The 2006 definition of "planet" by the
International Astronomical Union (IAU) states that, in the
solar
system, a
planet is a
celestial body that:
- is in orbit
around the Sun,
- has sufficient mass so that it assumes a
hydrostatic equilibrium (nearly round) shape, and
- has "cleared
the neighborhood" around its orbit.
A non-satellite
body fulfilling only the first two of these criteria is classified as a "dwarf
planet", whilst a non-satellite body fulfilling only the first criterion is
termed a "small
solar system body" (SSSB). The definition was a controversial one, and has
been both criticised and supported by different astronomers.
According to the definition there are currently
eight planets and
three dwarf planets known in the solar system. The definition distinguishes
planets from smaller bodies, and is not used outside the solar system, where
smaller bodies can't be found yet.
Exoplanets are covered separately under a complementary 2003 draft guideline
for the definition of planets, which distinguishes them from dwarf stars, which
are larger.
Reasons for the debate
Before the discoveries of the early 21st century, astronomers hadn't really
needed a formal definition for planets. With the discovery of
Pluto in 1930,
astronomers considered the
solar
system to have nine
planets, along
with thousands of smaller bodies such as asteroids and comets. Pluto was thought
to be larger than
Mercury. In 1978, the discovery of Pluto's moon
Charon radically changed this picture. By measuring Charon's orbital period,
astronomers could accurately calculate Pluto's mass for the first time, and they
found to be much smaller than expected.[1]
Pluto's mass was roughly one twentieth of Mercury's, making it by far the
smallest planet, smaller even than the
Moon, although it
was still over ten times as massive as the largest
asteroid,
Ceres. Then, in the 1990s, Astronomers began finding other objects at least
as far away as Pluto, now known as
Kuiper Belt Objects.[2]
Many of these shared some of Pluto's key orbital characteristics, and are now
called Plutinos.
Pluto came to be seen as the largest member of a new class of objects, and some
astronomers stopped referring to Pluto as a planet. New York City's newly
renovated
Hayden Planetarium famously did not include Pluto in its exhibit of the
planets.[3]
Then, starting in 2000, with the discovery of at least three bodies (Quaoar,
Sedna
and
Eris), all comparable to Pluto in terms of size and
orbit, it became
clear that they all would either have to be called planets or Pluto would have
to be reclassified. Astronomers also knew that more objects as large as Pluto
would be discovered, and the number of planets would start growing quickly. They
were also concerned about the classification of planets in other solar systems.
In 2006 the matter came to a head with the measurement of the size of 2003 UB313.
Eris (as it is now known), turned out to be slightly larger than Pluto, and so
was thought to be equally deserving of the status of 'planet'.
Historical Precedent
The refining understanding of Pluto was echoed by a debate in the 19th
century that began with the discovery of
Ceres on January 1, 1801. Astronomers immediately declared the tiny object
to be the missing planet between Mars and Jupiter, but within four years, the
discovery of two more tiny planets in similar orbits had cast doubt on this new
thinking. By 1851, the number of planets had grown to 23, and it was clear that
hundreds more would eventually be discovered. Astronomers began cataloging them
separately, and began calling them "asteroids"
instead of "planets."[4]
Early Proposals
Debate within the IAU led Julio Fernández and Gonzalo Tancredi of
Uruguay[5]
to suggest proposals to
redefine the term "planet"
so as to include other objects beyond the
traditional nine planets that had been historically considered part of the
solar
system.[6]
In its final form the proposal was denoted as Resolutions 5A, 5B, 6A and 6B
for GA-XXVI. Members of the IAU's General Assembly voted on the
proposal on
August 24, 2006
in Prague,
Czech Republic, with the vote removing Pluto's status as a planet and
reclassifying it as a
dwarf
planet.[7]
Pluto had long been exceptional among the planets, being small, distant, and
eccentric in orbit.
In its original form the redefinition would have kept Pluto as a planet and
recognized three new planets:
Ceres,
Charon, and
Eris. It was presumed that, after more observation and discussion,
astronomers would accept more objects in the
solar
system as meeting the new definition. On
August 22,
however, the original redefinition (which recognized twelve solar system
planets, including Pluto), was dealt a fatal blow in two open IAU meetings. Jay
Pasachoff of
Williams College, who attended both meetings, was quoted as saying, "I think
that today can go down as 'the day we lost Pluto' ".[8]
August 16
The IAU published the original definition proposal on
August 16,
2006. Its form
followed loosely the second of three options considered by a 19-member IAU-appointed
panel in 2005. It stated:
"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."
This definition would have led to three celestial bodies being recognized as
planets:
-
Ceres, which had been considered a planet at the time of its discovery, but
was subsequently treated as an asteroid
-
Charon, a moon of Pluto; the Pluto-Charon system would have been considered
a
double planet
-
Eris, a body in the
scattered disk of the outer solar system
A further twelve bodies, pending refinements of knowledge regarding their
physical properties, were possible candidates to join the list under this
definition. Some objects in this second list were more likely eventually to be
adopted as 'planets' than others. Despite what had been claimed in the media,[9]
the proposal did not necessarily leave the solar system with only twelve
planets.
Mike Brown, the discoverer of
Sedna
and Eris, has said that at least 53 known bodies in the solar system probably
fit the definition, and that a complete survey would probably reveal more than
200.[10]
The definition would have considered a pair of objects to be a
double planet system if each component independently satisfied the planetary
criteria and the common
center of gravity of the system (known as the
barycenter)
was located outside of both bodies.[11]
Pluto and Charon
would have been the only known double planet in the solar system. Other
planetary satellites (for example,
Earth's
moon) might be in
hydrostatic equilibrium, but would still not have been defined as a double
planet since the barycenter of the system lies within the more massive celestial
body (that is, Earth).
The term "minor planet" would have been abandoned, replaced by the categories
"Small Solar System Body" (SSSB) and a new classification of "pluton". The
former would have described those objects underneath the "spherical" threshold.
The latter would have been applied to those planets with highly inclined orbits,
large eccentricities and an orbital period of more than 200 years (that is,
those orbiting beyond Neptune). Pluto would have been the prototype for this
class. The term "dwarf
planet" would have been available to describe all planets smaller than the
eight "classical planets" in orbit around the Sun, though would not have been an
official IAU classification.[12]
The IAU did not make recommendations in the draft resolution on what separated a
planet from a
brown
dwarf.[13]
A vote on the proposal was scheduled for
August 24,
2006.[14]
Such a redefinition of the term "planet" could also have led to changes in
classification for the trans-Neptunian objects
2003 EL61,
2005 FY9,
Sedna,
Orcus,
Quaoar,
Varuna,
2002 TX300,
Ixion,
2002 AW197, and the asteroids
Vesta,
Pallas, and
Hygiea.
On 18
August the Division of Planetary Sciences of the
American Astronomical Society, the world's largest international
professional society of planetary scientists, endorsed the draft proposal.[15]
According to the IAU, the roundness condition generally results in the need
for a mass of at least 5×1020 kg, or diameter of at least 800 km.[12]
However, Mike Brown claims that these numbers are only right for rocky bodies
like asteroids, and that icy bodies like Kuiper Belt objects reach hydrostatic
equilibrium at much smaller sizes, probably somewhere between 200 and 400 km in
diameter.[16]
It all depends on the rigidity of the material that makes up the body, which is
in turn strongly influenced by its internal temperature.
Advantages
The proposed definition found support among many astronomers as it used the
presence of a physical qualitative factor (the object being round) as its
defining feature. Most other potential definitions depended on a limiting
quantity (e.g. a minimum size or maximum
orbital inclination) tailored for the solar system. According to members of
the IAU committee this definition did not use man-made limits but instead
deferred to "nature" in deciding whether or not an object was a planet.[17]
It also had the advantage of measuring an observable quality. Suggested
criteria involving the nature of formation would have been more likely to see
accepted planets later declassified as scientific understanding improved.
Additionally, the definition kept Pluto as a planet. Pluto's planetary status
was and is fondly thought of by many, and the general public could have been
alienated from professional astronomers; there was considerable uproar when the
media last suggested, in 1999, that Pluto might be demoted, which was a
misunderstanding of a proposal to catalog all trans-Neptunian objects uniformly.
[18]
Criticism of proposal
The proposed redefinition was criticized as ambiguous: Astronomer
Phil Plait[19]
and
NCSE writer
Nick
Matzke[20]
had both written about why they thought the redefinition was not, in general, a
good one. It defined a planet as orbiting a star, which would have meant that
any planet ejected from its star system or formed outside of one (a
rogue
planet or
interstellar planet) could not have been called a planet, even if it fit all
other definitions. A similar situation already applied to the term 'moon', such
bodies ceasing to be moons on being ejected from planetary orbit; this usage had
widespread acceptance.
Similarly the redefinition did not differentiate between planets and
brown
dwarf stars. Any attempt to clarify this differentiation was to be left
until a later date.
There had also been criticism of the definition of
double planet: at present the Moon is defined as a satellite of the Earth,
but over time the Earth-Moon
barycenter
will drift outwards (see
tidal acceleration) and could eventually become situated outside of both
bodies. This development would then upgrade the Moon to planetary status at that
time, according to the redefinition. The time taken for this to occur, however,
would be billions of years, long after many astronomers expect the Sun to expand
into a red
giant and destroy both Earth and Moon.[21]
In an 18
August
Science Friday interview, Mike Brown expressed doubt that a scientific
definition was even necessary:
- "The analogy that I always like to use is the word "continent".
You know, the word "continent" has no scientific definition ... they're just
cultural definitions, and I think the geologists are wise to leave that one
alone and not try to redefine things so that the word "continent" has a big,
strict definition."[22]
On 18
August,
Owen Gingerich, a historian and astronomer emeritus at
Harvard who
led the committee which generated the original definition, said that
correspondence he had received had been evenly divided for and against the
proposal.[23]
August 18
According to
Alan Boss
of the
Carnegie Institution of Washington, a subgroup of the IAU met on
August 18,
2006 and held a
straw poll
on the draft proposal: only 18 were in favour of it, with over 50 against. The
50 in opposition preferred an alternative proposal drawn up by
Uruguayan
astronomer
Julio Ángel Fernández.[23]
"(1) A planet is a celestial body that (a) is by far the largest object in its
local population[1], (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) does not produce
energy by any
nuclear fusion mechanism [3].
(2) According to point (1) the eight classical planets discovered before
1900, which move in nearly circular orbits close to the ecliptic plane are the
only planets of our Solar System. All the other objects in orbit around the Sun
are smaller than Mercury. We recognize that there are objects that fulfill the
criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf"
planets. Ceres as well as Pluto and several other large Trans-Neptunian objects
belong to this category. In contrast to the planets, these objects typically
have highly inclined orbits and/or large eccentricities.
(3) All the other natural objects orbiting the Sun that do not fulfill any of
the previous criteria shall be referred to collectively as "Small
Solar System Bodies".[4]
Definitions and clarifications
- The local population is the collection of objects that cross or closely
approach the orbit of the body in consideration.
- This generally applies to objects with sizes above several hundred
kilometers, depending on the material strength.
- This criterion allows the distinction between gas giant planets and brown
dwarfs or stars.
- 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.[24]"
Under the alternative proposal the current solar system would have remained
unchanged, but Pluto would have been demoted to a
dwarf
planet.
August 22
On 22
August the draft proposal was rewritten with two changes from the previous
draft.
The first was a generalisation of the name of the new class of planets
(previously the draft resolution had explicitly opted for the term 'pluton'),
with a decision on the name to be used postponed.
Many geologists had been critical of the choice of name for Pluto-like
planets,[25]
being concerned about the term
pluton which
has been used for years within the geological community to represent a form of
magmatic
intrusion; such formations are fairly common hunks of rock.[26][27]
Confusion was thought undesirable due to the status of planetology as a field
closely allied to geology.[28]
Further concerns surrounded use of the word pluton as in many European
languages such as
French and
Spanish, Pluto is itself called Pluton, potentially adding to
confusion.
The second change was a redrawing of the planetary definition in the case of
a double planet system. There had been a concern that, in extreme cases where a
double body had its secondary component in a highly eccentric orbit, there could
have been a drift of the barycenter in and out of the primary body, leading to a
shift in the classification of the secondary body between
satellite
and planet depending on where in its orbit the system was. Thus the definition
was reformulated so as to consider a double planet system in existence if its
barycenter lay outside both bodies for a majority of the system's orbital
period.
Later on the 22nd two open meetings were held which ended in an abrupt
about-face on the basic planetary definition. The position of astronomer
Julio Ángel Fernández gained the upper hand among the members attending and
was described as unlikely to lose its hold by the 24th. This position would
result in only eight major planets, with Pluto ranking as a "dwarf planet" or
"planetoid".[29]
The discussion at the first meeting was heated and lively, with IAU members in
vocal disagreement with one another over such issues as the relative merits of
static and
dynamic physics; the main sticking point was over whether or not to include
a body's orbital characteristics among the definition criteria. In an indicative
vote members heavily defeated the proposals on Pluto-like objects and double
planet systems, and were evenly divided on the question of hydrostatic
equilibrium. The debate was said to be "still open", with private meetings being
held ahead of a vote scheduled for the following day.[30]
At the second meeting of the day, following 'secret' negotiations, a
compromise began to emerge after the Executive Committee moved explicitly to
exclude consideration of
extra-solar planets and to bring into the definition a criterion concerning
the dominance of a body in its neighbourhood.[31]
August 24
Final draft
The final, third draft definition proposed on
24 August
was:
"The IAU...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”.
[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."
What exactly it meant to say that a body had "cleared
the neighbourhood around its orbit" or that it had a "nearly round" shape
was explicitly not defined. These descriptions were used partly to allow for
some "common
sense" judgement, but primarily to provide language that could readily be
used in communicating the definitions to lay audiences.
Plenary session debate
Voting on the definition took place at the Assembly plenary session during
the afternoon. Following a reversion to the previous rules on
15 August,
as a planetary definition is a primarily scientific matter every individual
member of the Union attending the Assembly was eligible to vote. The number
having registered their attendance at the Assembly at the time of the vote stood
at 2411,[32]
but out of the over a thousand who attended the session only 424 members chose
to vote or indicate their abstention on Resolution 6A (below).
The IAU Executive Committee presented four Resolutions to the Assembly, each
concerning a different aspect of the debate over the definition.[33]
Minor amendments were made on the floor for the purposes of clarification.
- Resolution 5A constituted the definition itself as stated above.
There was much discussion among members about the appropriateness of using the
expression "cleared the neighbourhood" instead of the earlier reference to
"dominant body", and about the implications of the definition for
satellites.
The Resolution was ultimately passed by a near-unanimous vote.
- Resolution 5B sought to amend the above definition by the insertion
of the word classical before the word planet in paragraph (1) and
footnote [1]. This represented a choice between having a set of three distinct
categories of body (planet, "dwarf planet" and SSSB) and the opening of an
umbrella of 'planets' over the first two such categories. The Resolution
proposed the latter option; it was defeated convincingly, with only 91 members
voting in its favour.
- Resolution 6A proposed a statement concerning Pluto: "Pluto is a
dwarf planet by the above definition and is recognized as the prototype of a new
category of trans-Neptunian objects." After a little quibbling over the grammar
involved and questions of exactly what constituted a "trans-Neptunian object",
the Resolution was approved by a vote of 237-157, with 30 people indicating
their absention. A
new category of dwarf planet was thus established.
- Resolution 6B sought to insert an additional sentence at the end of
the statement in 6A: "This category is to be called 'plutonian objects'." There
was no debate on the question, and in the vote the proposed name was defeated by
186-183; a proposal to conduct a re-vote was rejected. An IAU process will be
put in motion to determine the name for the new category.
On a literal reading of the Resolution, "dwarf planets" are by implication of
paragraph (1) excluded from the status of 'planet'. Use of the word planet
in their title may however cause some ambiguity.
Final definition
The final definition, as passed on
24 August 2006 is:
[34]
The IAU...resolves that planets and other bodies, except satellites, 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".
Footnotes:
[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.
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].
Footnote:
[1] An IAU process will be established to select a name for this category.
Objections
Criticism
Substance
There continues to be criticism regarding the wording of the final draft of
the definition. Notably, the lead scientist on
NASA's
robotic mission to Pluto,
Alan Stern,
contends that, like Pluto,
Earth,
Mars,
Jupiter and
Neptune have
not fully cleared their orbital zones either. Earth orbits with 10,000
near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000 Trojan
asteroids on its orbital path. "If Neptune had cleared its zone, Pluto wouldn't
be there," he added.[35]
However, his own earlier work on
neighborhood clearing supported the distinction between the largest eight
planets and the rest of the solar system.[36]
There is a substantial difference in the extent to which the neighborhood has
been cleared between Pluto and the eight planets. Also, Pluto's position is due
to the gravitational effects of Neptune as they are in
orbital resonance.
The debates have clarified that "clearing its orbit" refers to the process
that happened during the formation of the planets. It does not talk about the
presence of bodies that later strayed into the orbit after the accretions took
place.
The definition may be difficult to apply outside our solar system. Techniques
for identifying extrasolar objects generally cannot determine if an object has
"cleared its orbit," except indirectly via Stern and Levison's Λ parameter, and
provide limited information about when the objects were formed. The wording of
the new definition is
heliocentric in its use of the word
Sun instead of
star or
stars,
and is thus not applicable to the numerous
objects that have been identified in orbit around other stars.
Process
The final vote has come under criticism because of the relatively small
percentage of the 9000-strong membership who participated. Besides the fact that
most members do not attend the General Assemblies, this lack was also due to the
timing of the vote: the final vote was taken on the last day of the 10-day
event, after many participants had left or were preparing to leave. Of over
2,700 astronomers attending the conference, about 800 were present on the day
for the significant resolutions on a vote on a subsidiary resolution, the first
that required a count, only 424 votes were cast.[35]
There is also the issue of the many astronomers who were unable or who chose not
to make the trip to Prague and, thus, cast a vote. Astronomer Marla Geha has
clarified that not all members of the Union were needed to vote on the
classification issue: only those whose work is directly related to planetary
studies.[37]
Other
Some
astrologers have chosen not to follow the new definition.[38]
Impact
It is expected that the decision will have cultural and societal
implications. It will affect the “industry of astronomical artifacts and toys”.[39]
Educational books need to be revised. The decision was important enough to
prompt the editors of the 2007 edition of the
World Book Encyclopedia to hold off printing until a final result had been
reached.[39]
The new designation also has repercussion in the astrological world and finds
mixed receptions.[38]
