At the turn of the
21st century, the 'definition of "
planet"' became the subject of intense debate. Although the word itself dates back millennia, it had never had a formal definition until, beginning in 2005, a confluence of circumstances forced the astronomical community to take action.
From the end of the 19th century, "planet" had, without being defined, settled into a comfortable working term. It only applied to objects in the
Solar System; a number small enough that any differences could be dealt with on an individual basis. After 1992 however, astronomers began to discover many additional objects beyond the orbit of
Neptune, as well as hundreds of objects
orbiting other stars. These discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity. Some were nearly large enough to be
stars, while others were smaller than
Earth's moon. These discoveries challenged long perceived notions of what a planet could be.
The issue of a clear definition for "planet" came to a head in 2005 with the discovery of the
trans-Neptunian object Eris, a body larger than the smallest then-accepted planet,
Pluto. In its 2006 response, the
International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its
decision on the matter. This definition, which applies only to the Solar System, states that a planet is a body that orbits the
Sun, is large enough for its own gravity to make it round, and has "
cleared its neighbourhood" of smaller objects.
Pluto does not qualify as a planet under this definition, and the Solar System is thus considered to have eight planets:
Mercury,
Venus,
Earth,
Mars,
Jupiter,
Saturn,
Uranus, and
Neptune. The new category of
dwarf planet was created, currently including
Pluto,
Eris, and
Ceres. The IAU's decision has not resolved all controversies, however, and some in the astronomical community have rejected it outright. The issue of what constitutes a planet will likely remain contentious at least until 2009, when the IAU holds its next Congress in
Rio de Janeiro.
[1]
History
The word "planet" has meant many different things in its long life, often simultaneously. Throughout its history, use of the term was never strict and its meaning has twisted and blurred to include or exclude a variety of different objects.
While knowledge of the planets likely predates history, the word "planet" itself dates to
ancient Greece. The Greeks, like all ancient civilisations, believed the Earth to be stationary and at the centre of the universe in accordance with the
geocentric model, and that the objects in the sky, and indeed the sky itself, revolved around it. Greek astronomers employed the term ''asteres planetai'', "wandering stars",
[2][3] to describe those starlike lights in the heavens that moved over the course of the year, in contrast to the ''asteres aplanis'', the "fixed stars", which stayed motionless relative to one another. The five bodies currently called "planets" that were known to the Greeks are
Mercury, Venus, Mars, Jupiter, and Saturn.
Planets in antiquity
Graeco-Roman cosmology is commonly thought to have consisted of seven planets, with the Sun and the Moon counted among them (as is the case in modern
astrology); however, there is some ambiguity on that point: while many ancient astronomers contended that the
cosmos comprised seven planets, others refer to five planets, with the Sun and Moon as separate. As the 19th century German naturalist
Alexander von Humboldt noted in his work ''Cosmos,'' "Of the seven cosmical bodies which, by their continually varying relative positions and distances apart, have ever since the remotest antiquity been distinguished from the "unwandering orbs" of the heaven of the "fixed stars", which to all sensible appearance preserve their relative positions and distances unchanged, five only -Mercury, Venus, Mars, Jupiter and Saturn- wear the appearance of stars- "''cinque stellas errantes''"- while the Sun and Moon, from the size of their disks, their importance to man, and the place assigned to them in mythological systems, were classed apart."
[4]
In his ''
Timaeus'', written in roughly 360 BC,
Plato mentions, "the Sun and Moon and five other stars, which are called the planets".
[5] His student
Aristotle makes a similar distinction in his ''
On the Heavens'': "The movements of the sun and moon are fewer than those of some of the planets".
[6] In his ''Phaenomena'', which set to verse an astronomical treatise written by the philosopher
Eudoxus in roughly 350 BC,
[7] the poet
Aratus describes "those five other orbs, that intermingle with [the constellations] and wheel wandering on every side of the twelve figures of the Zodiac."
[8]
In his ''
Almagest'' written in the
2nd century,
Ptolemy refers to "the Sun, Moon and five planets."
[9] Hyginus explicitly mentions "the five stars which many have called wandering, and which the Greeks call Planeta."
[ Astra Planeta theoi.com ] Marcus Manilius, a Greek philosopher who lived during the time of
Caesar Augustus and whose poem ''
Astronomica'' is considered one of the principal texts for modern
astrology, says, "Now the
dodecatemory is divided into five parts, for so many are the stars called wanderers which with passing brightness shine in heaven."
[10]
Conversely, in his ''
Dream of Scipio'', written sometime around
53 BC,
Cicero has the spirit of
Scipio Africanus proclaim, "Seven of these spheres contain the planets, one planet in each sphere, which all move contrary to the movement of heaven."
[11] In his ''
Natural History'', written in
77 AD,
Pliny the Elder refers to "the seven stars, which owing to their motion we call planets, though no stars wander less than they do."
[12] Nonnus, the
5th century Greek poet, says in his ''
Dionysiaca'', "I have oracles of history on seven tablets, and the tablets bear the names of the seven planets."
Planets in the Middle Ages
Copernicus
Later medieval and Renaissance writers appear to have accepted the idea of seven planets. In his ''
Confessio Amantis'',
14th century poet
John Gower, referring to the planets' connection with the craft of
alchemy, writes, "Of the planetes ben begonne/The gold is tilted to the Sonne/The Mone of Selver hath his part...", indicating that the Sun and the Moon were planets.
[13] Even
Nicolaus Copernicus, who rejected the geocentric model, was ambivalent concerning whether the Sun and Moon were planets. In his ''
De Revolutionibus'', Copernicus clearly separates "the sun, moon, planets and stars";
[14] however, in his Dedication of the work to Pope Paul III, Copernicus refers to, "the motion of the sun and the moon... and of the five other planets."
[15]
Modern planets
William Herschel, discoverer of Uranus
Eventually, when Copernicus's
heliocentric model was accepted over the
geocentric, Earth was placed among the planets and the Sun was dropped. It could be said, therefore, that Earth was the first planet of the modern era.
In 1781, the astronomer
William Herschel was searching the sky for elusive
stellar parallaxes, when he observed what he termed a
comet in the constellation of
Taurus. Unlike stars, which remained mere points of light even under high magnification, this object's size increased in proportion to the power used. That this strange object might have been a planet simply did not occur to Herschel; the five planets beyond Earth had been part of humanity's conception of the universe since antiquity. However, unlike a comet, this object's orbit was nearly circular and within the ecliptic plane. Before Herschel announced his discovery of his "comet", his colleague, British
Astronomer Royal Nevil Maskelyne, wrote to him, saying, "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any coma or tail to it."
[16] The "comet" was also very far away, too far away for a mere comet to resolve itself. Eventually it was recognised as the seventh planet and named
Uranus after the father of Saturn.
Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of
Neptune in 1846, and presumed irregularities in Neptune's orbit subsequently led to the search which ultimately located
Pluto in 1930. Initially believed to be roughly the mass of the Earth, observation gradually shrank Pluto's estimated mass until it was revealed to be a mere five hundredth as large; far too small to have influenced Neptune's orbit at all.
[17] In 1989,
Voyager 2 determined the irregularities to be due to an overestimation of Neptune's mass.
[18]
Satellites
Christian Huygens, discoverer of Titan
When Copernicus placed the Earth among the planets, he also placed the Moon in orbit around the Earth, making the Moon the first
natural satellite to be discovered. When
Galileo discovered his four
satellites of Jupiter in 1610, they lent weight to Copernicus's argument, since if other planets could have satellites, then the Earth could too. However, there remained some confusion as to whether these objects were "planets"; Galileo initially intended to name them the "Medicean planets", in honour of his patrons, the
Medicis.
[19] Similarly,
Christiaan Huygens, upon discovering Saturn's largest moon
Titan in 1655, employed many terms to describe it, including "planeta", (planet) "stella" (star) "Luna" (moon), and the more modern "satellite".
[20] Giovanni Cassini, in announcing his discovery of Saturn's moons
Iapetus and
Rhea in 1671 and 1672, described them as ''Nouvelles Planetes autour de Saturne'' ("New planets around Saturn").
[21] However, when the "Journal de Scavans" reported Cassini's discovery of two new Saturnian moons in 1686, it referred to them strictly as "satellites".
[22] When William Herschel announced his discovery of two objects in orbit around Uranus, he referred to them as "satellites" and "secondary planets."
[23]
Minor planets
Giuseppe Piazzi, discoverer of Ceres
One of the unexpected results of
William Herschel's discovery of Uranus was that it appeared to validate
Bode's law, a mathematical function which generates the size of the
semimajor axis of planetary
orbits. Astronomers had considered the "law" a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801, astronomer
Giuseppe Piazzi found a miniature new world,
Ceres, lying at just the correct point in space. The object was hailed as a new planet.
[24]
Then in 1802,
Heinrich Olbers discovered
Pallas, a second "planet" at roughly the same distance from the Sun as Ceres. That two planets could occupy the same orbit was an affront to centuries of thinking; even
Shakespeare had ridiculed the idea ("Two stars keep not their motion in one sphere").
[25] In 1804, another world,
Juno, was discovered in a similar orbit.
24 In 1807, Olbers discovered a fourth object,
Vesta, at a similar orbital distance.
Herschel suggested that these four worlds be given their own separate classification,
asteroids (meaning "starlike" since they were too small for their disks to resolve and thus resembled
stars), though most astronomers preferred to refer to them as planets. This conception was entrenched by the fact that, due to the difficulty of distinguishing asteroids from yet-uncharted stars, those four remained the only asteroids known until 1845.
[ Call the Police! The story behind the discovery of the asteroids, , Keith, Cooper, Astronomy Now, ] Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets. With the arrival of more refined star charts, the search for asteroids resumed, and a fifth and sixth were discovered by
Karl Ludwig Hencke in 1845 and 1847.
By 1851 the number of asteroids had increased to 15, and a new method of classifying them, by affixing a number before their names in order of discovery, was adopted, inadvertently placing them in their own distinct category. Ceres became "(1) Ceres", Pallas became "(2) Pallas", and so on. By the 1860s, the number of known asteroids had increased to over a hundred, and observatories in Europe and the United States began referring to them collectively as "
minor planets", or "small planets", though it took the first four asteroids longer to be grouped as such.
24 To this day, "minor planet" remains the official designation for all small bodies in orbit around the Sun (whether asteroid or not), and each new discovery is numbered accordingly in the IAU's
Minor Planet Catalogue.
[26]
Pluto
Clyde Tombaugh, discoverer of Pluto
The long road from planethood to reconsideration undergone by Ceres is mirrored in the story of
Pluto, which was named a planet soon after its discovery by
Clyde Tombaugh in 1930. Uranus and Neptune had been declared planets based on their circular orbits, large masses and proximity to the ecliptic plane. None of these applied to Pluto; a tiny, icy world in a region of
gas giants with an orbit that carried it high above the
ecliptic and even inside that of Neptune. In 1978, astronomers discovered its largest moon,
Charon. This allowed astronomers to determine Pluto's size. It was found to be much tinier than expected, smaller even than the Earth's Moon. However, it was, as far as anyone could tell, unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-hypothesised
Kuiper belt (sometimes called the
Edgeworth-Kuiper belt), a band of icy debris that is the source for "short-period" comets—those with orbital periods of up to 200 years.
[27]
Pluto's orbit lay right in the middle of this band and thus its planetary status was thrown into question; the precedent set by Ceres in downgrading an object from planet status because of a shared orbit led many to conclude that Pluto must be reclassified as a minor planet as well.
Mike Brown of the
California Institute of Technology suggested that a "planet" should be redefined as "any body in the Solar System that is more massive than the total mass of all of the other bodies in a similar orbit."
[28] The eight planets over that mass limit would be referred to as "major planets". There was outcry at the prospect of Pluto's "demotion", and in 1999 the
International Astronomical Union clarified that it was not at that time proposing to change Pluto's status as a planet.
[29][30]
Image:EightTNOs.png|thumb|The relative sizes of the largest trans-Neptunian objects as compared to Earth.
#Earth
rect 646 1714 2142 1994 The Earth
#Eris and Dysnomia
circle 226 412 16 Dysnomia
circle 350 626 197 (136199) Eris
#Pluto and Charon
circle 1252 684 86 Charon
circle 1038 632 188 (134340) Pluto
#2005 FY9
circle 1786 614 142 (136472) 2005 FY9
#2003 EL61
circle 2438 616 155 (136108) 2003 EL61
#Sedna
circle 342 1305 137 (90377) Sedna
#Orcus
circle 1088 1305 114 (90482) Orcus
#Quaoar
circle 1784 1305 97 (50000) Quaoar
#Varuna
circle 2420 1305 58 (20000) Varuna
desc none
# - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
#Notes:
#Details on the new coding for clickable images is here:
#While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
#Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.
The discovery of several other
trans-Neptunian objects approaching the size of Pluto, such as
Quaoar and
Sedna, continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On
July 29,
2005, Mike Brown and his team announced the discovery of a trans-Neptunian object confirmed to be larger than Pluto,
[31] named
Eris.
[32]
In the immediate aftermath of the object's discovery, there was much discussion as to whether it could be termed a "
tenth planet". NASA even put out a press release describing it as such.
[33] However, acceptance of Eris as the tenth planet implicitly demanded a definition of planet that set Pluto as an arbitrary minimum size. Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognise Pluto's historical identity as a planet by "
grandfathering" it into the planet list.
[34]
IAU debate
Main articles: 2006 definition of planet
The eight
Planets and three dwarf planets of the Solar System. ''(Sizes to scale.)''
The discovery of
Eris forced the
IAU to act on a definition. In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of
Sedna in 2003, narrowed their choices to a shortlist of three, using
approval voting. The definitions were:
★ A planet is any object in
orbit around the Sun with a diameter greater than 2000 km. ''(eleven votes in favour)''
★ A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. ''(eight votes in favour)''
★ A planet is any object in orbit around the Sun that is dominant in its immediate neighborhood. ''(six votes in favour)''
[35][36]
Since no overall consensus could be reached, the committee decided to put these three definitions to a wider vote at the IAU General Assembly meeting in
Prague in August 2006,
[37] and on
August 24, the IAU put a final draft to a vote, which combined elements from two of the three proposals. It essentially created a medial classification between "planet" and "rock" (or, in the new parlance, "
small Solar System body"), called "
dwarf planet" and placed
Pluto in it, along with Ceres and Eris.
[38][39] The vote was passed, with 424 astronomers taking part in the ballot.
[40]
On
September 13,
2006, the IAU placed Eris, its moon Dysnomia, and Pluto into their
Minor Planet Catalogue, giving them the official minor planet designations
(134340) Pluto,
(136199) Eris, and
(136199) Eris I Dysnomia.
[41]
Acceptance of the definition
Plot of the current positions of all known Kuiper belt objects, set against the outer planets
Among the most vocal proponents of the IAU's decided definition are
Mike Brown, the discoverer of Eris, and
Steven Soter, professor of astrophysics at the
American Museum of Natural History.
In an article in the
January 2007 issue of ''
Scientific American'', Soter cited the definition's incorporation of current theories of the
formation and evolution of the Solar System; that as the earliest
protoplanets emerged from the swirling dust of the
protoplanetary disc, some bodies "won" the initial competition for limited material and, as they grew, their increased gravity meant that they accumulated more material, and thus grew larger, eventually outstripping the other bodies in the Solar System by a very wide margin. The asteroid belt, disturbed by the gravitational tug of nearby Jupiter, and the Kuiper belt, too widely spaced for its constituent objects to collect together before the end of the initial formation period, both failed to win the accretion competition.
When the numbers for the winning objects are compared to those of the losers, the contrast is quite striking; if we accept Soter's concept that each planet occupies an "orbital zone," then the least orbitally dominant planet, Mars, is larger than all other collected material in its orbital zone by a factor of 5100. Ceres, the largest asteroid, is only larger by a factor of 0.33; Pluto's ratio is even lower, at 0.07.
[ What is a Planet? Steven Soter ] Mike Brown asserts that this massive difference in orbital dominance leaves "absolutely no room for doubt about which objects do and do not belong."
[ The Eight Planets Michael E. Brown ]
Ongoing controversies
Despite the IAU's declaration, a number of critics remain unconvinced. The definition is seen by many as arbitrary and confusing, and a number of
Pluto-as-planet proponents, in particular
Alan Stern, head of
NASA's New Horizons mission to Pluto, have circulated a petition among astronomers to alter the definition. The claim is that, since less than 5 percent of astronomers voted for it, the decision was not representative of the entire astronomical community.
[42][43] Even with this controversy excluded, there remain several ambiguities in the definition.
Clearing the neighborhood
Main articles: Clearing the neighborhood
One of the main points at issue is the precise meaning of "cleared the neighborhood around its
orbit".
Alan Stern recently objected that "it is impossible and contrived to put a dividing line between dwarf planets and planets,"
[44] and that since neither Earth, Mars, Jupiter, nor Neptune have entirely cleared their regions of debris, none could properly be considered planets under the
IAU definition.
The asteroids of the inner Solar System; note the
Trojan asteroids (green), trapped into Jupiter's orbit by its gravity
Mike Brown counters these claims by saying that, far from not having cleared their orbits, the major planets completely control the orbits of the other bodies within their orbital zone. Jupiter may coexist with a large number of small bodies in its orbit (the
Trojan asteroids), but these bodies only exist in Jupiter's orbit because they are in the sway of the planet's huge gravity. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called
plutinos, into a 3:2 resonance, i.e., they orbit the Sun twice for every three Neptune orbits. The orbits of these objects are entirely dictated by Neptune's gravity, and thus, Neptune is gravitationally dominant.
Whatever definition of "clearing the neighborhood" is ultimately accepted by the IAU, it is still an ambiguous concept. Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona and organiser of the petition, explained the ambiguity to
National Public Radio. Since the definition does not categorise a planet by composition or formation, but, effectively, by its location, a Mars-sized or larger object beyond the orbit of Pluto would be considered a dwarf planet, since it would not have time to clear its orbit and would therefore be surrounded by objects of similar mass, whereas an object smaller than Pluto orbiting in isolation would be considered a planet.
[45]
Hydrostatic equilibrium
.jpg)
Proteus, the moon of
Neptune, is irregular, despite being larger than many spherical objects.
The
IAU's definition mandates that planets be large enough for their own
gravity to form them into a state of
hydrostatic equilibrium; this means that they will reach a shape that is, if not spherical, then
spheroidal. This distinction, as opposed to strict sphericity, is mandated by the fact that many large objects in the
Solar System, such as the planets Jupiter and Saturn, the moons
Mimas,
Enceladus and
Miranda, and the
Kuiper belt object 2003 EL61,
[46] have been distorted into oblate or prolate spheroids by rapid rotation or
tidal forces.
However, there is no one point at which an object can be said to have reached hydrostatic equilibrium. As Soter noted in his article,"How are we to quantify the degree of roundness that distinguishes a planet? Does gravity dominate such a body if its shape deviates from a spheroid by 10 percent or by 1 percent? Nature provides no unoccupied gap between round and nonround shapes, so any boundary would be an arbitrary choice."
Furthermore, objects made of ices, such as Enceladus and Miranda, assume that state more easily than those made of rock, such as Vesta and Pallas.
[47] Heat energy, from
gravitational collapse,
impacts, tidal forces, or
radioactive decay also factors into whether an object will be spherical or not; Saturn's icy moon Mimas is spheroidal, but Neptune's larger moon Proteus, which is similarly composed but colder because of its greater distance from the Sun, is irregular.
Double planets
Main articles: Double planet
The definition specifically excludes
satellites from the category of dwarf planet, though it does not directly define the term "satellite". In the original draft proposal, an exception was made for
Pluto and its largest satellite,
Charon, which possess a
barycenter outside the volume of either body. The initial proposal classified Pluto/Charon as a double planet, with the two objects orbiting the Sun in tandem. However, the final draft made clear that, double or not, both Pluto and Charon would be considered dwarf planets, not planets.
Under the same definition, the Earth-
Moon system is not formally recognized as a double planet, despite the Moon's large relative size, since the barycenter lies within the Earth. As the Moon is slowly
receding from the Earth, the Earth-Moon system may eventually become a double planet system on the basis of this barycentric definition.
Also, many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. Jupiter's moon
Ganymede and Saturn's moon
Titan are both larger in terms of diameter (though not mass) than
Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as
Io and
Triton demonstrate obvious and ongoing geological activity, and Ganymede has a
magnetic field. Just as
stars in orbit around other stars are still referred to as stars, so some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets.
[48] Indeed Mike Brown makes just such a claim in his dissection of the issue, noting that there is little case for describing an object 400 km across with little internal geological activity as a planet if a 5000 km object with methane lakes,
cryovolcanism and storms (i.e.
Titan) is called a moon.
Extrasolar planets and brown dwarfs
Main articles: Extrasolar planet,
Brown dwarf
The
IAU's definition of planet applies only to objects within our own
Solar System. The more than 200 extrasolar planets (planet-sized objects in
orbit around other
stars) were excluded as too complex an issue to be resolved during the congress. However, any future definition will need to include them, as their discovery has widened the debate on the nature of planethood in unexpected ways. Many of these planets are of considerable size, approaching the mass of small stars, while many newly-discovered brown dwarfs are conversely small enough to be considered planets.
[49]
Traditionally, the defining characteristic for starhood has been an object's ability to
fuse hydrogen in its core. However, stars such as brown dwarfs have always challenged that distinction. Too small to commence sustained hydrogen fusion, they have been granted star status on their ability to fuse
deuterium. However, due to the relative rarity of that
isotope, this process lasts only a tiny fraction of the star's lifetime, and hence most brown dwarfs would have ceased fusion long before their discovery.
[50] Binary stars and other multiple-star formations are common, and many brown dwarfs orbit other stars. Therefore, since they do not produce energy through fusion, they could be described as planets. Indeed, astronomer
Adam Burrows of the
University of Arizona claims that "from the theoretical perspective, however different their modes of formation, extrasolar giant planets and brown dwarfs are essentially the same."
[51] Burrows also claims that such stellar remnents as
white dwarfs should not be considered stars,
[52] a stance which would mean that an orbiting
white dwarf, such as
Sirius B could be considered a planet. However, the current convention among astronomers is that any object massive enough to have possessed the capability to fuse during its lifetime should be considered a star.
[53]
The confusion does not end with brown dwarfs. Maria Rosa Zapatario-Osorio et al. have discovered many objects in young
star clusters of masses below that required to sustain fusion of any sort (currently calculated to be roughly 13 Jupiter masses).
[54] These have been described as "
free floating planets" because current theories of Solar System formation suggest that planets may be ejected from Solar Systems altogether if their orbits become unstable.
[55]
However, it is also possible that these "free floating planets" could have formed in the same manner as stars.
[56] The material difference between a low-mass star and a large
gas giant is not clearcut; apart from size and relative temperature, there is little to separate a gas giant like Jupiter from its host star. Both have similar overall compositions: hydrogen and
helium, with trace levels of heavier
elements in their
atmospheres. The generally accepted difference is one of formation; stars are said to have formed from the "top down"; out of the gases in a nebula as they underwent gravitational collapse, and thus would be composed almost entirely of hydrogen and helium, while planets are said to have formed from the "bottom up"; from the accretion of dust and gas in orbit around the young star, and thus should have cores of
silicates or ices.
[57] As yet it is uncertain whether gas giants possess such cores. If it is indeed possible that a gas giant could form as a star does, then it raises the question of whether such an object, even one as familiar as Jupiter or Saturn, should be considered an orbiting low-mass star rather than a planet.
In 2003, the IAU officially released a statement
[58] to define what constitutes an extrasolar planet and what constitutes an orbiting star. To date, it remains the only official decision reached by the IAU on this issue.
CHXR 73 b, a star which lies at the border between planet and brown dwarf
Like defining a planet by having
cleared its neighbourhood, this definition creates ambiguity by making location, rather than formation or composition, the determining characteristic for planethood. A free-floating object with a mass below 13 Jupiter masses is a "sub-brown dwarf," whereas such an object in orbit round a fusing star is a planet, even if, in all other respects, the two objects may be identical.
This ambiguity was highlighted in December 2005, when the
Spitzer Space Telescope observed
Cha 110913-773444 (above), the least massive brown dwarf yet found, only eight times Jupiter's mass with what appears to be the beginnings of its own
star system. Were this object found in orbit round another star, it would have been termed a planet.
[59]
It was highlighted again in September 2006, when the
Hubble Space Telescope imaged
CHXR 73 b (left), an object orbiting a young companion star at a distance of roughly 200 AU. At 12 Jovian masses, CHXR 73 b is just under the threshold for deuterium fusion, and thus technically a planet; however, its vast distance from its parent star suggests it could not have formed inside the small star's
protoplanetary disc, and therefore must have formed, as stars do, from gravitational collapse.
[60]
Semantics
Finally, from a purely linguistic point of view, there is the dichotomy that the IAU created between 'planet' and 'dwarf planet'. The term 'dwarf planet' arguably contains two words, a noun (planet) and an adjective (dwarf). Thus, the term could suggest that a dwarf planet is a type of planet, even though the IAU explicitly defines a dwarf planet as ''not'' so being. By this formulation therefore, 'dwarf planet' and 'minor planet' are best considered
compound nouns. Benjamin Zimmer, of
languagelog.org, summarised the confusion: "The fact that the IAU would like us to think of dwarf planets as distinct from 'real' planets lumps the lexical item 'dwarf planet' in with such oddities as '
Welsh rabbit' (not really a rabbit) and '
Rocky Mountain oysters' (not really oysters)."
[ New planetary definition a "linguistic catastrophe"! ] As
Dava Sobel, the historian and popular science writer who participated in the IAU's initial decision in October 2006, noted in an interview with
National Public Radio, "A dwarf planet is not a planet, and in astronomy, there are dwarf stars, which are stars, and dwarf galaxies, which are galaxies, so it's a term no one can love, dwarf planet."
[61] Mike Brown noted in an interview with the Smithsonian that, "Most of the people in the dynamical camp really did not want the word "dwarf planet," but that was forced through by the pro-Pluto camp. So you’re left with this ridiculous baggage of dwarf planets not being planets."
[ Pluto's Planethood: What Now? ]
Conversely, astronomer Robert Cumming of the Stockholm Observatory notes that, "The name 'minor planet' been more or less synonymous with 'asteroid' for a very long time. So it seems to me pretty insane to complain about any ambiguity or risk for confusion with the introduction of 'dwarf planet'."
Notes
- Olbers proposed that these new discoveries were the fragments of a planet, later dubbed Phaeton, that had formerly revolved around the sun but had been destroyed by impact with a comet.
- Defined as the region occupied by two bodies whose orbits cross a common distance from the Sun, if their orbital periods differ less than an order of magnitude. In other words, if two bodies occupy the same distance from the Sun at one point in their orbits, and those orbits are of similar size, rather than, as a comet's would be, extending for several times the other's distance, then they are in the same orbital zone.[65]
- In 2002, in collaboration with dynamicist Harold Levison, Stern wrote, "we define an ''überplanet'' as a planetary body in orbit around a star that is dynamically important enough to have cleared its neighboring planetesimals ... And we define an ''unterplanet'' as one that has not been able to do so," and then a few paragraphs later, "our Solar System clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres."[66] While this may appear to contradict Stern's objections, Stern noted in an interview with Smithsonian Air and Space that, unlike the IAU's definition, his definition still allows unterplanets to be planets: "I do think from a dynamical standpoint, there are planets that really matter in the architecture of the solar system, and those that don’t. They’re both planets. Just as you can have wet and dry planets, or life-bearing and non-life-bearing planets, you can have dynamically important planets and dynamically unimportant planets."
- The density of an object is a rough guide to its composition: the lower the density, the higher the fraction of ices, and the lower the fraction of rock. The most dense of these objects, Vesta and Juno, are composed almost entirely of rock with very little ice, and have a density close to the Moon's, while the less dense, such as Proteus and Enceladus, are composed mainly of ice.[67][68]
See also
★
Timeline of discovery of Solar System planets and their natural satellites
★
List of solar system objects by planetary discriminant
★
Mesoplanet
★
Natural kind
★
Planemo
★
Planetar
★
Planetesimal
★
Pluto prototype
★
Definition of moon
References
1.
IAU Colloquia and Symposia sponsored by Division VIII
2. Definition of planet
3. Words For Our Modern Age: Especially words derived from Latin and Greek sources
4. Cosmos: A Sketch of a Physical Description of the Universe Alexander von Humboldt
5. Timaeus by Plato
6. On the Heavens by Aristotle, Translated by J. L. Stocks
7. Phaenomena Book I - ARATUS of SOLI
8. ARATUS, PHAENOMENA A. W. & G. R. Mair (translators)
9. The Almagest by Ptolemy, R. Gatesby Taliaterro (trans.), , , University of Chicago Press, 1952,
10. Marcus Malinius: Astronomica, GP Goold (trans.), , , Harvard University Press, 1977,
11. Roman Philospohy: Cicero: The Dream of Scipio Richard Hooker (translator)
12. Natural History vol 1, IH Rackham, , , William Heinemann Ltd., 1938,
13. The Seven Planets, P. Heather, , , , 1943
14. The text of Nicholas Copernicus' De Revolutionibus (On the Revolutions), 1543 C.E. Edward Rosen (trans.)
15. Dedication of the Revolutions of the Heavenly Bodies to Pope Paul III Nicholas Copernicus
16. William Herschel: Astronomer and Musician of 19 New King Street, Bath, Patrick Moore, , , PME Erwood, 1981,
17.
Planet Quest: The Epic Discovery of Alien Solar Systems, Croswell, Ken, , , Oxford University Press pp. 48, 66 (ISBN 0-19-288083-7), 1999,
18. Appendix 7: Hypothetical Planets Paul Schlyter
19. The Discovery of the Galilean Satellites
20. Systema Saturnium: Sive de Causis Miradorum Saturni Phaenomenon, et comite ejus Planeta Novo, Christiani Hugenii (Christiaan Huygens), , , Adriani Vlacq, 1659,
21. Decouverte de deux Nouvelles Planetes autour de Saturne, Giovanni Cassini, , , , 1673,
22. An Extract of the Journal Des Scavans. of April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris
23. An Account of the Discovery of Two Satellites Around the Georgian Planet. Read at the Royal Society, William Herschel, , , J. Nichols, 1787,
24. When did asteroids become minor planets? Hilton, James L.
25. King Henry the Fourth Part One in The Globe Illustrated Shakespeare: The Complete Works Annotated, William Shakespeare, , , Granercy Books, 1979,
26. The MPC Orbit (MPCORB) Database
27. The Kuiper Belt Weissman, Paul R.
28. A World on the Edge Brown, Mike.
29. The Status of Pluto:A clarification
30. Saving Planet Pluto Witzgall, Bonnie B.
31. The discovery of 2003 UB313, the 10th planet. Brown, Mike
32. DISCOVERY OF A PLANETARY-SIZED OBJECT IN THE SCATTERED KUIPER BELT M. E. Brown, C. A. Trujillo, and D. L. Rabinowitz
33. NASA-Funded Scientists Discover Tenth Planet
34. Topic - First Mission to Pluto and the Kuiper Belt; "From Darkness to Light: The Exploration of the Planet Pluto" Dr. Bonnie Buratti
35. Xena reignites a planet-sized debate McKee, Maggie
36. The Tenth Planet's First Anniversary Croswell, Ken
37. Planet Definition
38. IAU General Assembly Newspaper
39. The Final IAU Resolution on the Definition of "Planet" Ready for Voting
40. IAU 2006 General Assembly: Resolutions 5 and 6
41. Circular No. 8747 Central Bureau for Astronomical Telegrams, International Astronomical Union
42. Pluto demoted in highly controversial definition Robert Roy Britt
43. Pluto: Down But Maybe Not Out Robert Roy Britt
44. Pluto vote 'hijacked' in revolt Paul Rincon
45. Astronomers Prepare to Fight Pluto Demotion
46. 2003EL61 Brown, Michael E.
47. The Dwarf Planets Mike Brown
48. Solar System Voyage, Serge Brunier, , , Cambridge University Press, 2000,
49. IAU General Assembly: Definition of Planet debate
50. What is a planet? Basri, Gibor
51. The Theory of Brown Dwarfs and Extrasolar Giant Planets Burrows, Adam, Hubbard, W.B., Lunine, J., Leibert, James
52. Croswell p. 119
53. Planet Quest: The Epic Discovery of Alien Solar Systems, Croswell, Ken, , , Oxford University Press p. 119 (ISBN 0-19-288083-7), 1999,
54. Discovery of Young, Isolated Planetary Mass Objects in the Sigma Orionis Star Cluster Zapatero M. R. Osorio, V. J. S. Béjar, E. L. Martín, R. Rebolo, D. Barrado y Navascués, C. A. L. Bailer-Jones, R. Mundt
55. Timescales for Planetary Accretion and the Structure of the Protoplanetary disk, , J.J., Lissauer, Icarus, 1987
56. Rogue planet find makes astronomers ponder theory
57. Giant planet formation G. Wuchterl
58. Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union
59. A Planet With Planets? Spitzer Finds Cosmic Oddball Clavin, Whitney
60. Planet or failed star? Hubble photographs one of the smallest stellar companions ever seen
61. A Travel Guide to the Solar System
62. A Brief History of Asteroid Spotting
63. Olbers' planet: the history continues indeed
64. The Primordial Excitation and Clearing of the Asteroid Belt, Petit, J.-M.; Morbidelli, A.; Chambers, J., , , Icarus, 2001
65. What is a Planet? submitted to The Astronomical Journal, 16 August 2006
66. Regarding the criteria for planethood and proposed planetary classification schemes, , S. Alan, Stern, Highlights of Astronomy, 2002
67. A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites Righter, Kevin; Drake, Michael J.
68. Shapes and rotational properties of thirty asteroids from photometric data Johanna Torppa, Mikko Kaasalainen, Tadeusz Michałowski, Tomasz Kwiatkowski, Agnieszka Kryszczyńska, Peter Denchev, and Richard Kowalski
Bibliography and external links
★
Why Planets will never be defined: Robert Roy Britt on the outcome of the IAU's decision
★
Dwarfing Pluto An examination of the redefinition of Pluto from a linguistic perspective.
★
Q&A New planets proposal Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
★
David Jewitt's Kuiper Belt page- Pluto
★
Dan Green's webpage: What is a planet?
★
What is a Planet? Debate Forces New Definition
★
The Flap Over Pluto
★
"You Call That a Planet?: How astronomers decide whether a celestial body measures up."
★ David Darling. ''The Universal Book of Astronomy, from the Andromeda Galaxy to the Zone of Avoidance.'' 2003. John Wiley & Sons Canada (ISBN 0-471-26569-1), p. 394
★ ''Collins Dictionary of Astronomy'', 2nd ed. 2000. HarperCollins Publishers (ISBN 0-00-710297-6), p. 312-4.
★
Catalogue of Planetary Objects. Version 2006.0 O.V. Zakhozhay, V.A. Zakhozhay, Yu.N. Krugly, 2006
★
The New Proposal, Resolution 5, 6 and 7 2006-08-22
★
IAU 2006 General Assembly: video-records of the discussion and of the final vote on the Planet definition.
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