Pluto - The controversial Planet
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune.
It was the first Kuiper belt object to be discovered. It is the largest and
second-most-massive known dwarf planet in the Solar System and the
ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known
trans-Neptunian object by volume but is less massive than Eris, a dwarf
planet in the scattered disc. Like other Kuiper belt objects, Pluto
is primarily made of ice and rock[13] and is
relatively small—about one-sixth the mass of the Moon and one-third its volume. It has a
moderately eccentric and inclined orbit during which it ranges
from 30 to 49 astronomical units or AU (4.4–7.3 billion km) from the
Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding.
In 2014, Pluto was 32.6 AU from the Sun. Light from the Sun takes about 5.5
hours to reach Pluto at its average distance (39.4 AU).
Pluto
was discovered by Clyde Tombaugh in 1930, and was originally considered the ninth planet from the Sun. After 1992, its status as a planet fell
into question following the discovery of several objects of similar size in the
Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was
discovered, which led the International
Astronomical Union (IAU)
to define the term
"planet" formally
for the first time the following year. This
definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid).
Pluto
has five known moons: Charon (the
largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of
their orbits does not lie within either body. The IAU has not formalized a definition for
binary dwarf planets, and Charon is officially classified as a moon of
Pluto.
On July
14, 2015, the New Horizons spacecraft became
the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations
of Pluto and its moons.
Classification
From
1992 onward, many bodies were discovered orbiting in the same area as Pluto,
showing that Pluto is part of a population of objects called the Kuiper belt. This made its official status as a
planet controversial, with many questioning whether Pluto should be considered
together with or separately from its surrounding population. Museum and
planetarium directors occasionally created controversy by omitting Pluto from
planetary models of the Solar System. The Hayden Planetarium reopened—in February 2000, after
renovation—with a model of only eight planets, which made headlines almost a
year later.
As
objects increasingly closer in size to Pluto were discovered in the region, it
was argued that Pluto should be reclassified as one of the Kuiper belt objects,
just as Ceres, Pallas, Juno and Vesta eventually
lost their planet status after the discovery of many other asteroids. On July 29, 2005, astronomers at Caltech announced
the discovery of a new trans-Neptunian object, Eris, which was
substantially more massive than Pluto and the most massive object discovered in
the Solar System since Triton in
1846. Its discoverers and the press initially called it the tenth planet, although there was no official
consensus at the time on whether to call it a planet. Others in the astronomical community
considered the discovery the strongest argument for reclassifying Pluto as a
minor planet.
IAU classification
The
debate came to a head on August 24, 2006 with an IAU resolution that created an official definition for the
term "planet". According to this resolution, there are three main
conditions for an object to be considered a "planet":
1.
The object must be in orbit around the Sun.
2.
The object must be massive enough to be rounded by its own
gravity. More specifically, its own gravity should pull it into a shape of hydrostatic
equilibrium.
3.
It must have cleared the
neighborhood around
its orbit.
Pluto
fails to meet the third condition, because its mass is only 0.07 times that of
the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7
million times the remaining mass in its own orbit). The IAU further decided that bodies that,
like Pluto, meet criteria 1 and 2 but do not meet criterion 3 would be called dwarf planets. On September 13, 2006, the IAU
included Pluto, and Eris and its moon Dysnomia, in their Minor Planet Catalogue,
giving them the official minor planet designations "(134340) Pluto", "(136199)
Eris", and "(136199) Eris I Dysnomia". Had Pluto been included upon its discovery
in 1930, it would have likely been designated 1164, following 1163 Saga, which was discovered a month earlier.
There
has been some resistance within the astronomical community toward the
reclassification. Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, publicly derided the IAU
resolution, stating that "the definition stinks, for technical
reasons". Stern's
contention was that, by the terms of the new definition, Earth, Mars, Jupiter,
and Neptune, all of which share their orbits with asteroids, would be excluded. He argued that all big spherical moons,
including the Moon,
should likewise be considered planets. His
other claim was that because less than five percent of astronomers voted for
it, the decision was not representative of the entire astronomical community. Marc W. Buie, then at Lowell Observatory, voiced his
opinion on the new definition on his website and petitioned against the definition. Others have supported the IAU. Mike Brown,
the astronomer who discovered Eris, said
"through this whole crazy circus-like procedure, somehow the right answer
was stumbled on. It's been a long time coming. Science is self-correcting
eventually, even when strong emotions are involved."
Public
reception to the IAU decision was mixed. Although many accepted the
reclassification, some sought to overturn the decision with online petitions
urging the IAU to consider reinstatement. A resolution introduced by some
members of the California State Assembly facetiously called the IAU decision a
"scientific heresy". The New
Mexico House of Representatives passed a resolution in honor of Tombaugh, a
longtime resident of that state that declared that Pluto will always be
considered a planet while in New Mexican skies and that March 13, 2007, was
Pluto Planet Day. The Illinois Senate passed a similar resolution in 2009, on the
basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The
resolution asserted that Pluto was "unfairly downgraded to a 'dwarf'
planet" by the IAU. Some
members of the public have also rejected the change, citing the disagreement
within the scientific community on the issue, or for sentimental reasons,
maintaining that they have always known Pluto as a planet and will continue to
do so regardless of the IAU decision.
In
2006, in its 17th annual words of the year vote, the American Dialect Society voted plutoed as the
word of the year. To "pluto" is to "demote or devalue someone or
something".
Researchers
on both sides of the debate gathered on August 14–16, 2008, at the Johns
Hopkins University Applied Physics Laboratory for a conference that included back-to-back
talks on the current IAU definition of a planet. Entitled "The Great Planet
Debate", the
conference published a post-conference press release indicating that scientists
could not come to a consensus about the definition of planet. Just before the conference, on June 11,
2008, the IAU announced in a press release that the term "plutoid" would henceforth be used to refer to
Pluto and other objects that have an orbital semi-major axis greater than that of Neptune and enough
mass to be of near-spherical shape.
Orbit
Pluto's
orbital period is 248 Earth years. Its orbital characteristics are
substantially different from those of the planets, which follow nearly circular
orbits around the Sun close to a flat reference plane called the ecliptic. In contrast, Pluto's orbit is moderately inclined relative to the ecliptic (over 17°) and
moderately eccentric (elliptical). This eccentricity means a small region
of Pluto's orbit lies nearer the Sun than Neptune's. The Pluto–Charon barycenter came to perihelion on September 5, 1989, and was last closer to the Sun than Neptune
between February 7, 1979, and February 11, 1999.
In the
long term, Pluto's orbit is chaotic. Although computer simulations can be used to
predict its position for several million years (both forward and backward in time), after intervals longer than the Lyapunov time of 10–20 million years, calculations become
speculative: Pluto is sensitive to immeasurably small details of the Solar
System, hard-to-predict factors that will gradually change Pluto's position in
its orbit.
Relationship with Neptune
Despite
Pluto's orbit appearing to cross that of Neptune when viewed from directly
above, the two objects' orbits are aligned so that they can never collide or
even approach closely. There are several reasons why.
At the
simplest level, one can examine the two orbits and see that they do not
intersect. When Pluto is closest to the Sun, and hence closest to Neptune's
orbit as viewed from above, it is also the farthest above Neptune's path.
Pluto's orbit passes about 8 AU above
that of Neptune, preventing a collision. Pluto's ascending and descending nodes, the points at which
its orbit crosses the ecliptic, are currently separated from Neptune's by over
21°.
This alone
is not enough to protect Pluto; perturbations from the planets (especially Neptune) could
alter aspects of Pluto's orbit (such as its orbital precession)
over millions of years so that a collision could be possible. Some other
mechanism or mechanisms must therefore be at work. The most significant of
these is that Pluto lies in the 2:3 mean-motion resonance with Neptune: for every two orbits that Pluto makes around
the Sun, Neptune makes three. The two objects then return to their initial
positions and the cycle repeats, each cycle lasting about 500 years. This
pattern is such that, in each 500-year cycle, the first time Pluto is near perihelion, Neptune is over 50° behind Pluto. By Pluto's second perihelion,
Neptune will have completed a further one and a half of its own orbits, and so
will be a similar distance ahead of Pluto. Pluto and Neptune's minimum
separation is over 17 AU, which is greater than Pluto's minimum separation
from Uranus (11 AU).
The 2:3
resonance between the two bodies is highly stable, and is preserved over millions
of years. This
prevents their orbits from changing relative to one another; the cycle always
repeats in the same way, and so the two bodies can never pass near each other.
Thus, even if Pluto's orbit were not inclined, the two bodies could never collide.
Other factors
Numerical
studies have shown that over periods of millions of years, the general nature
of the alignment between the orbits of Pluto and Neptune does not change. There are several other resonances and
interactions that govern the details of their relative motion, and enhance
Pluto's stability. These arise principally from two additional mechanisms
(besides the 2:3 mean-motion resonance).
First,
Pluto's argument of perihelion,
the angle between the point where it crosses the ecliptic and the point where
it is closest to the Sun, librates around
90°. This
means that when Pluto is closest to the Sun, it is at its farthest above the
plane of the Solar System, preventing encounters with Neptune. This is a direct
consequence of the Kozai mechanism, which relates the eccentricity of an orbit
to its inclination to a larger perturbing body—in this case Neptune. Relative
to Neptune, the amplitude of libration is 38°, and so the angular separation of
Pluto's perihelion to the orbit of Neptune is always greater than 52° (90°–38°). The
closest such angular separation occurs every 10,000 years.
Second,
the longitudes of ascending nodes of the two bodies—the points where they cross
the ecliptic—are in near-resonance with the above libration. When the two
longitudes are the same—that is, when one could draw a straight line through
both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and hence it
comes closest to the Sun when it is highest above Neptune's orbit. This is
known as the 1:1
superresonance. All the Jovian planets, particularly Jupiter, play a role in
the creation of the superresonance.
To
understand the nature of the libration, imagine a polar point of view, looking
down on the ecliptic from a distant vantage point where the planets orbit counterclockwise. After passing the ascending node,
Pluto is interior to Neptune's orbit and moving faster, approaching Neptune
from behind. The strong gravitational pull between the two causesangular momentum to be transferred to Pluto, at Neptune's
expense. This moves Pluto into a slightly larger orbit, where it travels
slightly more slowly, according to Kepler's third law.
As its orbit changes, this has the gradual effect of changing the perihelion
and longitude of Pluto's orbit (and, to a lesser degree, of Neptune). After
many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently
speeded up, that Neptune begins to catch up with Pluto at the opposite side of
its orbit (near the opposing node to where we began). The process is then
reversed, and Pluto loses angular momentum to Neptune, until Pluto is
sufficiently speeded up that it begins to catch Neptune again at the original
node. The whole process takes about 20,000 years to complete.
Geology
Pluto's Charon Moon
Pluto's Spinning moons
Due to
Pluto's distance from Earth, in-depth study from Earth is difficult. On July
14, 2015, NASA's New Horizons space probe flew
through the Pluto system, and the information it gathered is still being
transmitted to Earth.
Surface
Pluto's
surface is composed of more than 98 percent nitrogen ice,
with traces of methane and carbon monoxide. Nitrogen and carbon monoxide are most
abundant on the anti-Charon face of Pluto (around 180° longitude, where Tombaugh Regio is located), whereas methane is most
abundant near 300° east. Pluto's
surface is quite varied, with large differences in both brightness and color. Pluto is one of the most contrastive bodies
in the Solar System, with as much contrast as Saturn's moonIapetus. The
color varies between charcoal black, dark orange and white. Pluto's color is more similar to that of Io with
slightly more orange, significantly less red than Mars. Notable geographical
features include Tombaugh Regio, or the "Heart" (a large
bright area on the side opposite Charon), Cthulhu Regio, or the "Whale" (a large dark
area on the trailing hemisphere), and the "Brass Knuckles"
(a series of equatorial dark areas on the leading hemisphere). The New Horizons science team summarized initial findings as
"Pluto displays a surprisingly wide variety of geological landforms,
including those resulting from glaciological and surface–atmosphere interactions as well
as impact, tectonic, possible cryovolcanic, and mass-wasting processes."
Internal structure
Pluto's
density is 1.860±0.013 g/cm3.[k] Because
the decay of radioactive elements would eventually heat the ices enough for the
rock to separate from them, scientists expect that Pluto's internal structure
is differentiated, with the rocky material having settled into a dense core surrounded
by a mantle of
water ice. The diameter of the core is hypothesized to be approximately 1700 km, 70%
of Pluto's diameter. It is
possible that such heating continues today, creating a subsurface ocean layer of
liquid water some100 to 180 km thick at the core–mantle boundary.
Mass
and size
Pluto's
diameter is 2372 km and its
mass is 1.31×1022 kg, which is less than 17.82% of the Moon (less than 0.24% of Earth). Its surface area is 1.665×107 km2, or
roughly the same surface area as Russia. Its surface gravity is 0.063g (compared to 1g for Earth).
The
discovery of Pluto's satellite Charon in 1978
enabled a determination of the mass of the Pluto–Charon system by application
of Newton's
formulation of Kepler's third law. Observations of Pluto in
occultation with Charon allowed scientists to establish Pluto's diameter more
accurately, whereas the invention of adaptive opticsallowed them to determine its shape
more accurately.
With
less than 0.2 lunar masses, Pluto is much less massive than the terrestrial planets,
and also less massive than seven moons, Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton. The mass is much less than thought before
Charon was discovered.
Pluto
is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the
largest object in the asteroid belt. It is less massive than the dwarf
planet Eris, a trans-Neptunian object discovered in 2005, though Pluto has a
larger diameter of 2372 km compared
to Eris's approximate diameter of 2326 km.
Determinations
of Pluto's size had been complicated by its atmosphere, and hydrocarbon haze. In March 2014, Lellouch, de Bergh et al.
published findings regarding methane mixing ratios in Pluto's atmosphere
consistent with a Plutonian diameter greater than 2360 km, with a
"best guess" of 2368 km. On July 13, 2015, images from NASA's New Horizons mission Long Range Reconnaissance Imager
(LORRI), along with data from the other instruments, determined Pluto's
diameter to be 2,370 km (1,470 mi), which was later revised to be
2,372 km (1,474 mi) on July 24.
Atmosphere
Pluto
has a thin atmosphere consisting
of nitrogen (N2), methane (CH4),
and carbon monoxide (CO), which are in equilibrium with their ices on Pluto's surface. The surface pressure ranges from 0.65 to
2.4 Pa (6.5 to
24 μbar), roughly
one million to 100,000 times less than Earth's atmospheric pressure. Pluto's
elliptical orbit is predicted to have a major effect on its atmosphere: as
Pluto moves away from the Sun, its atmosphere should gradually freeze out. When
Pluto is closer to the Sun, the temperature of Pluto's solid surface increases,
causing the ices to sublimate.
Just like sweat cools the body as it evaporates from the skin, this sublimation
cools the surface of Pluto. The
atmosphere is about 300 kilometers high.
The
presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion,
with average temperatures 36 K warmer 10 km above the surface. The lower atmosphere contains a higher
concentration of methane than its upper atmosphere.
Even
though Pluto is receding from the Sun, in 2002, the atmospheric pressure
(0.3 Pa) was higher than in 1988, because in 1987, the north pole of Pluto
came out of the shadow for the first time in 120 years, causing extra nitrogen
to start sublimating from the polar region. It will take decades for this
nitrogen to condense out of the atmosphere as it freezes onto Pluto's now
continuously dark south pole.
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