The New Planets
New planet Xena is about the size of Pluto and further out in the Kuniper Belt.
Pluto's largest moon, Charon, will now be classified as a double planet with Pluto.
Ceres, the third, is between Mars and Jupiter. Ceres was classified as a planet in 1801 when it was discovered, but later had its status revokes when the asteroid belt between Mars and Jupiter was discovered.
The International Astronomical Union will put the matter to a vote of its 2,500 members on August 24, 2006.
Proposed Planet Definition
Under the proposal, anything that is round by virtue of its mass (something that happens only for large objects in nature because substantial gravity is required to do so) and orbits a star rather than another planet, is a planet, unless it is a star. According to the IAU:
The shape of objects with mass above 5 x 10^20 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.
Star status starts with brown dwarft stars which are believed to have a cutoff with bodies "about 13 times more massive than Jupiter." Jupiter itself has a mass of about 1,321 times that of Earth and a mean radius of about 43,441 miles. The Sun has a mass of about 333,000 times the mass of Earth and a mean radius of about 432,500 miles. Thus, planets generally, by virtue of the laws of physics, have radii of very roughly between 200 and 200,000 miles. Gravity won't make smaller ones round, and gravity will make larger ones go nuclear.
This is a sound definition.
The other way the committee could have gone would have been to allow satellites which otherwise meet the definition to qualify as planets. But, this would greatly increase the number of planets and would fail to recognize the nature derived structure of the solar system in which big planets have satellites which they dominate.
Classes of Planets
Planets would be themselves divided into three classes, the big eight called "classical planets," plutons, and dwarf planets like Ceres. There might be 50 to 100+ planets in the solar system by the new definition.
According to the IAU:
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).
The IAU believes that:
Perhaps as many as a dozen or two new planets in the IAU category called “plutons" remain to be discovered.
This categorization is a valid one, because plutons are, as a class, meaningfully different from other planets, and this arbitrary definition seems to capture that difference, even though a more generalizable definition, rather than one based on an arbitrary number of years might be preferrable.
A double planet would be defined as:
A pair of objects, which each independently satisfy the definition of “planet” are considered a “double planet” if they orbit each other around a common point in space that is technically known as the “barycentre”. In addition, the definition of “double planet” requires that this “barycentre” point must not be located within the interior of either body.
While this decision has the primary effect of elevating Charon from satellite status to planet status, is one of two physically sensible definitions that preserve the distinction between a planet and a satellite of another planet. The only other sensible alternative that would have maintained this distinction, which would not have changed the status quo, would have been to designate the largest planet in any double or multiple planet system the primary planet, and to label all other planets in the system satellites of the largest one.
Also, because it appears that Pluto and Charon may have a quite rare relationship, this is a definition that doesn't seem likely to have a big effect. I don't know what the precedent is in multiple star systems, where an analagous rule should be used.
Small Solar System bodies
The rejects that orbit the sun that fail to meet planet status would be called “small Solar System bodies.” According to the IAU:
This collection includes the category of objects we continue to call asteroids and comets. This collection also currently includes, near-Earth objects (NEOs), Mars- and Jupiter-Trojan asteroids, most Centaurs and most Trans-Neptunian Objects (TNOs). In the new system of IAU definitions, the term “minor planet” is no longer used.
This default category is as good as any. And, the prior useage that called these objects minor planets probably gave too much credit to big rocks floating around the solar system.
The demise of the term minor planets requires the invention of a new term for small planets that won't be confused with the historical useage of the term "minor planet." While not technically defined, dwarf planets would be used as a "descriptive category":
A dwarf planet is a term generally used to describe any planet that is smaller than Mercury. Note that the term “dwarf planet” is simply a descriptive category and not an IAU definition. Terms such as “terrestrial planets” and “giant planets” are additional examples of descriptive categories that are not IAU definitions.
Mercury has a mean radius of 1,516 miles, which makes it larger than the Moon (at 1,079 miles), Pluto at 742 miles, and Xena of similar size. Pluto would be just another dwarf planet under the new rules, but for its special pluton status. Thus, in light of the definition of planet itself, this means that dwarf planets would generally have a radius of 240 to 1,515 miles.
This definition is justified on the ground that:
The classical planets are those recognized by sky watchers and astronomers starting from the beginning of human history until the year 1900 A.D. . . . The classical planets can be numbered by their distance from the Sun, and there is no change in their order. Plutons, on the other hand, may due to their high eccentricity change their relative distances from the Sun with time (and hence their order).
In short, while Pluto is the first and largest of the plutons discovered, the committee felt that it has more in common with other plutons than with the other eight planets.
All of the proposed new candidates for planet status (other than Xena) are smaller than Pluto and hence are dwarf planets (footnotes omitted):
Object -- Unofficial diameter estimate
*2003 EL61 2000×1000×1200 km
*2005 FY9 1500±300 km
*(90377) Sedna 1200-1800 km
*(90482) Orcus 1000±200 km
*(50000) Quaoar ~1000 km
*(20000) Varuna 600 ± 150 km
*(55636) 2002 TX300 <700 km
*(28978) Ixion 500±100 km
*(55565) 2002 AW197 700±100 km
*(4)Vesta 578×560×458 km
*(2) Pallas 570×525×500 km
(10) Hygiea 500×400×350 km
The IAU doesn't break these candidates down into plutons and non-pluton dwarft planets, but comments that identify which belong in which category would be appreciated.
The dwarf planet defintion is the most arbitrary of the lot, which explains its non-official status. It has little scientific basis, yet a lot of practical importance.
Kids, and for that matter, most college students and astronomy hobbiests, will not memorize the names of several dozen to a hundred planets. Similarly, few people will even learn the names of all the plutons. But, everybody will learn the names of the classical planets.
Whether people learn the names of non-pluton dwarf planets probably will depend on how many exist. If Ceres is it, or there are only a couple more, people will probably learn all of them by name. If there are lots of candidate non-pluton dwarf planets out there, people probably won't learn their names.
A "smaller than Mercury" definition excludes plutons Pluto, Charon and Xena, despite current useage that calls Pluto a planet. In contrast, a smaller than Pluto definition would make Xena a tenth classical planet, but would make almost all other prospective planets dwarf planets. But, a Pluto standard for dwarf planet, would mean that Pluto is a non-dwarf planet, while co-double planet Charon is not. And, a Charon standard for a dwarf, non-dwarft planet divide would open the door to quite a few more non-dwarf planets.
The argument for eight classical planets is that they can be detected with the naked eye or only minimal optical equipment and have less ecentric orbits, while Pluto, Charon, Xena and the other plutons cannot be so easily detected and is more irregular in its orbit. On the other hand, if you are going to have an arbitrary classification like dwarf planet and non-dwarf planet that has no real basis in science, there is something to be said for respecting current common useage (i.e. the status quo) in doing so.
One suspects that this dwarf planet undefinition was necessary to please anti-Pluto factions on the committee in question, with the pluton category enshrining the special place of Pluto as the first known pluton as a consolation prize.
If the dwarf planet threshold were sight at Pluto level, in contrast, there would be two classical Plutons, and the remaining plutons would be dwarf plutons.