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Photograph of the
crescent of the planet Neptune (top) and its moon
Triton (center), taken by
Voyager 2 during its 1989 flyby
The
definition of planet, since the word was coined by the
ancient Greeks, has included within its scope a wide range of celestial bodies.
Greek astronomers employed the term
asteres planetai
(ἀστέρες πλανῆται), "wandering stars", for star-like objects which
apparently moved over the sky. Over the millennia, the term has included
a variety of different objects, from the
Sun and the
Moon to
satellites and
asteroids.
By the end of the 19th century the word
planet, though it had yet to be defined, had become a working term applied only to a small set of objects in the
Solar System. 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 January 2005 with the discovery of the
trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet,
Pluto. In its August 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 during a meeting in
Prague in the
Czech Republic. This definition, which applies only to the Solar System, states that a planet is a body that orbits the Sun, is
massive enough for its
own gravity to make it round, and has "
cleared its neighbourhood"
of smaller objects around its orbit. Under this new definition, Pluto
and the other trans-Neptunian objects do not qualify as planets. The
IAU's decision has not resolved all controversies, and while many
scientists have accepted the definition, some in the astronomical
community have rejected it outright.
History
Planets in antiquity
While knowledge of the planets predates history and is common to most civilizations, the word
planet dates back to
ancient Greece. Most Greeks believed the Earth to be stationary and at the center of the universe in accordance with the
geocentric model and that the objects in the sky, and indeed the sky itself, revolved around it. (An exception was
Aristarchus of Samos who put forward an early version of
Heliocentrism.) Greek astronomers employed the term
asteres planetai (ἀστέρες πλανῆται), "wandering stars",
[1][2] to describe those starlike lights in the heavens that moved over the course of the year, in contrast to the
asteres aplaneis (ἀστέρες ἀπλανεῖς), the "
fixed stars",
which stayed motionless relative to one another. The five bodies
currently called "planets" that were known to the Greeks were those
visible to the naked eye:
Mercury,
Venus,
Mars,
Jupiter, and
Saturn.
Graeco-Roman
cosmology commonly considered 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, as many ancient
astronomers distinguished the five star-like planets from the Sun and
Moon. 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.[3]
In his
Timaeus, written in roughly 360 BC,
Plato mentions, "the Sun and Moon and five other stars, which are called the planets".
[4] 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".
[5] In his
Phaenomena, which set to verse an astronomical treatise written by the philosopher
Eudoxus in roughly 350 BC,
[6] 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."
[7]
In his
Almagest written in the 2nd century,
Ptolemy refers to "the Sun, Moon and five planets."
[8] Hyginus explicitly mentions "the five stars which many have called wandering, and which the Greeks call Planeta."
[9] Marcus Manilius, a Latin writer 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]
The single view of the seven planets is found in
Cicero's
Dream of Scipio, written sometime around 53 BC, where the spirit of
Scipio Africanus
proclaims, "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."
[9]
Planets in the Middle Ages
Medieval and Renaissance writers generally accepted the idea of seven planets. The standard medieval introduction to astronomy,
Sacrobosco's De Sphaera, includes the Sun and Moon among the planets,
[13] the more advanced
Theorica planetarum presents the "theory of the seven planets,"
[14] while the instructions to the
Alfonsine Tables show how "to find by means of tables the mean
motuses of the sun, moon, and the rest of the planets."
[15] 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.
[16] 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";
[17]
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."
[18]
Earth
Eventually, when Copernicus's
heliocentric model was accepted over the
geocentric,
Earth
was placed among the planets and the Sun and Moon were reclassified,
necessitating a conceptual revolution in the understanding of planets.
As the
historian of science Thomas Kuhn noted in his book,
The Structure of Scientific Revolutions:
[19]
The Copernicans who denied its traditional title 'planet' to the sun
... were changing the meaning of 'planet' so that it would continue to
make useful distinctions in a world where all celestial bodies ... were
seen differently from the way they had been seen before... Looking at
the moon, the convert to Copernicanism ... says, 'I once took the moon
to be (or saw the moon as) a planet, but I was mistaken.'
Copernicus obliquely refers to Earth as a planet in
De Revolutionibus
when he says, "Having thus assumed the motions which I ascribe to the
Earth later on in the volume, by long and intense study I finally found
that if the motions of the other planets are correlated with the
orbiting of the earth..."
[17] Galileo also asserts that Earth is a planet in the
Dialogue Concerning the Two Chief World Systems: "[T]he Earth, no less than the moon or any other planet, is to be numbered among the natural bodies that move circularly."
[20]
Modern planets
William Herschel, discoverer of Uranus
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. As the asteroids
had yet to be discovered, comets were the only moving objects one
expected to find in a telescope.
[21]
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."
[22]
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 a search which did not find the perturbing object (it was later
found to be a mathematical artefact caused by an overestimation of
Neptune's mass) but did find
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.
[21] In 1989,
Voyager 2 determined the irregularities to be due to an overestimation of Neptune's mass.
[23]
Satellites
When Copernicus placed Earth among the planets, he also placed the Moon in orbit around Earth, making the Moon the first
natural satellite to be identified. When
Galileo discovered his four
satellites
of Jupiter in 1610, they lent weight to Copernicus's argument, because
if other planets could have satellites, then Earth could too. However,
there remained some confusion as to whether these objects were
"planets"; Galileo referred to them as "four planets flying around the
star of Jupiter at unequal intervals and periods with wonderful
swiftness."
[24] 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" (attendant).
[25] 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").
[26] However, when the "Journal de Scavans" reported Cassini's discovery of
two new Saturnian moons in 1686, it referred to them strictly as
"satellites", though sometimes Saturn as the "primary planet".
[27]
When William Herschel announced his discovery of two objects in orbit
around Uranus in 1787, he referred to them as "satellites" and
"secondary planets".
[28] All subsequent reports of natural satellite discoveries used the term "satellite" exclusively,
[29] though the 1868 book "Smith's Illustrated Astronomy" referred to satellites as "secondary planets".
[30]
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.
[31]
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").
[32] Even so, in 1804, another world,
Juno, was discovered in a similar orbit.
[31] 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.
[31]
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.
[33][34] Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets.
[31] 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.
[34]
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.
[31]
To this day, "minor planet" remains the official designation for all
small bodies in orbit around the Sun, and each new discovery is numbered
accordingly in the IAU's
Minor Planet Catalogue.
[35]
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 and 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 Pluto's largest moon,
Charon,
which allowed them to determine its mass. Pluto was found to be much
tinier than anyone had expected: only one sixth the mass of Earth's
Moon. However, as far as anyone could yet tell, it was unique. Then,
beginning in 1992, astronomers began to detect large numbers of icy
bodies beyond the orbit of Neptune that were similar to Pluto in
composition, size, and orbital characteristics. 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.
[36]
Pluto's orbit lay within this band and thus its planetary status was
thrown into question. Many scientists concluded that tiny Pluto should
be reclassified as a minor planet, just as Ceres had been a century
earlier.
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."
[37] Those objects under that mass limit would become minor planets. In 1999,
Brian G. Marsden of
Harvard University's
Minor Planet Center suggested that Pluto be given the
minor planet number 10000 while still retaining its official position as a planet.
[38][39] The prospect of Pluto's "demotion" created a public outcry, and in response the
International Astronomical Union clarified that it was not at that time proposing to remove Pluto from the planet list.
[40][41]
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
more massive than Pluto,
[42] named
Eris.
[43]
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.
[44]
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.
[45]
IAU definition
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:
Michael E Brown, discoverer of Eris
- 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 neighbourhood. (six votes in favour)[46][47]
Since no 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,
[48]
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.
[49][50] The vote was passed, with 424 astronomers taking part in the ballot.
[51][52][53]
“ |
The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, 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)
shape2, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3, 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 recognised as the prototype of a new category of trans-Neptunian objects.
|
” |
Artistic comparison of
Pluto,
Eris,
Makemake,
Haumea,
Sedna,
2002 MS4,
2007 OR10,
Quaoar,
Salacia,
Orcus, and
Earth along with the
Moon.
The IAU also resolved that "
planets and
dwarf planets are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets.
[53]
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.
[54] Other
possible dwarf planets, such as
2003 EL61,
2005 FY9,
Sedna and
Quaoar, were left in temporary limbo until a formal decision could be reached regarding their status.
On June 11, 2008, the IAU executive committee announced the
establishment of a subclass of dwarf planets comprising the
aforementioned "new category of trans-Neptunian objects" to which Pluto
is a prototype. This new class of objects, termed
plutoids,
would include Pluto, Eris and any other future trans-Neptunian dwarf
planets, but excluded Ceres. The IAU also determined that, for naming
purposes, only those TNOs with an
absolute magnitude brighter than H = +1 would be allowed into the category. To date, only two other TNOs, 2003 EL
61 and 2005 FY
9, meet the absolute magnitude requirement, while other potential dwarf planets, such as Sedna, Orcus and Quaoar, do not.
[55] On July 11, 2008, the Working Group on Planetary Nomenclature included 2005 FY
9 in the plutoid class, naming it
Makemake.
[56] On September 17, 2008, 2003 EL
61 joined the category with the name
Haumea.
[57]
Acceptance of the IAU definition
Among the most vocal proponents of the IAU's decided definition are
Mike Brown, the discoverer of Eris;
Steven Soter, professor of astrophysics at the
American Museum of Natural History; and
Neil deGrasse Tyson, director of the
Hayden Planetarium.
In the early
2000s,
when the Hayden Planetarium was undergoing a $100 million renovation,
Tyson refused to refer to Pluto as the ninth planet at the planetarium.
[58]
He explained that he would rather group planets according to their
commonalities rather than counting them. This decision resulted in Tyson
receiving large amounts of hate mail, primarily from children.
[59] In 2009, Tyson
wrote a book detailing the demotion of Pluto.
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 striking; if Soter's concept that each planet
occupies an "orbital zone"
[b]
is accepted, 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 object in the asteroid belt, only accounts for
one third of the material in its orbit; Pluto's ratio is even lower, at
around 7 percent.
[60]
Mike Brown asserts that this massive difference in orbital dominance
leaves "absolutely no room for doubt about which objects do and do not
belong."
[61]
Ongoing controversies
Despite
the IAU's declaration, a number of critics remain unconvinced. The
definition is seen by some as arbitrary and confusing. 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.
Stern's claim is that, since less than 5 percent of astronomers voted
for it, the decision was not representative of the entire astronomical
community.
[51][62] Even with this controversy excluded, however, there remain several ambiguities in the definition.
Clearing the neighbourhood
One of the main points at issue is the precise meaning of "cleared the neighbourhood around its
orbit".
Alan Stern objects that "it is impossible and contrived to put a dividing line between dwarf planets and planets,"
[63]
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.
[c]
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.
[61]
In October 2015, astronomer
Jean-Luc Margot of the
University of California Los Angeles proposed a metric for orbital zone clearance derived from whether an object can clear an orbital zone of extent 2
√3 of its
Hill radius
in a specific time scale. This metric places a clear dividing line
between the dwarf planets and the planets of the solar system.
[64]
The calculation is based on the mass of the host star, the mass of the
body, and the orbital period of the body. An Earth-mass body orbiting a
solar-mass star clears its orbit at distances of up to 400
astronomical units
from the star. A Mars-mass body at the orbit of Pluto clears its orbit.
This metric, which leaves Pluto as a dwarf planet, applies to both the
Solar System and to extrasolar systems.
[64]
Some opponents of the definition have claimed that "clearing the
neighbourhood" is an ambiguous concept. Mark Sykes, director of the
Planetary Science Institute in Tucson, Arizona, and organiser of the
petition, expressed this opinion to
National Public Radio.
He believes that the definition does not categorise a planet by
composition or formation, but, effectively, by its location. He believes
that a Mars-sized or larger object beyond the orbit of Pluto would not
be considered a planet, because he believes that it would not have time
to clear its orbit.
[65]
Brown notes, however, that were the "clearing the neighbourhood"
criterion to be abandoned, the number of planets in the Solar System
could rise from eight to
more than 50, with hundreds more potentially to be discovered.
[66]
Hydrostatic equilibrium
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 round,
ellipsoidal
shape. Up to a certain mass, an object can be irregular in shape, but
beyond that point gravity begins to pull an object towards its own
centre of mass until the object collapses into an ellipsoid. (None of the large objects of the Solar System are truly spherical. Many are
spheroids, and several, such as the larger moons of Jupiter and Saturn and the dwarf planet
Haumea, have been further distorted into ellipsoids by rapid rotation or
tidal forces, but still in hydrostatic equilibrium.
[67])
However, there is no precise 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."
[60]
Furthermore, the point at which an object's mass compresses it into an
ellipsoid varies depending on the chemical makeup of the object. Objects
made of ices,
[d] such as Enceladus and Miranda, assume that state more easily than those made of rock, such as Vesta and Pallas.
[66] Heat energy, from
gravitational collapse,
impacts, tidal forces, or
radioactive decay,
also factors into whether an object will be ellipsoidal or not;
Saturn's icy moon Mimas is ellipsoidal, but Neptune's larger moon
Proteus,
which is similarly composed but colder because of its greater distance
from the Sun, is irregular. In addition, the much larger Iapetus is
ellipsoidal but does not have the dimensions expected for its current
speed of rotation, indicating that it was once in hydrostatic
equilibrium but no longer is.
[68]
Double planets and moons
The definition specifically excludes
satellites from the category of dwarf planet, though it does not directly define the term "satellite".
[53] 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, even though they
are similar in relative size, only Pluto would currently be classified
as a dwarf planet.
[53]
A diagram illustrating the
Moon's co-orbit with the
Earth
However, some have suggested that the Moon nonetheless deserves to be called a planet. In 1975,
Isaac Asimov noted that the timing of the Moon's orbit is in tandem with the Earth's own orbit around the Sun—looking down on the
ecliptic, the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right.
[69]
Also many moons, even those that do not orbit the Sun directly, often
exhibit features in common with true planets. There are 19 moons in the
Solar System that have achieved hydrostatic equilibrium and would be
considered planets if only the physical parameters are considered. Both
Jupiter's moon
Ganymede and Saturn's moon
Titan are larger 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, some
astronomers argue that objects in orbit around planets that share all
their characteristics could also be called planets.
[70][71][72] Indeed, Mike Brown makes just such a claim in his dissection of the issue, saying:
[61]
It is hard to make a consistent argument that a 400 km iceball should
count as a planet because it might have interesting geology, while a
5000 km satellite with a massive atmosphere, methane lakes, and dramatic
storms [Titan] shouldn't be put into the same category, whatever you
call it.
However, he goes on to say that, "For most people, considering round
satellites (including our Moon) "planets" violates the idea of what a
planet is."
[61]
Alan Stern
has argued that location should not matter and that only geophysical
attributes should be taken into account in the definition of a planet,
and proposes the term
satellite planet for a planet-sized satellite.
[73]
Extrasolar planets and brown dwarfs
The discovery since 1992 of
extrasolar planets, or planet-sized objects around other stars (3,767 such planets in 2,816
planetary systems including 628
multiple planetary systems as of 1 May 2018),
[74]
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.
[75] 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.
[76] As yet it is uncertain whether gas giants possess such cores, though the
Juno mission
to Jupiter could resolve the issue. 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 should be considered an orbiting low-mass star rather
than a planet.
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.
[77] 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."
[78] Burrows also claims that such stellar remnants as
white dwarfs should not be considered stars,
[79] 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 sustain atomic fusion during its lifetime should be
considered a star.
[80]
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).
[81] These have been described as "
free floating planets" because current theories of Solar System formation suggest that planets may be ejected from their
star systems altogether if their orbits become unstable.
[82] However, it is also possible that these "free floating planets" could have formed in the same manner as stars.
[83]
In 2003, a working group of the IAU released a position statement
[84]
to define what constitutes an extrasolar planet and what constitutes a
brown dwarf. To date, it remains the only guidance offered by the IAU on
this issue. The 2006 planet definition committee did not attempt to
challenge it, or to incorporate it into their definition, claiming that
the issue of defining a planet was already difficult to resolve without
also considering extrasolar planets.
[85]
“ |
- Objects with true masses
below the limiting mass for thermonuclear fusion of deuterium
(currently calculated to be 13 Jupiter masses for objects of solar metallicity)
that orbit stars or stellar remnants are "planets" (no matter how they
formed). The minimum mass required for an extrasolar object to be
considered a planet should be the same as that used in the Solar System.
- Substellar objects with true masses above the limiting mass for
thermonuclear fusion of deuterium are "brown dwarfs", no matter how they
formed nor where they are located.
- Free-floating objects in young star clusters with masses below the
limiting mass for thermonuclear fusion of deuterium are not "planets",
but are "sub-brown dwarfs" (or whatever name is most appropriate).
|
” |
CHXR 73 b, an object which lies at the border between planet and brown dwarf
This definition makes 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 around a fusing star is a planet, even if, in all
other respects, the two objects may be identical. Further, in 2010, a
paper published by Burrows, David S. Spiegel and John A. Milsom called
into question the 13-Jupiter-mass criterion, showing that a brown dwarf
of three times solar
metallicity could fuse deuterium at as low as 11 Jupiter masses.
[86]
Also, the 13 Jupiter-mass cutoff does not have precise physical
significance. Deuterium fusion can occur in some objects with mass below
that cutoff. The amount of deuterium fused depends to some extent on
the composition of the object.
[86] The
Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 M
Jup in the observed mass spectrum reinforces the choice to forget this mass limit,".
[87] The
Exoplanet Data Explorer
includes objects up to 24 Jupiter masses with the advisory: "The 13
Jupiter-mass distinction by the IAU Working Group is physically
unmotivated for planets with rocky cores, and observationally
problematic due to the
sin i ambiguity."
[88] The
NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.
[89]
Another criterion for separating planets and brown dwarfs, rather
than deuterium burning, formation process or location, is whether the
core
pressure is dominated by
coulomb pressure or
electron degeneracy pressure.
[90][91]
Planetary-mass stellar objects
The ambiguity inherent in the IAU's definition was highlighted in December 2005, when the
Spitzer Space Telescope observed
Cha 110913-773444 (above), only eight times Jupiter's mass with what appears to be the beginnings of its own
planetary system. Were this object found in orbit around another star, it would have been termed a planet.
[92]
In September 2006, 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.
[93]
In 2012, Philippe Delorme, of the
Institute of Planetology and Astrophysics of
Grenoble in France announced the discovery of
CFBDSIR 2149-0403; an independently moving 4-7 Jupiter-mass object that likely forms part of the
AB Doradus moving group, less than 100 light years from Earth. Although it shares its spectrum with a
spectral class T brown dwarf, Delorme speculates that it may be a planet.
[94]
In October 2013, astronomers led by Dr. Michael Liu of the
University of Hawaii discovered
PSO J318.5-22, a solitary free-floating
L dwarf estimated to possess only 6.5 times the mass of Jupiter, making it the least massive
sub-brown dwarf yet discovered.
[95]
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
Language Log
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)."
[96] 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."
[97]
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."
[98]
Conversely, astronomer Robert Cumming of the Stockholm Observatory
notes that, "The name 'minor planet' [has] 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'."
[96]