The IAU draft definition of "planet" and "plutons"
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html

************ EMBARGOED UNTIL 16 August 2006, 8:00 CEST ************

The IAU draft definition of “planet” and “plutons”

16-August-2006, Prague 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.

Notes for editors
More detailed information about the implications of the draft Resolution can be found in the other documents accompanying this press release (at http://www.iau2006.org/):

• “Questions and Answers about the Planet Definition”
• Composition of the Planet Definition Committee and bios
• Miscellaneous high-res graphics illustrating the 12 planets and current planet candidates are also available.
• Three articles for the General Assembly newspaper written by Ron Ekers, Owen Gingerich and Roberts Williams

The IAU is the international astronomical organisation that brings together distinguished astronomers from all nations of the world. IAU’s mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. Founded in 1919, the IAU is the world’s largest professional body for astronomers. The IAU General Assembly is held every three years and is one of the largest and most diverse meetings in the astronomical community’s calendar.

Science contacts (the persons below can all be reached via the IAU GA Press Office, see bottom)
Owen Gingerich
IAU Planet Definition Committee Chair
Tel: via the Press Room +420-261-177-075

Iwan Williams
President, IAU Division III Planetary Systems Sciences
Tel: via the Press Room +420-261-177-075

Ron Ekers
IAU President
Tel: via the Press Room +420-261-177-075

Catherine Cesarsky
IAU President-Elect and member of the Planet Definition Committee
Tel: via the Press Room +420-261-177-075

Richard Binzel
Member of the Planet Definition Committee
Tel: via the Press Room +420-261-177-075

Dava Sobel
Author and historian, member of the Planet Definition Committee
via the IAU Press Office
Tel: via the Press Room +420-261-177-075

Mike Brown
Discoverer of 2003 UB313 and a host of other planet candidates
Via Caltech Media Relations
Tel: +1-626-395-3226
E-mail: mbrown@caltech.edu

PIO contact
Lars Lindberg Christensen
IAU Press Officer
IAU GA 2006 Press office, Meeting Room 3.2
Prague Congress Center
Tel: +420-261-177-075
Cellular: +49-173-3872-621
E-mail: lars@eso.org

Links
Additional material in this release package may be found in: http://www.iau2006.org/
• Draft Resolution
• Question and Answer sheet
• General Assembly Newspaper articles
• Committee composition

The IAU Web page: http://www.iau.org

IAU News during the 2006 General Assembly: http://www.iau2006.org

IAU General Assembly: http://www.astronomy2006.com

Free registration for the media: http://www.astronomy2006.com/media-accreditation.php

Images of the Pluto system: http://hubblesite.org/gallery/album/search.php?method=and&format=normal&sort=score&config=picturealbum&restrict=entire_collection%2Fpr&exclude=&words=pluto&Submit=Search+site&page=1

Image of 2003 UB313:
http://hubblesite.org/gallery/album/search.php?method=and&format=normal&sort=score&config=picturealbum&restrict=entire_collection%2Fpr&exclude=&words=ub313&Submit=Search+site

Image of Ceres:
http://hubblesite.org/gallery/album/search.php?method=and&format=normal&sort=score&config=picturealbum&restrict=entire_collection%2Fpr&exclude=&words=ceres&Submit=Search+site

Images

iau0601a: The new Solar System? [artist’s impression]
The world’s astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the lower end of the planet scale – what defines the difference between “planets” and “solar system bodies”. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, our Solar System will consist of 12 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313. The three new proposed planets are Ceres, Charon (Pluto’s companion) and 2003 UB313. There is no change in the planetary status of Pluto.

In this artist’s impression the planets are drawn to scale, but without correct relative distances.

Credit: The International Astronomical Union/Martin Kornmesser

iau0601b: Three new planets? [artist’s impression]
The world’s astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the lower end of the planet scale – what defines the difference between “planets” and “solar system bodies”. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, three of the bodies in the Solar System will be assigned new status as planets: Ceres, Charon (Pluto’s companion) and 2003 UB313. There is no change in the planetary status of Pluto.

In this artist’s impression the planets are drawn to scale, but without correct relative distances.

Credit: The International Astronomical Union/Martin Kornmesser

iau0601c: Planet candidates in the Solar System [artist’s impression]
The world’s astronomers, under the auspices of the International Astronomical Union (IAU), have concluded two years of work defining the lower end of the planet scale – what defines the difference between “planets” and “solar system bodies”. If the definition is approved by the astronomers gathered 14-25 August 2006 at the IAU General Assembly in Prague, our Solar System will consist of 12 planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313.

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. A number of these planet candidates are shown here.

In this artist’s impression the planet candidates are drawn to scale, but without correct relative distances.

Credit: The International Astronomical Union/Martin Kornmesser

iau0601d: The new Solar System? [unannotated]
As iau0601a but without annotations.

Credit: The International Astronomical Union/Martin Kornmesser

iau0601e:
As iau0601b but without annotations.

Credit: The International Astronomical Union/Martin Kornmesser

iau0601f:
As iau0601c but without annotations.

Credit: The International Astronomical Union/Martin Kornmesser

Draft Resolution 5 for GA-XXVI: Definition of a Planet
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html

Draft Resolution 5 for GA-XXVI: Definition of a Planet

Contemporary observations are changing our understanding of the Solar System, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries force us to create a new definition, which we can make using currently available scientific information. (Here we are not concerned with the upper boundary between “planet” and “star”.)

The IAU therefore resolves that planets and other Solar System bodies be defined in the following way:

(1) 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) shape1, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.2

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”3

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.

(4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.4

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1 This generally applies to objects with mass above 5 x 1020 kg and diameter greater than 800 km. An IAU process will be established to evaluate planet candidates near this boundary.

2 For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are “satellites”. Under this definition, Pluto’s companion Charon is a planet, making Pluto-Charon a double planet.

3 If Pallas, Vesta, and/or Hygeia are found to be in hydrostatic equilibrium, they are also planets, and may be referred to as “dwarf planets”.

4 This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets. In the new nomenclature the concept “minor planet” is not used.

Planet Definition Questions & Answers Sheet
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_Q_A.html

“Planet Definition” Questions & Answers Sheet

The following Question and Answer sheet may help to interpret the “IAU Resolution 5 for GA-XXVI” provided this is passed during the 2nd Session of the 2006 IAU General Assembly.

Q: What is the origin of the word “planet”?
A: The word “planet” comes from the Greek word for “wanderer”, meaning that planets were originally defined as objects that moved in the sky with respect to the background of fixed stars.

Q: Why is there a need for a new definition for the word “planet”?
A: 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. (Noting that historically Pluto has been recognized as “the ninth planet.”) Thus these discoveries have rightfully called into question whether or not they should be considered as new “planets.”

Q: How did astronomers reach a consensus for a new definition of “planet”?
A: The world’s astronomers, under the auspices of the International Astronomical Union, have had official deliberations on a new definition for the word “planet” for nearly two years. The results of these deliberations were channelled to a Planet Definition Committee comprising 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.”

Q: What new terms are proposed as official IAU definitions?
A: There are two new terms being proposed for use as official definitions of the IAU. The terms are: “planet” and “pluton”.

Q: What is the proposed new definition of “planet”?
A: The new definition of “planet” recognizes and utilizes the capabilities of modern science. The new definition is based on the principle that we no longer need to rely on the simple definition from past millennia that an object is a “planet” if it moves against the background of fixed stars. Instead we can utilize our modern ability to measure the physical properties of an object to determine its true nature. An object is thus defined as a planet based on its intrinsic physical nature. 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.

Q: What is the exact wording of the official IAU proposed definition of “planet” in “Resolution 5 for GA-XXVI”?
A: “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.”

Q: Does an object have to be in orbit around a star in order to be called a “planet”?
A: Yes.

Q: Does a body have to be perfectly spherical to be called a “planet”?
A: No. For example, the rotation of a body can slightly distort the shape so that it is not perfectly spherical. Earth, for example, has a slightly greater diameter measured at the equator than measured from pole to pole.

Q: Based on this new definition, how many planets are there in our Solar System?
A: There are currently 12. Eight are the classical planets Mercury through Neptune. Three are in a newly defined (and growing in number) category called “plutons”, for which Pluto is the prototype. One is Ceres, which may be described as a dwarf planet.

Q: What are the 12 planets?
A: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313 (provisional name).

Q: How are these 12 planets categorized?
A: There are eight “classical planets”: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Ceres is a planet, but because it is smaller than Mercury, one may describe it as a “dwarf planet”. A new category of planet is now defined: “plutons”. Pluto, Charon, and 2003 UB313 fall into the growing category of planets called “plutons”.

Q: What are the classical planets?
A: 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 are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Note that the term “classical planet” is only a historical reference and not an IAU definition.

Q: What is a dwarf planet?
A: 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.

Q: What is a “pluton”?
A: A pluton is a new category of planet now being defined by the IAU. A "pluton" is an object satisfying the technical (hydrostatic equilibrium shape in the presence of self-gravity) definition of "planet." 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.

Q: Where does the name “pluton” come from?
A: The name “pluton” comes from Pluto itself. Pluto is the first object discovered that is a large spherical body, and therefore a planet, orbiting beyond Neptune.

Q: Is a “pluton” a planet?
A: Yes.

Q: Is Pluto a “pluton”?
A: Yes.

Q: Is Pluto a planet?
A: Yes. In fact, Pluto’s large companion named Charon is also large enough and massive enough to satisfy the definition of “planet”. Because Pluto and Charon are gravitationally bound together, they are actually now considered to be a “double planet.”

Q: Is Pluto a dwarf planet?
A: If one describes any planet smaller than Mercury as a “dwarf planet”, then Pluto could be called a dwarf planet. Note that in terms of an IAU definition, Pluto is a “pluton.” “Dwarf planet” is simply a descriptive category like “terrestrial planet” and “giant planet.” None of these descriptive categories are formally defined by the IAU.

Q: What is a “double planet”?
A: 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.

Q: What is a “satellite” of a planet?
A: For a body that is large enough (massive enough) to satisfy the definition of “planet”, an object in orbit around the planet is called a “satellite” of the planet if the point that represents their common centre of gravity (called the “barycentre”) is located inside the surface of the planet.

Q: The Earth’s moon is spherical. Is the Moon now eligible to be called a “planet”?
A: No. The Moon is a satellite of the Earth. The reason the Moon is called a “satellite” instead of a “planet” is because the common centre of gravity between the Earth and Moon (called the “barycentre”) resides below the surface of the Earth.

Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called planets?
A: No. All of the large satellites of Jupiter (for example, Europa) and Saturn (for example, Titan) orbit around a common centre of gravity (called the “barycentre”) that is deep inside of their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is what defines large orbiting bodies such as Europa, Titan, etc. to be “satellites” rather than planets.

Q: Why is Pluto-Charon a “double planet” and not a “planet with a satellite”?
A: Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of “planet”. The reason they are called a “double planet” is that their common centre of gravity is a point that is located in free space outside the surface of Pluto. Because both conditions are met: each body is “planet-like” and each body orbits around a point in free space that is not inside one of them, the system qualifies to be called a “double planet.”

Q: Pluto has at least two recently discovered additional satellites that are smaller than Charon. If these smaller satellites also orbit the “barycentre”, does this make Pluto a “quadruple planet”?
A: No. The two newly discovered smaller bodies in orbit around Pluto are too small and not massive enough for their self-gravity to force them in to a spherical shape. Therefore neither of these bodies independently satisfies the definition of “planet.” The fact that their size (mass) and shapes does not qualify them as planets implies that they must be called satellites, even though the center of gravity (called the “barycentre”) about which they orbit is located outside the surface of Pluto.

Q: Can there be triple planets or quadruple planets?
A: Yes, but none are currently known.

Q: Is Ceres a planet?
A: Yes.

Q: Didn’t Ceres used to be called an asteroid or minor planet?
A: Historically, Ceres was called a “planet” when it was first discovered (in 1801) orbiting in what is known as the asteroid belt between Mars and Jupiter. Because 19th century astronomers could not resolve the size and shape of Ceres, and because numerous other bodies were discovered in the same region, Ceres lost its planetary status. For more than a century, Ceres has been referred to as an asteroid or minor planet.

Q: Why is Ceres now being called a “planet”?
A: An object in orbit around a star is now being called a “planet” if it is large enough (or more technically, massive enough) for its own gravity to pull it into a nearly spherical shape. Recent Hubble Space Telescope images that resolve the size and shape of Ceres show it to be nearly spherical. More technically, Ceres is found to have a shape that is in a state of hydrostatic equilibrium under self-gravity. Therefore Ceres is a planet because it satisfies the IAU definition of “planet.” [Published reference for shape of Ceres: P. Thomas et al. (2005), Nature 437, 224-227. Dr. Peter Thomas is at Cornell University.]

Q: Is Ceres a dwarf planet?
A: If one describes any planet smaller than Mercury as a “dwarf planet”, then Ceres could be called a dwarf planet. Note that “dwarf planet” is simply a descriptive category like “terrestrial planet” and “giant planet.” None of these descriptive categories are formally defined by the IAU.

Q: Is Ceres a “pluton”?
A: No.

Q: What is 2003 UB313?
A: “2003 UB313” is a provisional name given to a large object discovered in 2003 that resides in an orbit around the Sun beyond Neptune.

Q: Is 2003 UB313 a planet?
A: Yes.

Q: Why is 2003 UB313 a planet?
A: Recent Hubble Space Telescope images have resolved the size of 2003 UB313 showing it to be as large as, or larger than Pluto. Any object having this size, and any reasonable estimate of density, is understood to have sufficient mass that its own gravity will pull it into a nearly spherical shape determined by hydrostatic equilibrium. Therefore, 2003 UB313 is a planet because it satisfies the IAU definition of “planet.” [Published reference: M. Brown et al. (2006). Astrophysical Journal 643, L61-L63. Dr. Michael Brown is at the California Institute of Technology.]

Q: Will the new planet 2003 UB313 receive a name? When?
A: Yes. The International Astronomical Union has the official authority to assign names to objects in space. This object has been popularly called “Xena”, but this is not an official IAU name. A decision and announcement of the new name are likely not to be made during the IAU General Assembly in Prague, but at a later time.

Q: Has the IAU ever named a planet?
A: No, so far not.

Q: Is 2003 UB313 a “pluton”?
A: Yes.

Q: Is 2003 UB313 a dwarf planet?
A: If one describes any planet smaller than Mercury as a “dwarf planet”, then 2003 UB313 could be called a dwarf planet. Note that “dwarf planet” is simply a descriptive category like “terrestrial planet” and “giant planet.” None of these descriptive categories are formally defined by the IAU.

Q: What is an object called that is too small to be a “planet”?
A: All objects that orbit the Sun, which are too small (not massive enough) for their own gravity to pull them into a nearly spherical shape are now collectively referred to as “small Solar System bodies.” 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.

Q: Is the term “minor planet” still to be used?
A: No. The term “minor planet” is no longer to be used for official IAU purposes. The term will be replaced by “small Solar System bodies.”

Q: Why is the term “minor planet” being replaced by “small Solar System bodies”?
A: Under the new definition of “planet”, nearly all objects currently called “minor planets” are not planets. For IAU purposes, a definition and name is needed that clearly distinguishes between objects that are officially recognized as planets and those that are not.

Q: For any newly discovered object, how will a decision be reached on whether or not to officially call it a “planet.”
A: The decision on whether or not an object is officially a “planet” will be made by a review committee within the IAU. The review process will be an evaluation, based on the best available data, of whether or not the physical properties of the object satisfy the definition of “planet.” It is likely that for many objects, a period of time of several years may be required in order for sufficient data to be gathered.

Q: Are there additional “planet” candidates currently being considered?
A: Yes.

Q: Does this mean there will be more than 12 planets in our Solar System?
A: Almost certainly yes.

Q: When will additional new planets likely be announced?
A: When the responsible committee has had time to work on the issues after the resolution has been voted upon. Most likely any further new planet announcements will not be made until after the General Assembly in Prague.

Q: How many more new planets are there likely to be?
A: Perhaps as many as a dozen or two new planets in the IAU category called “plutons” remain to be discovered. We estimate this number based on our understanding of the current discovery statistics. This understanding is subject to change as new data continue to be gathered.

Q: When is an object too large to be called a “planet”?
A: The new definitions proposed by the IAU seek only to define the lower boundary between an object that is a “planet” or a “small Solar System body.” At this time there is no official IAU definition in place or proposed that defines the upper limit for when an object is, for example a “planet” or a “brown dwarf.” This limit is generally thought to be about 13 times more massive than Jupiter, but is subject to discussion.

Q: Is the new definition for “planet” intended to apply also to objects discovered in orbit around other stars?
A: Yes.

Q: Are objects that have planetary sizes and masses, but which are free floating in space (and not orbit a star) officially “planets” by the IAU definition?
A: No. At this time there is no official IAU definition in place that addresses this class of objects.

Q: Is a “pluton” a dwarf planet?
A: If one describes any planet smaller than Mercury as a "dwarf planet", then any pluton smaller than Mercury could be called a dwarf planet. Note that "dwarf planet" is simply a descriptive category like "terrestrial planet" and "giant planet." None of these descriptive categories are formally defined by the IAU.

Q: What is the difference between a “pluton” and a “plutino”?
A: A “plutino” is a small body that has a similar orbit to Pluto, that is, it has the same 248 year orbital period as Pluto. Plutino is not an official IAU description. All currently known plutinos are too small to be planets. Plutinos are not plutons.

Q: Is Pluto the ninth planet?
A: Historically it was indeed the ninth planet to be discovered, but now Pluto is also known as the first pluton, with its moon Charon being the second pluton. 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).

IAU Planet Definition Committee
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_newspaper.html

IAU Planet Definition Committee
By Ron Ekers, President of the IAU

The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919. The various IAU Working Groups normally handle this process, and their decisions primarily affect the professional astronomers. But from time to time the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.
The boundary between (major) planet and minor planet has never been defined and the recent discovery of other “Trans-Neptunian Objects” (TNOs), including some larger than Pluto, triggered the IAU to form a Working Group on “Definition of a Planet” from its Division III members. While there was general agreement on all the scientific issues related to Solar System dynamics and physical properties of planets, the IAU Division III Working Group could not agree on aspects that were related to social and cultural issues, such as the status of Pluto. In order to include these broader aspects, the IAU Executive Committee (EC) formed a new committee whose membership had backgrounds in history, science publishing, writing and education as well as in planetary science.

Terms of Reference
The Planet Definition Committee of the IAU Executive Committee was charged with:
(i) discussing the broader social implications of any new definition of a planet and recommending a course of action that balances the scientific facts with the need for social acceptance of any change;
(ii) addressing the status of Pluto, and of the newly discovered TNOs in the light of recommendation (i);
(iii) considering whether the current naming procedures for planets and minor planets have exacerbated the problem of defining a planet and recommending whether revisions are needed; and
(iv) attempting to frame these recommendations as a resolution, or resolutions, that could be put before the Prague GA in August 2006 for possible adoption.

The Path to Defining Planets
By Owen Gingerich, Harvard-Smithsonian Center for Astrophysics/IAU EC “Planet Definition” Committee chair

Celestial nomenclature has long been fraught with controversy. Galileo proposed to name the large satellites of Jupiter the “Medicean planets”; William Herschel named his new planet after the English monarch, George III; Hevelius honoured the defender of Vienna with “Scutum Sobieski”; and Bode named a northern constellation after the comet hunter Charles Messier. None of these appellations have stood the test of time except for the fragment “Scutum”.

At its inaugural meeting in 1922, the IAU standardized the constellation names and abbreviations. More recently IAU Committees or Working Groups have certified the names of asteroids, satellites, and planetary and satellite features. Until now, however, the IAU has never named a planet, and it has been unclear whether there are potential planets to be named.

How, in fact, should the word “planet” be defined?

This was the controversial question facing the committee established by the IAU Executive Committee with the charge to recommend a definition for an IAU resolution. The seven members represented a spectrum of opinion and expertise. We all knew that modern scientific advances have taught us that the Solar System is a far more complicated place than William Herschel and his contemporaries ever imagined, not only containing an assortment of planets, asteroids, and comets, but rocks, gravel, dust, and ions. We met in Paris for a vigorous discussion 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.

On the scientific side, we wanted to avoid arbitrary cut-offs simply based on distances, periods, magnitudes, or neighbouring objects. One physical criterion seemed pre-eminent: was the object shaped by hydrostatic equilibrium, that is, was it basically a round object? This criterion became the basis of our proposed definition. Objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be considered to be in hydrostatic equilibrium, but borderline cases would have to be established by observation. Even among these round Solar System objects there is a distinct difference between the major planets, whose orbits lie near the ecliptic plane, and those smaller objects with more eccentric, tilted orbits. Had astronomers realized in 1930 that Pluto was smaller than our Moon and with a mass well under 1% that of the Earth, perhaps some special designation would have been devised for it. On the cultural/historical side, combined with contemporary science, our committee felt that the time was ripe to recognize Pluto as the prototype of a different sort of planet. Consequently, we propose to distinguish between the eight classical planets discovered before 1900, and a new class of Trans-Neptunian Objects, for which we recommend the name “plutons.”

The question immediately arises about the status of Pluto. Although Pluto remains a planet by the proposed definition, it will generally be
preferable to call it a pluton to emphasise its role as the prototype for a physically distinct category of planetary bodies.
Specialists will at once recall that there are over a hundred so-called “plutinos,” Trans-Neptunian lumps of rock, ice, and snow, each with the period 248 years (thus in a 3/2 resonance with the period of Neptune). These faint objects are in general not plutons. Plutons are at present very rare objects: Pluto, Charon, 2003 UB313, and perhaps several more, and anyone who finds a new pluton should be appropriately celebrated.

Savvy astronomers will notice that our definition also makes Ceres a planet, and if Pallas, Vesta, and Hygeia are found to be in hydrostatic equilibrium, they will also have to be considered planets. Without making a formal definition, we suggest that it might be convenient simply to refer to these small round members of our inner Solar System as “dwarf planets.”

Did our committee think of everything, including extra-solar system planets? Definitely not! Science is an active enterprise, constantly bringing new surprises. Undoubtedly some future IAU committee will have to revisit this question and define the upper limit for "planet", probably well before 2106!

The Process of making a Resolution on the Definition of a Planet

By Robert Williams, Space Telescope Science Institute, Vice President of the IAU

Statements of scientific importance are expressed by the IAU in resolutions of the General Assembly. Although resolutions are non-binding they do represent the consensus scientific judgment of the members, and are arrived at by a process that involves member input and debate. As explained in the accompanying articles the question of the definition of a planet is of great interest within the Union and among the public, and Division III and the Executive Committee are attempting to set forth criteria that define planets and provide for a nomenclature for the different Solar System objects.

A Working Group under Division III was established to formulate a recommendation on the definition of a planet that could be put before the Executive Committee. Although that Working Group did not achieve a clear consensus, it did succeed in defining the important criteria and framing the discussion of issues to be considered. The EC studied the Division III Working Group report and decided to form its own advisory group, the Planet Definition Committee, to attempt to resolve the issue in a manner that had a solid scientific basis and which might achieve consensus support among members of the Union. Prof. Gingerich has described the work of the Planet Definition Committee, whose report has been received by the EC and used as a basis for framing the draft resolution that is now being put before the General Assembly. The current draft of the resolution “The Definition of a Planet” that has been approved by the EC and the Resolutions Committee appears with these articles.

The process by which resolutions are considered by the IAU is set forth in the Working Rules. It involves consideration by the Resolutions Committee and the Executive Committee, and discussion by the General Assembly before a vote taken in the second business meeting of the GA. Because of the potential impact of this resolution the EC is undertaking extra measures to assure full discussion of the draft during the General Assembly that will allow for possible revisions to the current version before it is presented to the GA at the closing business meeting. They include a discussion and debate of the resolution by Division III-Planetary Sciences at its scheduled meeting this Friday, 18 August. In addition, the EC is convening an extraordinary plenary session of the General Assembly to take place next Tuesday, 22 August, during the lunch break, which will be devoted entirely to a discussion of the draft resolution, and after which a “sense of the meeting” vote will be taken on the resolution as presented. We are fully aware of the potential difficulty in achieving a consensus on this complex issue, and we wish to provide ample opportunity for input from members in the formulation of the final resolution to be considered next week.

The key events that bear on the substance of the final resolution to be presented at the closing business meeting, and in which all IAU members are encouraged to participate, are (1) the discussion at the meeting of Division III on Friday, 18 August at 11:00 am in Club B, and (2) the Plenary Session on the Definition of a Planet on Tuesday, 22 August at 12:45 pm in Forum Hall. The Closing Session of the GA will be held Thursday 24 August at 14:00 in the Congress Hall and here the final resolution will be presented, discussed, and voted upon.

The EC reiterates our desire to benefit from members’ input into this issue by your participation in these events, which are an important part of the IAU’s mission to communicate the discoveries of astronomy to the public.

Members of the Planet Definition Committee
http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_committee.html

INTERNATIONAL ASTRONOMICAL UNION
UNION ASTRONOMIQUE INTERNATIONALE

Members of the Planet Definition Committee

Dr. Richard Binzel is Professor of Earth, Atmospheric and Planetary Science at MIT and a specialist in asteroids and outer solar system small bodies, and is also a well known and respected educator and science writer.

Dr. André Brahic is Professor at Université Denis Diderot (Paris VII) and is Director of the Laboratory Gamma-gravitation of the Commissariat a l’Energie Atomique. He specializes in planetary rings, and has co-discovered the rings and arcs of Neptune. For the French-speaking public, André Brahic is one of the best known popularisers of science and astronomy, having authored a number of books.

Dr. Owen Gingerich [chair], Professor of Astronomy and History of Science Emeritus at the Harvard-Smithsonian Center for Astrophysics, is an esteemed historian of astronomy with a broad perspective, and a prize-winning educator.

Dava Sobel is the author of the very successful books Longitude, The Planets, and Galileo's Daughter. She has a solid background in, and knowledge of, the history of science, astronomy in particular.

Dr. Junichi Watanabe is an Associate Professor and also Director of the Outreach Division of NAOJ. He is a solar system astronomer and highly appreciated in Japan as interpreter and writer of astronomy for the public and students. He has strong connections with amateur astronomers, science editors, school teachers and journalists.

Dr. Iwan Williams, Queen Mary University of Londo, is an expert on the dynamics and physical properties of Solar System objects. He is the current President of IAU Division III (Planetary Systems Sciences).

Dr Catherine Cesarsky, Director General of ESO and President-Elect of the IAU, took part in the work of the committee, bringing in the perspective of the IAU Executive as well as that of an astronomer at large.

The Final IAU Resolution on the definition of "planet" ready for voting
http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html

24. August 2006, Prague

At the second session of the 2006 International Astronomical Union (IAU) General Assembly, which will be held 14:00 Thursday 24 August, members of the IAU will vote on the Resolutions presented below. There will be separate sequential votes on Resolution 5A and Resolution 5B. Similarly, there will be separate votes on Resolutions 6A and 6B.

Following active discussion among IAU scientists at the IAU 2006 General Assembly in Prague, draft Resolution 6b (issued 16 August 2006) has been updated and amended.

IAU President Ron Ekers says: "IAU's rules for proposing resolutions are based on an open democratic process and it is a great pleasure for the IAU Executive Committee to see the level of engagement of so many astronomers here. We want to engage as broad a part of the IAU community as possible in the decision-making process to give this Resolution the best chance to be passed."

Below are the full texts of "IAU Resolution 5a for GA-XXVI", "IAU Resolution 5b for GA-XXVI" and "IAU Resolution 6a for GA-XXVI" and "IAU Resolution 6b for GA-XXVI". The voting will take place in four steps.

The voting on these Resolutions is expected to end today (Thursday 24 August) between 15:30 and 16:00 CEST. This is a rough estimate.

According to the revised Statutes approved at the First Session of the General Assembly last week, scientific issues such as Resolutions are decided by majority of those IAU members present and voting at the business meeting. Thus the scientific resolutions, including those on the definition of solar system bodies, will be presented and decided by voting of the individual members. Yellow ballots will be handed out to all IAU members at the entrance. Members will vote by raising these ballots in the air; the number of raised ballots will be counted. The result of the vote should be known shortly thereafter and will be communicated in a public statement.

Notes for editors
A press conference about the Closing Ceremony of the General Assembly, including the results of the planet-definition vote, will be held at 18:00, in Meeting Room 3.3 of the Prague Congress Center. (It will NOT be possible for journalists to ring in to this conference: they must be there in person.)

The panel for the press conference will be:

Ron Ekers (outgoing IAU President)
Catherine Cesarsky (incoming IAU President, Member of the Planet Definition Committee)
Jan Palous (Chair of the National Organising Committee)
Richard Binzel (Member of the Planet Definition Committee)
Karel van der Hucht (incoming Secretary General)

This press conference will conclude around 18:30 CEST.

The IAU is the international astronomical organisation that brings together distinguished astronomers from all nations of the world. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. Founded in 1919, the IAU is the world's largest professional body for astronomers. The IAU General Assembly is held every three years and is one of the largest and most diverse meetings on the astronomical community's calendar.

Contacts
Following the vote, some of the members of the planet definition committee will be available for interviews (after the final vote):

Richard Binzel
Member of the Planet Definition Committee
Prague Conference Center, Meeting Room 3.1
Tel: +420-261-177-110
Cell: +420-776-806-297 (during the General Assembly)

Junichi Watanabe
Member of the Planet Definition Committee
Prague Conference Center, Meeting Room 3.3
Tel: +420-261-177-081
Cell: +420-776-806-265 (during the General Assembly)

Iwan Williams
President, IAU Division III Planetary Systems Sciences
Prague Conference Center, Meeting Room 244
Tel: +420-261-177-064
Cell: +420-776-175-769 (during the General Assembly)

Owen Gingerich
Chair of the IAU Planet Definition Committee
Tel: via the Press Room +420-261-177-075

Professor Ron Ekers
IAU President
Tel: via the Press Room +420-261-177-075

Catherine Cesarsky
IAU President-Elect and member of the Planet Definition Committee
Tel: via the Press Room +420-261-177-075

PIO contact
Lars Lindberg Christensen
IAU Press Officer
IAU GA 2006 Press office, Meeting Room 3.2
Prague Congress Center
Tel: +420-261-177-075/+420-261-222-130
Cellular: +49-173-3872-621
E-mail: lars@eso.org

(Inicio)
Links
Programme for the Closing Ceremony: http://www.astronomy2006.com/second-session-and-closing-ceremony.php
Live public webcast of the Closing Ceremony: http://astronomy2006.com/tv/
The IAU Web page: http://www.iau.org
IAU News during the 2006 General Assembly: http://www.iau2006.org
IAU General Assembly: http://www.astronomy2006.com
Free registration for the media: http://www.astronomy2006.com/media-accreditation.php

RESOLUTIONS
Resolution 5A is the principal definition for the IAU usage of "planet" and related terms. Resolution 5B adds the word "classical" to the collective name of the eight planets Mercury through Neptune.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. Resolution 6B introduces the name "plutonian objects" for this class. The Merriam-Webster dictionary defines "plutonian" as:
Main Entry: plu • to • ni • an
Pronunciation: plü-'tO-nE-&n
Function: adjective
Usage: often capitalized
: of, relating to, or characteristic of Pluto or the lower world

After having received inputs from many sides -- especially the geological community -- the term "Pluton" is no longer being considered.

IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A planet 1 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 neighbourhood 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 2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

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

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1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

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RESOLUTION 5B
Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading:

(1) A classical planet 1 is a celestial body . . .

and

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1 The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

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IAU Resolution: Pluto

RESOLUTION 6A
The IAU further resolves:

Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

RESOLUTION 6B
The following sentence is added to Resolution 6A:

This category is to be called "plutonian objects."

IAU 2006 General Assembly: Result of the IAU Resolution votes
http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html

24. August 2006, Prague

The first half of the Closing Ceremony of the 2006 International Astronomical Union (IAU) General Assembly has just concluded. The results of the Resolution votes are outlined here.

It is official: The 26th General Assembly for the International Astronomical Union was an astounding success! More than 2500 astronomers participated in six Symposia, 17 Joint Discussions, seven Special Sessions and four Special Sessions. New science results were vigorously discussed, new international collaborations were initiated, plans for future facilities put forward and much more.

In addition to all the exciting astronomy discussed at the General Assembly, six IAU Resolutions were also passed at the Closing Ceremony of the General Assembly:

Resolution 1 for GA-XXVI : "Precession Theory and Definition of the Ecliptic"
Resolution 2 for GA-XXVI: "Supplement to the IAU 2000 Resolutions on reference systems"
Resolution 3 for GA-XXVI: "Re-definition of Barycentric Dynamical Time, TDB"
Resolution 4 for GA-XXVI: "Endorsement of the Washington Charter for Communicating Astronomy with the Public"
Resolution 5A: "Definition of 'planet' "
Resolution 6A: "Definition of Pluto-class objects"

The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as 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 neighbourhood around its orbit.

This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name). More "dwarf planets" are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The "dwarf planet" Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects.

Below are the planet definition Resolutions that were passed.

Notes for editors
A press conference about the Closing Ceremony of the General Assembly, including the results of the planet-definition vote, will be held at 18:00, in Meeting Room 3.3 of the Prague Congress Center. (It will NOT be possible for journalists to ring in to this conference: they must be there in person.)

The panel for the press conference will be:

Ron Ekers (outgoing IAU President)
Catherine Cesarsky (incoming IAU President, Member of the Planet Definition Committee)
Jan Palous (Chair of the National Organising Committee)
Richard Binzel (Member of the Planet Definition Committee)
Karel van der Hucht (incoming Secretary General)

This press conference will conclude around 18:30 CEST.

The IAU is the international astronomical organisation that brings together distinguished astronomers from all nations of the world. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. Founded in 1919, the IAU is the world's largest professional body for astronomers. The IAU General Assembly is held every three years and is one of the largest and most diverse meetings on the astronomical community's calendar.

Contacts
Following the vote, some of the members of the planet definition committee will be available for interviews (after the final vote):

Richard Binzel
Member of the Planet Definition Committee
Prague Conference Center, Meeting Room 3.1
Tel: +420-261-177-110
Cell: +420-776-806-297 (during the General Assembly)

Junichi Watanabe
Member of the Planet Definition Committee
Prague Conference Center, Meeting Room 3.3
Tel: +420-261-177-081
Cell: +420-776-806-265 (during the General Assembly)

Iwan Williams
President, IAU Division III Planetary Systems Sciences
Prague Conference Center, Meeting Room 244
Tel: +420-261-177-064
Cell: +420-776-157-769 (during the General Assembly)

Owen Gingerich
Chair of the IAU Planet Definition Committee
Tel: via the Press Room +420-261-177-075

Professor Ron Ekers
IAU President
Tel: via the Press Room +420-261-177-075

Catherine Cesarsky
IAU President-Elect and member of the Planet Definition Committee
Tel: via the Press Room +420-261-177-075

PIO contact
Lars Lindberg Christensen
IAU Press Officer
IAU GA 2006 Press office, Meeting Room 3.2
Prague Congress Center
Tel: +420-261-177-075/+420-261-222-130
Cellular: +49-173-3872-621
E-mail: lars@eso.org

(Inicio)
Links
Programme for the Closing Ceremony: http://www.astronomy2006.com/second-session-and-closing-ceremony.php
Live public webcast of the Closing Ceremony: http://astronomy2006.com/tv/
Live press webcast of the Closing Ceremony (press only, please do not distribute): http://www.astronomy2006.com/tv-press
The IAU Web page: http://www.iau.org
IAU News during the 2006 General Assembly: http://www.iau2006.org
IAU General Assembly: http://www.astronomy2006.com
Free registration for the media: http://www.astronomy2006.com/media-accreditation.php

RESOLUTIONS
Resolution 5A is the principal definition for the IAU usage of "planet" and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.

IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

RESOLUTION 5A
The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A "planet" 1 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 neighbourhood 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 2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

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

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1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2 An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3 These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

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IAU Resolution: Pluto

RESOLUTION 6A
The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.1