IAU Proposes 3 New Planets

IZ Reloaded writes "Sources tell SPACE.com that the International Astronomical Union is preparing to include three new entries to the current list of planets in our solar system. From the article: The asteroid Ceres, which is round, would be recast as a dwarf planet in the new scheme. Pluto would remain a planet and its moon Charon would be reclassified as a planet. Both would be called "plutons," however, to distinguish them from the eight "classical" planets. A far-out Pluto-sized object known as 2003 UB313 would also be called a pluton."

Comments

Re:One issue

[goober]

Also by the definition since Charon would be a planet... wouldn't the moon need to be its own planet?

Charon differs from Luna because Pluto and Charon jointly orbit around a point outside either of their bodies, whereas Luna orbits a point inside the Earth. Pluto and Charon are therefore (currently) technically a twin planet system.

That's no moon

[Namarrgon]

It's a b... I mean, it's actually part of a double-planet system, orbiting around a common point in space (unlike all other moons in our solar system). And Ceres is an asteroid with a name, thank you very much.

In answer to your second question, since August 24, if the vote passes.

Re:That's no moon

[john83]

All two body systems orbit a common point. If you cleared all the other gunk out of the solar system, the earth and the sun would orbit a common point. It would just happen to be very, very deep in the sun because of the disparity of mass. I don't see your argument there, unless you're saying that the common point is outside Pluto and that this isn't true of other systems.

Re:That's no moon

[isorox]

unless you're saying that the common point is outside Pluto and that this isn't true of other systems.

That's exactly what he's saying

Re:That's no moon

[mrxak]

Look at these animations. Second from the left is Pluto and Charon. That's why they're both planets. The definition being proposed is rather elegant, I think. It leaves it up to gravity to determine what a planet is, and catches special cases like Pluto and Ceres rather nicely too. You have your 8 classical planets in nice orbits, you have your N plutons in crazy orbits, and you have your really small dwarf planets like Ceres that never quite made it.

Re:That's no moon

[SpryGuy]

Actually, it's more due to the fact that in its orbit around the earth and around the sun, the moon is NEVER falling AWAY from the Sun. It always falls towards it.

It seems to me that might be a useful definition to consider... and it would make more sense for the Moon to also be classified as a planet, than for Charon (for example) to be classified as a planet while the moon (many times larger) isn't.

Frankly, though, I think the whole thing is a mess. Pluto, Charon, Quoar, Xena, and all the rest, are Kuiper Belt Objects, just like Ceres is an asteroid. In particular, Pluto, Charon, Quoar, Xena, and the others KBOs are all in highly elliptical orbits, outside the plane of the ecliptic. Why can't the definition of a planet include the plane of the ecliptic? We'd have 8 planets, and then a mess of KBOs.

Thats no moon...

[The Creator]

(Nor a space station)

It's not orbiting Pluto, but instead a point between itself and Pluto. If the mass of Pluto was higher, so that their common center of gravity was inside pluto, then Chauron whould indeed be a moon.

Planetary Categories

[Rob Carr]

New Scientist has the complete set of proposed categories for planets:

• Planet: A round thing orbiting a star. More precisely, according to the draft definition: "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."
• Pluton: A planet orbiting beyond Neptune, taking more than 200 Earth years to circle the Sun. So far, it would include Pluto; Pluto's former moon, Charon; and "Xena" (2003 UB313).
• Satellite: Anything orbiting a planet, as long as the mutual centre of gravity does not fall outside the planet. Includes several bodies much larger than many planets, such as Jupiter's moon Ganymede (diameter: 5262 kilometres).
• Small solar system body: Anything orbiting the Sun that's not a planet or a satellite. Most asteroids and comets would be SSSBs. Currently called minor planets.

Unofficial categories of planet:

• Dwarf planet: A planet smaller than Mercury (diameter: 4879 kilometres), which is the smallest uncontested planet. Would include the former asteroid Ceres; Pluto; Charon; and Xena.
• Giant planet: Jupiter, Saturn, Uranus, Neptune.
• Classical planet: The four giant planets plus the familiar four rocky, terrestrial planets: Mercury, Venus, Earth, Mars.

Re:What the pluton?

[KiloByte]

The whole difference between a "planet" and a "moon" is a fallacy. It assumes things can orbit only a physical object, and not an immaterial object like a center of mass. The "official" definition fails not only in the obvious Pluton-Charon case, but even for Sun-Jupiter (putting the smaller bodies aside for now). We orbit not the Sun, but the center of mass of the Solar system, which is actually outside the Sun itself.

Thus, with the difference between "planets" and "moons" away, the classification that matters is:
* pieces of rock (Mercury, Venus, Earth, Moon, Mars, Phobos, Deimos, Europa, ...)
* sub-stellar balls of gas (Jupiter, Saturn, Neptune, Uranus)
* pieces of dirty ice

And to make it even harder, there is absolutely no reasonable boundary between "almost big enough to fuse" and "one particle". The difference between a "pebble" and a "boulder" isn't tangible.

Mike Brown's take on this

[call -151]

Mike Brown, leader of teams that have discovered 2003UB313 and 11 other objects that meet the proposed definition of planet, has the following on his webpage now:

The IAU proposal officially recognizes only 12 planets; where does the number 53 come from?

By the proposed IAU definition, anything large enough to be pulled by its own gravity into the shape of a sphere and which is in orbit around a star is a planet. The proposal officially recognizes 12 planets (the nine previously recognized plus Ceres and Pluto's moon Charon plus 2003 UB313) creates a complex committee procedure for an object to become officially recognized. This part of the proposal is perhaps the weakest. In no other area of astronomy is there a definition for a class of objects and then a committee that has to decide if an object fits the definition. There are simply definitions. If an object fits the definition it is part of the class. If the IAU proposal is accepted then scientifically all of the spherical objects out there are indeed classified as planets, regardless of how long it takes for a committee to officiailly declare them to be so.

A relatively simple analysis show that there are currently 53 known objects in the solar system which are likely round. Another few hundred will likely be discovered in the relatively near future. Regardless of what the official count is from the IAU proposal these object all fit the scientific definition of the word planet and if the scientific definition is to have any credibility they should all generally be considered planets.

What should the public think about 53 planets?

Most people, when first confronted with a proposal to make 44 new planets in the solar system, seem to react by looking blankly for a second, then shaking their heads and muttering something about astronomers being crazy. Astronomers are not actually crazy, at least most of them. Astronomers have needed a good scientific definition of the word "planet" for many years now and this one works well for scientists. It doesn't, however, work terribly well for the rest of the world. The solution is the one that should have happened long ago: a divorce of the scientific term "planet" for the cultural term "planet." No one expects school children to name the 53 planets (most, in fact, don't even have names). If I were a school teacher I would teach 8, or 9, or perhaps 10 planets and then say "scientists consider many more things to be planets too" and use that opportunity to talk about how much more there is in the solar system. But at the end of the day I would talk about 8 or 9 or 10. Not 53.

Culture and science have always meant something different when they use the word planet, and with this new scientific definition so clearly far removed from what the rest of the world things a planet is there will no longer be any need to confuse the scientific word with the cultural one.

How am I going to vote on the IAU resolution?

This one is easy to answer. I am not an IAU member, I took no part in drafting the resolution, and I get no vote. If I were to vote, however, I would have to decide that while the definition itself is viable the extra non-scientific beauracratic barrage attached to the resolution would doom it for me.

Re:What the pluton?

[jc42]

The Earth and the Moon also revolve around a common point, which is inside the Earth.

A perspective I've read on this is that our moon's orbit is everywhere concave with respect to the sun. So it's more accurate to interpret the Earth-Luna pair as not really orbiting each other, but rather sharing a solar orbit. Two bodies that are close together in the same orbit do swap places periodically; there are several known cases of this in the Jupiter and Saturn systems. From a rotating frame of reference, they appear to be orbiting each other. But viewed in a static frame, they appear to be swapping the lead periodically. So the Earth-Luna pair could be more accurately considered a binary planet pair in a common orbit.

It's all rather nitpicky anyway. As numerous astronomers have pointed out here, they mostly don't use such vague terms as "planet". And an orbit isn't really a property of the bodies in an orbit; it's a property of the system.

The "debate" is basically a media event, based on people who take their grade-school science classes too seriously, and think that for some reason that the Solar System must contain exactly nine "planets".

Re:Solar Center of Mass

[Steve B]

Shouldn't the Sun have a hell of a tide due to Jupiter?

Tidal effects (the difference between gravitational acceleration at the near and far sides of an object) vary with the inverse cube of distance.

Definitions

[j_w_d]

The difference between a "pebble" and a "boulder" isn't tangible.

"Pebble" has a formal scientific definition of small alluvial material from 4 to 64 mm diameter. "Boulders" are more than 256 mm diameter. Assuming the piece is a standard stony material such granite, drop a pebble of granite on your left toe and a boulder of granite on your right. I believe you will quite clearly note tangible, tactile differences. You might also try carrying a boulder in your back pocket and a pebble in a front pocket in your pants, or the other way around, if your are insecure with your girlfriend. Whenever you try sitting I again suspect you will note "tangible" differences.