Is there any definition of a planet that could possibly include Pluto, but not 2003UB313 (Xena)?

Question

2003UB313 is farther away from the Sun, but it's larger than Pluto.

The center of gravity between Pluto and Charon is between the bodies, so isn't Charon as much of a planet as Pluto?

Answer 1

Hmm, well you could say everything out to a certain distance from the sun can be a planet if it is large enough; that would give you Pluto and not Xena.

Pluto was the former planet and Charon the moon because Pluto was larger and so conventionally Charon would be the moon instead of Pluto. Sometimes Pluto and Charon were considered a double planet system since they are tidally locked and always show the same side to one another. Also Pluto now has two other moons, but they are much smaller than Charon.

Answer 2

Yes, there is a definition that would include Pluto as a planet...and maybe include or exclude (don't know yet) "Xena". To be a planet, there must be some hint of an atmosphere. Pluto has a faint atmosphere. And, the latest observations of Pluto's sister, Charon, suggest that it may also have an incredibly faint atmosphere. So there ya' go...10 planets.
As a footnote, I'm fine with Pluto's "elevation" to mascot and #1 leader of the new dwarf planets, but I think Charon must be defined as a dwarf planet just like Pluto since the barycenter is located outside of either body, a double dwarf planet system.

Answer 3

The definition of a planet should depend on WHAT it is, not WHERE it is. It should be based on the properties of the object itself, not on the properties of the system it is contained in. By the IAU s definition, any and all objects floating around in interstellar space not bound to a star, are not planets. That s ABSURD. Think about it! If we discovered a spherical mass in space of equal or more massive than earth and of equal or larger diameter to Earth, THAT is a planet. If we found that same object in the Ort Cloud, it s a plane, not an Ort Object. Kuiper Belt Object is a property/attribute of only our own solar system, the Kuiper Belt is nothing more than Sol s second asteroid belt going out from the star.

Answer 4

any kuiper belt object will be a dwarf planet, if it is round. charon also fits the definition of a dwarf planet because pluto and charon are considered a binary system.

(1) A classical planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(3) All other objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html

Answer 5

That's a great question! I think if you went by physical characteristics you'd be hard pressed to find a way to include Pluto and exclude "Xena" and other as-yet-to-be-found objects also bigger than Pluto.

Aside from distance, which is too arbitrary in my opinion, I can't think of anything! Which, to me, is a good reason to not keep Pluto as a planet.

Answer 6

No you can't....even given the new classification scheme that the IAU have come up with. The whole problem with resolutions 5 and 6 are that they're ambiguous at best, nonsensical at worst. First up, the definitions say nothing about planets orbiting other stars.....how do you classify an exoplanet with the given scheme??. You can't. You have no way of classifying them because the scheme deals only with our own Solar System. That's the first flaw. Secondly, talking about clearing its orbit of debris. That being the case, very few of the objects in this Solar System can be called "classical planets". For example, the Earth itself is surrounded by upwards of 10000 NEA's (Near Earth Asteroids), then you have to account for the small body of trojans either side of Earth in its orbit. Certainly hasn't cleared its orbit of debris. Same goes for Jupiter, and Neptune. Saturn is also accompanied by its own retinue of flotsam. So does Mars (the Amor asteroids), Venus and Mercury (less so, with the Atens). So there's flaw two. And what about exoplanets in systems whose star still has substantial debris clouds surrounding the star??. Thirdly, the orbiting of the Sun......now whilst that is correct for us, the Sun being the centre of mass in the system (same goes for nearly all single stars), what about multiple systems where the centre of mass isn't a single star but a point in space between the stars of the system. What do you call a planetary sized object that orbits a point in empty space, then?? Same problems goes for most red dwarf stars. Being so small, the centre of mass between them and most of the mass in a solar system similar to ours would lie several tens of thousand miles OUTSIDE the visible surface of the star. There you have the same point in empty space problem again. Flaw three. Now for flaw four....orbital geometry. One of the gripes some astronomers had with Pluto was its orbital geometry. That being the case, what about all those exoplanets with non circular orbits. Many have orbital geometries even more extreme than Pluto..... 16CygAb for example. Can't get too more elliptical an orbit than it has without becoming either hyperbolic or parabolic.

Since this decision was only resolved by the votes of 424 delegates (most had already left the conference), it really can't be classed as a majority vote of all the astronomers either at the conference or worldwide for that matter. So the decision should be declared void and a new vote taken.

What should be the definition of a planet is something like this....

1. A planet is a body that through self gravitation has assumed a spherical shape.

2. Through its process of formation, it should now be a fully differentiated body with distinct layers of varying compositions and densities.

3. It should be in orbit about the centre of mass of the stellar system within which it resides. Be it a single star or the centre of mass in a multiple star system.

4. The object's diameter must be of a size equal to and greater than ..... (an arbitrary size agreed upon by ALL astronomers.....say 2000kms), and a mass equal to or less than 13 Jupiter masses.

That above definition avoids problems such as orbital geometry, clearing of debris in orbit, surface conditions of the body and sets a minimum size limit, plus excludes any object orbiting a planet or the centre of mass between a planet and another body. It also differentiates between a planet of Jupiter or larger size upto the point where it can be classed as a Brown Dwarf. It's far more precise than the rather ambiguous one the IAU came up with.

Answer 7

2003UB313 is the least of your problems. Jupiter has four moons which are more massive than Pluto. And Pluto is substantially different from the other planets - it has a highly irregular shape and angle of orbit, which hints strongly that it was formed in a way completely different from the other major planets.

If you want to retain Pluto in spite of these differences, there are plenty of completely arbitrary definitions of 'planet' you might adopt. Some of them include:

- Solar bodies no smaller than 0.002 Earth's mass and discovered before 1931.

- Solar bodies whose names in order correspond to the first letters of the phrase, "My very excellent mother just sent us nine pizzas."

- Solar bodies which fit the IAU definition of a planet or share the name of a Disney character.

But the question is do you really want to adopt completely odd and arbitrary definitions to include things that are obviously different just for reasons of tradition?

Answer 8

In science, one holds certain facts to be truth, until they are proven false. Pluto is a Kuiper Belt object, not a planet. To fight otherwise is to tend towards sentimentality, not observed fact.