IAU Division III

Sciences des Systemes Planetaires

Planetary Systems Sciences

The Status of Pluto

Press Release
The IAU has issued a Press Release describing the limited decisions that have been made and reaffirming that Pluto is considered a planet.

Media Attention
Extensive media reporting on this issue, both print and electronic over the last ten days, has varied from excellent to abysmal. Errors in the media include a) errors of fact about what has been decided, b) misidentification of people making proposals, c) inverted chronology of events. The following describes the issues.

This note added 1999 Jan 24, mfa

Why Has a Question Arisen?
For some time it has been recognized that we did not completely understand how to fit Pluto into our picture of the planetary system. Over the last few years, the accumulated information on Pluto and the discovery of an increasing number of other objects in the outer solar system with orbital characteristics very similar to those of Pluto have been discussed within the community of minor-planet researchers. It has become clear that, in many respects (but not all respects), Pluto is more similar to these outer-solar-system objects than it is to the other 8 planets and it may be reasonable to assign to Pluto a number in a consistent cataloging system including the other members of this class. This question has recently come to the forefront because the orbits of some of these objects are now sufficiently well determined that it is reasonable to begin including them in the catalog of orbits of what are now generically known as minor planets. The first of these other objects was discovered in 1992 and there are now (1998/12/30) 84 of these other Trans-Neptunian Objects (TNOs) known. A regularly updated complete list is kept at the Minor Planet Center. The objects are designated there only by their preliminary (upon discovery) designations which follow the standard system of preliminary designations for minor planets. None has yet been assigned a permanent designation (catalog number) or name.

Historical View of Pluto
Pluto has been known as the ninth planet of our solar system since it was discovered by Clyde Tombaugh at Lowell Observatory in 1930. On the other hand, it has been clear for decades that Pluto did not fit in with the pattern of the other planets -

All the other planets in the outer solar system are gaseous, giant planets whereas Pluto is a small solid object,
Pluto is more than a factor 2 smaller than any other planet,
Pluto's orbit is by far the most inclined with respect to the plane of the solar system and also the most eccentric with only the eccentricity of Mercury's orbit being even close,
Pluto's orbit is the only planetary orbit which crosses that of another planet (during 1999 Pluto will again cross Neptune's orbit, thus regaining its status as the most distant planet), and
Pluto's satellite, Charon, is larger in proportion to its planet than any other satellite in the solar system.

These aspects of Pluto that do not fit the planetary pattern have been known for some time and most astronomers have long recognized that Pluto did not fit well into our picture of planetsa and their origin. For example, D. Morrison and T. Owen, in their textbook The Planetary System (first edition, 1988, Addison-Wesley), suggested (p371) "Perhaps Pluto should be considered the largest asteroid rather than the smallest planet." (Pluto has more than twice the diameter of Ceres, the largest asteroid.) They then summarized the outer planets, saying (p375) "Pluto is an anomalous object, resembling an icy satellite more closely than the other planets." This illustrates the widespread realization that there were difficulties understanding the role of Pluto in the solar system and it is how many of us have taught the subject, both to science majors and to non-science majors, since even long before that textbook was published -- although Pluto is generally considered to be a planet it just does not fit properly into the general picture of planets and their formation.

Other Trans-Neptunian Objects
It was only with the discovery in 1992 of the object 1992 QB1, by D. Jewitt and J. Luu, that astronomers found the first other member of a group to which Pluto apparently belongs. 1992 QB1 is a smallish object (thought to be comparable in size to a moderately large asteroid although its size is still only roughly estimated) in an approximately circular orbit almost 1.5 times further from the sun than the orbit of Neptune. A belt of objects beyond Pluto had originally been proposed by G. P. Kuiper (1951, in the book Astrophysics, edited by J. A. Hynek, McGraw-Hill; an earlier qualitative prediction by Edgeworth was apparently ignored and essentially unrecognized until rediscovered in the mid-1990s) as the result of the expected gradual outward decrease of material in the disk from which the planets formed. This "Kuiper Belt" of objects was subsequently invoked by several dynamicists as the only plausible source of many of the known short-period comets but it was not until 1992 that the first such object was discovered. In recent years, with the discovery of a large number of TNOs, it has become clear that there are at least two types of TNOs, both objects in roughly circular orbits entirely outside Neptune's orbit (the classical Kuiper-belt objects, KBOs) and objects in eccentric orbits that cross the orbit of Neptune with a 3:2 ratio of orbital periods such that they never get close to Neptune. Dynamically Pluto appears in every way to be typical of this second class of Trans-Neptunian Objects and the other members of the group are now often called Plutinos. Pluto still does not fit perfectly as a TNO, in as much as it is far larger than any of the other objects discovered in these orbits and it is the only one known to have a satellite, but there are fewer discrepancies between Pluto and the other TNOs than between Pluto and the other planets in the outer solar system. Thus Pluto should normally be considered as a TNO for many purposes.

For purposes of making catalogues of related objects, it is clear to many astronomers that Pluto should be included in any list that includes TNOs and IAU Division III has recommended that this be the case. The details of the cataloging are still under discussion but it has been agreed among all the relevant bodies in the IAU that Pluto will be number 1 in a sequential listing of all TNOs.

So What is Pluto?
The question naturally arises, then, as to whether Pluto is really a "planet". The IAU has never officially defined what constitutes a planet and has primarily used historical practice in accepting the eight planets that were known when the IAU was created and accepting Pluto as the ninth when it was discovered not long after the formation of the IAU. When the asteroid Ceres was first discovered in 1801, astronomers widely considered it to be a planet. Several years later, after the asteroids Pallas, Vesta, and Juno had also been discovered, it became clear that Ceres was the first discovered member of the asteroid belt and astronomers generally stopped referring to Ceres as a planet. (Those events preceded the existence of the IAU by a full century.) Pluto, on the other hand, has been known as a planet, albeit one which did not fit very well, for almost 70 years. For reasons of historical precedent, therefore, it will continue to be thought of as a planet by many people, both astronomers and non-astronomers.

Throughout astronomy there are examples of bodies which can sensibly be put into two or more groups or classes of bodies. There are, for example, three objects in our solar system that have dual classifications, both as minor planets and as comets. Should Pluto also be considered a planet for reasons other than the historical ones?

What Is a Planet?
The word planet comes from a Greek word meaning "wanderer" and was used by the Greeks to describe things that appeared like stars but moved in the sky relative to the stars. The Greeks used separate names for the sun, the moon, and for comets (the last name being derived from the Greek meaning roughly a hairy star), i.e. for the things that appeared different from stars and most Greeks did not consider Earth to be one of the planets. Since the time of Galileo and his first telescope, we have known that at least some planets look qualitatively different from the stars. Furthermore, many of the asteroids and all the newly discovered TNOs now do fit the classical, Greek definition. Clearly this Greek usage is no longer appropriate and our understanding of what constitutes a planet has evolved considerably over the last two millennia - adding Earth, discovering Uranus and Neptune, discovering but then discounting Ceres, and then discovering Pluto.

For many years we have been using the term Minor Planets both to refer to the objects traditionally called asteroids (typically in orbits between the orbits of Mars and Jupiter) and to refer to a wide variety of other small bodies in the solar system, including the objects that approach very close to Earth (NEOs, many of which may be extinct comets) and those that travel out into the regime of the giant, gaseous planets (Centaurs). This term has been used for all objects that are neither planets, nor satellites, nor comets. The approach implicitly assumed that we knew all the planets and that we would be discovering only smaller objects throughout the solar system, although this was in no way a condition for the terminology or for undertaking cataloging of such objects. Extensive surveys have not found any objects as large as Pluto even to distances considerably beyond the orbit of Pluto but they may well exist. Since we can not measure the sizes of most such objects directly, estimates of the size are usually based on the brightness but this requires assuming the reflectivity of the bodies and this is highly variable among the objects in the outer solar system. As it has become clear that far beyond Pluto we might find other objects comparable in size to Pluto, and as we have considered the cataloging of Pluto among the TNOs, various definitions for a planet (or perhaps of a major planet as opposed to a minor planet) have been proposed. The definitions all refer to bodies independently orbiting the sun, i.e., to objects that are not satellites:

Bodies that are above some arbitrary size (diameters of 1000 km and 2000 km have both been proposed; Ceres would not be a planet and Pluto would be a planet under either definition),
Bodies that are above some arbitrary mass (masses have been proposed that lie between the masses of Ceres and Pluto),
Bodies that are sufficiently large that self-gravity would overcome the structural strength of the material and make them roughly spherical except for oblateness caused by rotation (several of the largest asteroids, as well as Pluto, would be planets by this criterion), and
Bodies that dominate the mass at a particular range of distances from the sun, such as containing more than 90% of the total mass between 0.8 and 1.2 times the body's average distance from the sun, or more than 90% of the mass on orbits that cross the orbit of the body or massive enough to sweep up all the other material in its neighborhood (neither Ceres nor Pluto would be a planet by most versions of this criterion).

Note that both of the definitions based on physical processes imply a clear break with recent historical practice while the two arbitrary criteria (a round number for the size or mass of the boundary) have generally been proposed with numerical values chosen to preserve the historical definitions. Under the arbitrary definitions or under the definition based on self-gravity, Pluto would still be considered a planet in addition to being the first known Trans-Neptunian Object. There is at present not a consensus as to which of these definitions would be most useful to astronomers in thinking about the solar system so we are left with the historical precedent that Pluto is considered a planet. Until there is a consensus that one of the physical definitions is clearly the must useful approach in thinking about the solar system, the IAU will not "demote" Pluto or "promote" Ceres.

The Process Within the IAU
IAU Division III has an ongoing discussion of whether it is even necessary to make an explicit definition of what constitutes a planet and, if so, what that definition should be. Such a definition should be sufficiently general that it deals not only with potential future discoveries in our own solar system but also deals properly with extrasolar planets. In the case of extrasolar planets, theoreticians generally distinguish between planets and brown dwarfs. Some astronomers make this distinction based on the internal structure and what processes have occurred there (whether or not deuterium burning has occurred; a break that occurs at about 10 or 15 times the mass of Jupiter or .01 to .015 times the mass of the sun). Other astronomers, however, make the distinction based on the mechanism by which the object formed -- planets form in the disc around a forming star whereas brown dwarfs form by fragmentation of the cloud that is collapsing to form a star. Only recently have theoreticians realized that the distinction based on formation mechanisms may allow overlapping masses for planets and brown dwarfs and that the two definitions may lead to inconsistencies in what we should call a planet orbiting another star. As is the case with Pluto, cataloging the objects orbiting other stars now must also proceed, regardless of whether we call them brown dwarfs or planets.

The scientific definition of a planet is really a scientific matter and therefore can only be decided when there is a consensus among the experts in the field as to what definition makes physical sense and is useful to astronomers. Similarly, the scientific definition of a TNO is also a scientific matter, but in this case there is already a consensus among those experts (there is only a partial overlap among the groups of relevant experts) that Pluto is, at least dynamically, a member of the TNO group.

The decision on cataloging Pluto as a planet, or as a minor planet, or as a TNO, or as several of the above, is a practical matter rather than a matter of scientific definition and therefore is more amenable to a simple vote, although again a consensus, i.e. a very strong majority, is being sought among the Executive Committee of IAU Division III to ensure that the practical matters of ease of cataloging and utility of the catalog are met together with the goal of not misleading the public. In particular, because of the public perception, the IAU Executive Committee will be consulted before a decision is implemented. Because one can not predict when consensus will be reached, it is difficult to give a specific date by which a decision will be reached. Any decision, however, will not alter either the true nature of Pluto or the historical record of its having been generally considered a planet; a decision will be made if it provides useful distinctions to aid future work in astronomy.

Communications
Members of the IAU who wish to suggest a different approach or wish to discuss the matter with the IAU officers should contact the president(s) of the commission(s) to which they belong.

Acknowledgements

Although the material above was prepared entirely by the undersigned and any errors are entirely my fault, I would acknowledge significant input that influenced this presentation from numerous individiduals.

I'm Tired and I Wanna Go Back to:

IAU .............................. IAU Division III ............. IAU Commission 15

Department of Astronomy ............. M. F. A'Hearn .............

Created: 1998/12/30, M. A'Hearn; Last Update: 1999/01/24, mfa