posted 08-16-2006 07:18 AM               

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The IAU draft definition of "planet" and "plutons"

The world's astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the difference between "planets" and the smaller "solar system bodies" such as comets and asteroids. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, our Solar System will include 12 planets, with more to come: eight classical planets that dominate the system, three planets in a new and growing category of "plutons" - Pluto-like objects - and Ceres. Pluto remains a planet and is the prototype for the new category of "plutons."

With the advent of powerful new telescopes on the ground and in space, planetary astronomy has gone though an exciting development over the past decade. For thousands of years very little was known about the planets other than they were objects that moved in the sky with respect to the background of fixed stars. In fact the word "planet" comes from the Greek word for "wanderer". But today hosts of newly discovered large objects in the outer regions of our Solar System present a challenge to our historically based definition of a "planet".

At first glance one should think that it is easy to define what a planet is - a large and round body. On second thought difficulties arise, as one could ask "where is the lower limit?" - how large, and how round should an asteroid be before it becomes a planet - as well as "where is the upper limit?" - how large can a planet be before it becomes a brown dwarf or a star?

IAU President Ron Ekers explains the rational behind a planet definition: "Modern science provides much more knowledge than the simple fact that objects orbiting the Sun appear to move with respect to the background of fixed stars. For example, recent new discoveries have been made of objects in the outer regions of our Solar System that have sizes comparable to and larger than Pluto. These discoveries have rightfully called into question whether or not they should be considered as new 'planets.' "

The International Astronomical Union has been the arbiter of planetary and satellite nomenclature since its inception in 1919. The world's astronomers, under the auspices of the IAU, have had official deliberations on a new definition for the word "planet" for nearly two years. IAU's top, the so-called Executive Committee, led by Ekers, formed a Planet Definition Committee (PDC) comprised by seven persons who were astronomers, writers, and historians with broad international representation. This group of seven convened in Paris in late June and early July 2006. They culminated the two year process by reaching a unanimous consensus for a proposed new definition of the word "planet."

Owen Gingerich, the Chair of the Planet Definition Committee says: "In July we had vigorous discussions of both the scientific and the cultural/historical issues, and on the second morning several members admitted that they had not slept well, worrying that we would not be able to reach a consensus. But by the end of a long day, the miracle had happened: we had reached a unanimous agreement."

The part of "IAU Resolution 5 for GA-XXVI" that describes the planet definition, states "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." Member of the Planet Definition Committee, Richard Binzel says: "Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet."

According to the new draft definition, two conditions must be satisfied for an object to be called a "planet." First, the object must be in orbit around a star, while not being itself a star. Second, the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape. The shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation.

If the proposed Resolution is passed, the 12 planets in our Solar System will be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313. The name 2003 UB313 is provisional, as a "real" name has not yet been assigned to this object. A decision and announcement of a new name are likely not to be made during the IAU General Assembly in Prague, but at a later time. The naming procedures depend on the outcome of the Resolution vote. There will most likely be more planets announced by the IAU in the future. Currently a dozen "candidate planets" are listed on IAU's "watchlist" which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The IAU draft Resolution also defines a new category of planet for official use: "pluton". Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). Plutons also typically have orbits that are far from being perfectly circular (technically referred to as having a large orbital eccentricity). All of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets.

The draft "Planet Definition" Resolution will be discussed and refined during the General Assembly and then it (plus four other Resolutions) will be presented for voting at the 2nd session of the GA 24 August between 14:00 and 17:30 CEST.

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posted 08-16-2006 12:28 PM               

Rather than creating a precise delineation of what is and isn't a planet, this rule comes up with the broadest possible definition -- "round things that go around stars." (And, while answering the planet question for small bodies in our system, it leaves issues on the other end of the spectrum, which will have to be dealt with as we discover more extrasolar worlds: a planet has to orbit a star, but not be a star. As we find bodies that hug the line between gas giant and brown dwarf, how do we decide what's a planet and what's a star? Easy -- planets are the ones that aren't stars.)

Regardless of size and shape, Ceres has more in common with asteroids than with planets, and Pluto and UB313 have more in common with KBOs than with planets. Rather than create these artificial delineations, we should create definitions that group like objects together. Mercury, Venus, Earth and Mars are like each other. They are rocky planets. Jupiter, Saturn, Uranus and Neptune are like each other. They are gaseous planets. While those two groups are diferent, I have no problem with both being grouped under the term planet, nor with that term being applied to extrasolar bodies that fit within those two categories. Asteroids are asteroids. KBOs are KBOs. Comets are comets. It's that simple. (Well, almost that simple -- extrasolar KBOs aren't KBOs unless you define Kuiper Belt in a way that could apply to other stellar systems.)

This proposal also creates the possibility that as we get better info about the Kuiper Belt (and then the Oort Cloud), we could end up with just a huge number of planets, which I don't like. I think our solar system should never have more planets than the average person could reasonably be expected to remember.

The one thing I do like? Even though it's wrong, I do think the idea of having a double planet system in our solar system is kinda cool.

Since the old "My Very Educated Mother Just Showed Us Nine Planets" would no longer work, perhaps:

Many Varied "Experts" Manning Committees Just Shallowly Undermined Natural Planetary Categories, Unbelievably.

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All These Worlds Space Blog | Hatbag.net
"America's challenge of today has forged man's destiny of tomorrow." - Commander Eugene Cernan, Apollo 17 Mission, 11 December 1972

[Edited by DavidH (August 16, 2006).]

posted 08-16-2006 03:38 PM               

For example, New Horizons, which is presently 3.24 AU from the Sun and 28.56 AU from Pluto, was launched as a mission to study to the last planet of our solar system. If the vote is confirmed next Thursday, New Horizons will instead fly to the 10th and 11th planets in our solar system and the only two (as of now) to comprise a dual-planetary system.

posted 08-16-2006 05:11 PM            

(1) Since the whole process involves a certain arbitrariness, why on earth (no pun intended) can't they set a realistic arbitrary lower size limit, say 1,000 miles or perhaps 2,000 kilometers. That would keep Pluto as a planet and allow "Xena" to become the 10th planet (which is logical, since it seems to be slightly bigger than Pluto).

(2) Ceres is not a planet. It is a minor planet, a planetoid, or an asteroid (take your pick of the terminology). The mere fact that it is a spherical body isn't good enough. It's too small.

(3) Charon is not a planet. It is a moon in orbit around a planet. OK, it is the largest moon in relation to its parent planet, but if Pluto-Charon is to be seen as a double-planet, then Earth-Moon is also a double planet (and is often described as such). Of course, to add our Moon to the list of planets would simply underline the complete absurdity of making Charon a planet. (By the way, I am aware that, technically, Pluto and Charon revolve around the centre of mass of the Pluto-Charon system, but the same applies to the Earth-Moon system).

posted 08-16-2006 05:58 PM               

quote:

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Originally posted by Blackarrow:
By the way, I am aware that, technically, Pluto and Charon revolve around the centre of mass of the Pluto-Charon system, but the same applies to the Earth-Moon system.

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In regards to your points (1) and (2), both I suppose are related, as the IAU has set a lower size limit, or rather mass, which is dictated by the process of the candidate planet's formation. ("The shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation.")

posted 08-18-2006 07:32 AM               

I propose that we quickly launch a couple space missions to retrieve these plaques before anyone sees them.

Perhaps we could just launch a can of spray paint to cover the plaque over. Or maybe we could change Voyager's tragectory so that it appears to be coming from another solar system.

posted 08-18-2006 03:09 PM               

The proposal adds to the IAU proposed-definition of a planet by suggesting that for a world to be a planet it must be "by far the largest body in its population of bodies". This would disqualify Pluto, Charon and Ceres from full-fledged planethood.

posted 08-19-2006 12:11 AM            

Culturally, we have nine planets. Pluto, culturally, is a planet. Scientifically, it's a KBO or a Trans-Neptunian object or whatever.