the first double planet in the solar system.

Source: www.newscientistspace.com/article/dn9761

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I like the idea of a double planet system. It's kinda cool to have one, and reflects the writings of science fiction stories by the ton...

[Rocketman]

...and there a a lot more than that which could be round.

Rhadamanthus, Huya, Deucalion, Logos, Chaos, Ixion...


Would much more neat in my opinion to have forty-seven planets, rather than nine.

www.nineplanets.org/kboc.html

[Centaur]

The inclusion of Pluto’s satellite Charon in the list of 12 planets seems ludicrous. Many other planetary satellites, including the Moon, are far larger. The argument seems to be that the Pluto-Charon barycenter lies above Pluto's surface. For the Moon and the other large satellites the barycenter lies within the main planet. Future students memorizing the list of planets will have a hard time understanding the difference that caused the Moon to be left out.

I would think that the shape of a planetary satellite's orbit relative to the Sun would be a greater consideration than barycenter. As with most non-retrograde satellites, Charon describes an epitrochoid relative to the Sun. In particular this is the type of epitrochoid that is alternately convex and concave with points of inflection (wavelike), e.g. Jupiter’s Ganymede and Callisto. Other satellites describe epitrochoids that cross over themselves (loop-the-loop), e.g. Jupiter’s Io and Europa.

The Moon is unique in that its orbit is perpetually concave toward the Sun. We think of the Earth as orbiting the Sun while being perturbed by the Moon. Normally we consider the Moon as orbiting the Earth while being perturbed by the Sun. But more so than other satellites, the Moon could also be thought of as orbiting the Sun while being perturbed by the Earth.

I would refer to the Moon as a planet before I would do so for Charon. Geophysically, I have always thought of the Moon as the first extraterrestrial planet visited by astronauts. Historically, there were seven planets (wandering stars): Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. Although in most contexts the term referred only to the latter five, as the Sun and Moon were considered to be of elevated importance to ancient naked-eye observers.

In truth, I am not calling for the Moon and other large planetary satellites to be included in the list of major planets. I’m just asking that Charon be crossed off the list. It makes those seven committee members who agreed to it look silly. They almost have me wondering if they intended to draw up a list so ridiculous that it might make others stop insisting on the inclusion of Pluto.

By the way, there is only one Moon. The proper term for bodies orbiting planets is satellite. Yes, the generic word moon has entered the popular language. This has led some to ask why the Moon has no name of its own. It has a name in English: Moon. Of course when the Moon was first called so, there were no others to consider. Today, even some astronomers use the term imprecisely. But the enlightened ones realize the original error. The launching of artificial satellites beginning nearly 50 years ago may have caused some writers to mistakenly believe that all satellites must be artificial, leading to the greatly increased generic use of the term moon.

[Chicago Astronomer - Astro Joe]

Curt,

I agree with you on this one.

Charon should not be considered a planet. I rather like the idea of a double planet in our system, but in this case, Charon should just be considered a Satellite of Pluto...along with Nix, Hydra and any others to be discovered. I think maybe it just might be politics and a compromise of sorts.

A while ago, I too fell into the "Moon" labeling. Curt pointed it out with logic and clarity, and I have seen the light. No more will I call orbiting bodies of planets Moons, but refer to them as satellites...and have been since.