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Definition of planet

2007 Schools Wikipedia Selection. Related subjects: The Planets

   Photograph of the planet Neptune and its moon Triton, taken by Voyager
   2 as it entered the outer solar system.
   Enlarge
   Photograph of the planet Neptune and its moon Triton, taken by Voyager
   2 as it entered the outer solar system.

   The definition of "planet" has been the subject of intense debate in
   recent years. Although the word itself dates back millennia, there was
   no official scientific definition of a "planet" until the early 21st
   century.

   Traditionally, the term was only applied to objects in the Solar
   System, and any differences there 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 are nearly large enough to be stars, while others are
   smaller than our Moon, and they have 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 accepted planet, Pluto. In response, the
   International Astronomical Union (IAU), recognised by astronomers as
   the international body responsible for resolving issues of astronomical
   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.

History

   The planets as understood before the acceptance of the heliocentric
   model.
   Enlarge
   The planets as understood before the acceptance of the heliocentric
   model.

   The word "planet" has meant many things in its long life, some of them
   contradictory. When originally coined by the ancient Greeks, a planet
   was any object that appeared to wander against the field of fixed stars
   that made up the night sky (asteres planetai "wandering stars"). This
   included not only the five "classical" planets, that is, Mercury,
   Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon (the "
   seven heavenly objects"). However, a distinction was occasionally made
   in terminology; the "five planets" (excluding the Sun and the Moon)
   were referred to alongside the "seven planets" (including the Sun and
   the Moon), so that the term "planet", even at this early stage, had
   acquired ambiguity.

   Eventually, when the heliocentric model was accepted over the
   geocentric, Earth was placed among their number and the Sun was
   dropped, and after Galileo discovered his four satellites of Jupiter,
   the Moon was also eventually reclassified. However, the Galilean
   satellites of Jupiter (in 1610), Saturn's satellite Titan in 1659, and
   Iapetus and Rhea in 1673 were initially described as "planets", not
   "moons"; the word "moon" at that time only referred to Earth's Moon.

   In 1781, the astronomer William Herschel was searching the sky for
   binary stars when he observed what he termed a comet in the
   constellation of Taurus. That this strange object might have been a
   planet simply did not occur to him; 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. Eventually it was recognised as the seventh
   planet and named Uranus.

   Gravitationally induced irregularities in Uranus's observed orbit led
   eventually to the discovery of Neptune in 1846, and calculation errors
   that were thought to be irregularities in Neptune's orbit led to the
   search which ultimately located Pluto in 1930. Pluto was later
   discovered to be too small to have caused those irregularities, which
   Voyager 2 determined were due to an overestimation of Neptune's mass.

   Pluto rendered obsolete such prior considerations as roughly circular
   orbit, orbit-perturbing mass, and lying within the ecliptic, as none of
   them applied to it. Astronomers had therefore to look elsewhere for a
   definition.

Minor planets

   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.

   Then in 1802, Heinrich Olbers discovered Pallas, a second "planet" at
   roughly the same distance from the Sun as Ceres. The idea that two
   planets could occupy the same orbit was an affront to centuries of
   thinking. Some years later, another world, Juno, was discovered in a
   similar orbit. Over the following decades, several more were
   discovered, all within relatively the same orbital distance.

   Herschel suggested that these 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. Science textbooks in
   1828, after Herschel's death, still numbered the asteroids among the
   planets. By 1851, the number of asteroids had increased to 15, and a
   new method of classifying them, by adding a number before their names,
   was adopted, inadvertently placing them in their own distinct category.
   By the 1860s, observatories in Europe and the United States began
   referring to them as " minor planets", or "small planets", though it
   took the first four asteroids longer to be grouped as such.
   The relative sizes of the largest trans-Neptunian objects as compared
   to Earth.
   Enlarge
   The relative sizes of the largest trans-Neptunian objects as compared
   to Earth.

   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 in 1930. Pluto was an anomaly: a tiny, icy world in a region
   of gas giants with an orbit that carried it high above the plane of the
   ecliptic and even inside that of Neptune. 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, like Halley, with orbital periods of up to
   200 years.

   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." 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.

   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 an object confirmed to be larger than Pluto, named Eris.

   Although its discoverers (and many in the news media) immediately
   referred to it as the tenth planet, it is officially designated as a
   dwarf planet - although the Minor Planet Centre has given it a full
   minor planet designation: 136199 Eris.

IAU debate

   Eris, Charon and Ceres; the three new planets submitted under the draft
   proposal. The final proposal excluded them from planethood.
   Enlarge
   Eris, Charon and Ceres; the three new planets submitted under the draft
   proposal. The final proposal excluded them from planethood.

   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 neighbourhood. (six votes in favour)

   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, 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 among them. The vote
   was passed, though only 424 astronomers took part in the ballot.


   Definition of planet

    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)
    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.


   Definition of planet

Ongoing controversies

   Despite the IAU's declaration, a number of issues continue to be
   unresolved. 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. Even with this
   controversy excluded, 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 recently objected that "it
   is impossible and contrived to put a dividing line between dwarf
   planets and planets," 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. This would
   appear to contradict his earlier published work, in which he 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."

   In his article on the subject, Steven Soter gives one possible
   explanation. Essentially, "clearing the neighbourhood" is Mike Brown's
   definition by mass dominance: if an orbiting body is more than a
   hundred times more massive than the remaining collective mass in its
   "orbital zone", then it is a planet. Two bodies can be said to occupy
   the same "orbital zone" if their orbits cross a common distance from
   the star, or "primary", and their orbital periods differ less than an
   order of magnitude. In other words, if two bodies occupy the same
   distance from a star at one point in their orbits, and those orbits are
   similar, rather than, as a comet's would be, extending for several
   times the other's distance, then they are in the same orbital zone.

   Assuming this definition of "neighbourhood" is the one 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.

Hydrostatic equilibrium

   The asteroid Vesta illustrates the broad boundary between "irregular" &
   "spheroid".
   Enlarge
   The asteroid Vesta illustrates the broad boundary between "irregular" &
   " spheroid".

   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
   the planets Jupiter and Saturn, the moons Mimas, Enceladus and Miranda,
   and the Kuiper belt object 2003 EL[61], have been distorted into oblate
   or prolate spheroids by rapid rotation or tidal forces. However,
   deciding which objects in the solar system are spheroid is more
   complicated than it seems. In mathematical terms, spheroids consist of
   an ellipse rotated around one axis. Consequently they have two axes of
   equal length and one that is either longer or shorter; they resemble
   spheres that have been deformed (by stretching or squashing) in one
   dimension. A section through one axis will produce a circle, and a
   section through the other two axes will produce an ellipse.

   All spheroids, however, have the points on their surfaces joined by
   smooth curves (which form the elliptical or circular sections). On a
   topographically irregular body this can only be an approximation;
   however, taking such irregularity into account, a definite contrast
   exists between bodies, such as Enceladus, which are essentially
   spheroidal, and irregular bodies, like Neptune's moon Proteus, whose
   limbs do not show smooth curvature.

   If one uses this mathematical basis to define a spheroid, then the
   boundary between spheroidal and irregular objects within the solar
   system frays noticeably, as this table illustrates:
     __________________________________________________________________

   Object Dimensions (km) Mass (10^19 kg) Density (g/cm^3)^* Shape
   1 Ceres 975 × 909 95 2.08 Spheroid
   4 Vesta 578 × 560 × 478 27 3.4 Spheroid
   2 Pallas 570 × 525 × 500 22 2.8 Irregular
   Enceladus 513.2 × 502.8 × 496.6 10.8 1.61 Spheroid
   10 Hygiea 500 × 385 × 350 10 2.76 Irregular
   Miranda 480 × 468.4 × 465.8 6.59 1.20 Spheroid
   Proteus 436 × 416 × 402 5.0 1.3 Irregular
   Mimas 414.8 × 394.4 × 381.4 3.84 1.17 Spheroid
   511 Davida 326.1 3.6 2.0 Irregular
   704 Interamnia 316.6 3.3 2.0? Irregular
   Nereid 340 3.1 ? Irregular
   3 Juno 290 × 240 × 190 3.0 3.4 Irregular

   ^*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.
     __________________________________________________________________

   Plainly, there is no clear mass or size boundary dividing those objects
   in the solar system which could be considered "spheroids" and those
   which are obviously irregular. The irregular objects Pallas, Hygeia and
   Proteus are all larger than regular objects, such as Miranda and Mimas.
   Also, as demonstrated by the dimensions listed in the table, the term
   "spheroid" is, in any case, fairly loose. Vesta, by the above
   forumulation, could be considered either a spheroid or irregular. (see
   image)

   Also, there is no one point at which an object can be said to have
   reached hydrostatic equilibrium. Objects made of ices, such as
   Enceladus and Miranda, assume that state more easily than those made of
   rock, such as Vesta and Pallas. 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

   A telescopic image of Pluto and Charon.
   Enlarge
   A telescopic image of Pluto and Charon.

   The current 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. Rather than one orbiting the other,
   both orbit each other like the tips of a spinning baton. The initial
   proposal therefore classified Pluto/Charon as a double planet: 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.

   According to the current definition, Earth and its Moon, since their
   barycenter lies within the Earth, are excluded from double planet
   status. Nonetheless they could be considered as such since, though the
   Moon orbits the Earth, the timing of its 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.
   A diagram illustrating the Moon's co-orbit with the Earth.
   Enlarge
   A diagram illustrating the Moon's co-orbit with the Earth.

   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.

   It could be argued that, just as stars in orbit around other stars are
   still referred to as stars, thus objects in orbit around planets that
   share all their characteristics could also be called planets.

Extrasolar planets and brown dwarfs

   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, conversely, are small
   enough to be considered planets.
   The brown dwarf Gliese 229B in orbit around its star.
   Enlarge
   The brown dwarf Gliese 229B in orbit around its star.

   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. 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." Similarly, an orbiting white dwarf, such as
   Sirius B, since it too has ceased fusion, 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.

   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). 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. One could therefore argue
   that the original criterion that a planet must orbit a star should
   instead be amended to indicate that it must have originated in orbit
   around a star. This, however, would make large captured satellites such
   as Neptune's Triton into planets.
   The solitary sub-brown dwarf Cha 110913-773444 (middle), the least
   massive brown dwarf yet found, set to scale against the Sun (left) and
   the planet Jupiter (right).
   Enlarge
   The solitary sub-brown dwarf Cha 110913-773444 (middle), the least
   massive brown dwarf yet found, set to scale against the Sun (left) and
   the planet Jupiter (right).

   However, it is also possible that these "free floating planets" could
   have formed in the same manner as stars. 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. 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 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.


   Definition of planet

    1. 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/size required for an extrasolar object to be considered a
       planet should be the same as that used in our Solar System.
    2. 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.
    3. 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).


   Definition of planet

   Like defining a " minor planet" by clearing 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, 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.

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. Arguably, this can confuse people. This is met by the
   observation that, if so, the term 'minor planets' (which are also not
   planets) would share the same difficulties, although this term has been
   in use for many years, and that therefore 'dwarf planet' (and 'minor
   planet') is best considered as being a compound noun. 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' [sic] (not really a rabbit) and 'Rocky Mountain oysters' (not
   really oysters)." As Dava Sobel, 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 ["dwarf planet" is] a term no
   one can love."

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