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Pluto
Pluto
Discovery
Discovered by
Clyde W. Tombaugh
Discovered on
February 18, 1930
Orbital characteristics (Epoch J2000)
Semi-major axis
5,906,376,272 km
39.481 686 77 AU
Orbital circumference
36.530 Tm
244.186 AU
Eccentricity
0.248 807 66
Perihelion
4,436,824,613 km
29.658 340 67 AU
Aphelion
7,375,927,931 km
49.305 032 87 AU
Orbital period
90,613.3055 d
(248.09 a)
Synodic period
366.73 d
Avg. Orbital Speed
4.666 km/s
Max. Orbital Speed
6.112 km/s
Min. Orbital Speed
3.676 km/s
Inclination
17.141 75°
(11.88° to Sun's equator)
Longitude of the
ascending node
110.303 47°
Argument of the
perihelion
113.763 29°
Number of satellites
3
Physical characteristics
Diameter
2390 km
(19% of Earth, or
1485 mi)
Surface area
1.795×107 km²
(0.033 Earths)
Volume
7.15×109 km³
(0.0066 Earths)
Mass
(1.305±0.007)×1022 kg
(0.0021 Earths)
Mean density
2.03±0.06 g/cm³
Equatorial gravity
0.58 m/s²
(0.059 g)
Escape velocity
1.2 km/s
Rotation period
−6.387230 d
(6 d 9 h 17 m 36 s)
Rotation velocity
47.18 km/h (at the equator)
Axial tilt
119.59° (to orbit)
112.78° (to the ecliptic)
Right ascension
of North pole
133.045±0.02°
(8 h 52 min 11 s)
Declination
-6.145±0.02°
Albedo
0.49–0.66 (varies by 35%)
Surface temp.
min mean max
33 K 44 K 55 K
Atmospheric characteristics
Atmospheric pressure
0.30 pascals (summer maximum)
Composition
nitrogen, methane
Pluto (IPA: / plu təʊ/), designated (134340) Pluto in the Minor Planet Center
catalogue, is the second-largest known dwarf planet in the solar system. It orbits
between 29 and 49 AU from the Sun, and was the first Kuiper Belt Object to be
discovered. Approximately one-fifth the mass of the Earth's Moon, Pluto is primarily
composed of rock and ice. It has an eccentric orbit that is highly inclined with respect
to the planets and takes it closer to the Sun than Neptune during a portion of its orbit.
Pluto and its largest satellite, Charon, could be considered a binary system because
they are closer in size than any of the other known planetoid/moon combinations in
the solar system, and because the barycentre of their orbits does not lie within either
body. However, the International Astronomical Union (IAU) has yet to formalize a
definition for binary dwarf planets, so Charon is regarded as a moon of Pluto. Two
smaller moons, Nix and Hydra, were discovered in 2005. Pluto is smaller than several
of the natural satellites or moons in our solar system (see the list of solar system
objects by radius).
From its discovery by Clyde Tombaugh in 1930, Pluto was considered the ninth
planet from the Sun. In the late 20th and early 21st century, many similar objects were
discovered in the outer solar system, most notably the Trans-Neptunian object Eris
which is slightly larger than Pluto. In August 2006 the IAU redefined the term
"planet", and classified Pluto, Ceres, and Eris as dwarf planets. Pluto is also classified
as the prototype of a family of trans-Neptunian objects. After the reclassification,
Pluto was added to the list of minor planets and given the number 134340.
Discovery
Discovery photographs of Pluto
In 1930 Clyde Tombaugh was working on a project searching for a ninth planet at
Lowell Observatory. Tombaugh's work was to systematically take pictures of the
celestial sky in pairs, one to two weeks apart, then look for objects that had moved
between images. On February 18, 1930, Tombaugh discovered a possible moving
object on photographic plates taken on January 23 and January 29 of that year. A
lesser-quality photo taken on January 20 helped confirm the movement. After the
observatory worked to obtain further confirmatory photographs, news of the
discovery was telegraphed to the Harvard College Observatory on March 13, 1930.
Pluto would later be found on photographs dating back to March 19, 1915.
Relations to Neptune and Uranus
The history of how Pluto was discovered is intertwined with the discoveries of
Neptune and Uranus. In the 1840s, using Newtonian mechanics, Urbain Le Verrier,
and John Couch Adams had correctly predicted the position of the then-undiscovered
planet Neptune after analysing perturbations in the orbit of Uranus. Theorizing the
perturbations were caused by the gravitational pull of another planet, Johann Gottfried
Galle discovered Neptune on September 23, 1846.
Observations of Neptune in the late 19th century had astronomers starting to speculate
that Neptune's orbit too was also being disturbed by another planet in a similar
manner that Neptune was disturbing Uranus. By 1909, William H. Pickering and
Percival Lowell had suggested several possible celestial coordinates for such a planet.
In May 1911, the Bulletin of the Astronomical Society of France published
calculations by Indian astronomer V.B. Ketakar which predicted a location for an
undiscovered planet.
Physical characteristics
Diagram of Pluto (top left) and its moons (top right) compared in size, albedo and
color index with the largest plutinos: Orcus (bottom left) and Ixion (bottom right).
Many details about Pluto remain unknown, mainly due to the fact that it has not yet
been visited up close by spacecraft. Pluto's distance from Earth makes in-depth
investigation difficult.
Mass and size
Pluto's volume is about 0.66% that of Earth's
Pluto's diameter and mass were incorrectly overestimated for many decades after its
discovery. Initially it was thought to be relatively large, with a mass comparable to
Earth, but over time the estimates were revised sharply downward as observations
were refined.
The discovery of its 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.
Originally it was believed that Pluto was larger than Mercury but smaller than Mars,
but that calculation was based on the premise that a single object was being observed.
Once it was realized that there were two objects instead of one, the estimated size of
Pluto was revised downward. Observations were able to determine Pluto's diameter
when it is at occultation with Charon, and its shape can be resolved by telescopes
using adaptive optics.
Pluto (bottom right) compared in size to the largest satellites in the solar system (from
left to right and top to bottom): Ganymede, Titan, Callisto, Io, The Moon, Europa, and
Triton.
Among the objects of the Solar System, Pluto is not only smaller and much less
massive than any planet, but at less than 0.2 lunar masses it is also smaller and less
massive than seven of the moons: Ganymede, Titan, Callisto, Io, The Moon, Europa
and Triton. Pluto is more than twice the diameter and a dozen times the mass of
Ceres, a dwarf planet in the asteroid belt. However, it is smaller than trans-Neptunian
Kuiper belt object Eris, discovered in 2005.
Planetary status controversy
Pluto's official status as a planet has been a constant subject of controversy, fueled by
the past lack of a clear definition of planet, since at least as early as 1992, when the
first Kuiper Belt Object, (15760) 1992 QB1, was discovered. Since then, further
discoveries intensified the debate in the 21st century.
New discoveries ignite debate
Pluto compared to Eris, 2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar, and Varuna
compared to Earth (artist's impressions; no detailed photographs exist).
Continuing advances in telescope technology allowed for further discoveries of TransNeptunian objects in the 21st century, some of comparable size to that of Pluto. In
2002, 50000 Quaoar was discovered, with a 1,280 kilometers diameter, making it a bit
more than half the size of Pluto. In 2004, the discoverers of 90377 Sedna placed an
upper limit of 1,800 kilometers on its diameter, near Pluto's diameter of 2,320
kilometers.
On July 29, 2005, a Trans-Neptunian object later named Eris was announced, which
on the basis of its magnitude and simple albedo considerations is assumed to be
slightly larger than Pluto. This was the largest object discovered in the solar system
since Neptune in 1846. Discoverers and media 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 to be the strongest
argument for reclassifying Pluto as a minor planet.
The last remaining distinguishing feature of Pluto was now its large moon, Charon,
and its atmosphere; these characteristics are probably not unique to Pluto: several
other Trans-Neptunian objects have satellites; and Eris' spectrum suggests that it has a
similar surface composition to Pluto, as well as a moon, Dysnomia, discovered in
September 2005. Trans-Neptunian object 2003 EL61 (nicknamed "Santa") has two
moons (one of which is nicknamed "Rudolph") and is the fourth largest TNO behind
Eris, Pluto, and 2005 FY9 (nicknamed "Easterbunny").
IAU Decision
The debate came to a head in 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 a sphere by its own gravitational
force. 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. The IAU further resolved that Pluto be
classified in the simultaneously created dwarf planet category, and that it act as
prototype for a yet-to-be-named category of trans-Neptunian objects, in which it
would be separately, but concurrently, classified.
Prior to this decision several other definitions had been proposed, some of which
might have ruled out planetary status for Earth or Mercury or may have classified
several of the asteroids as planets. This version was democratically chosen in a
successful attempt at avoiding these non-traditional results.
Impact of the IAU decision
There has been resistance amongst the astronomical community towards the
reclassification, dubbed the "Great Pluto War" by some astronomers. Alan Stern,
principal investigator with NASA's "New Horizons" mission to Pluto, has publicly
derided the IAU resolution, stating that "the definition stinks" albeit "for technical
reasons." Stern's current contention is that by the terms of the new definition Earth,
Mars, Jupiter and Neptune, all of which share their orbits with asteroids would be
excluded. However, his own published writing has supported the new list of planets,
as "our solar system clearly contains" eight planets that have cleared their
neighborhoods. 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."
Among the general public, reception is mixed amidst widespread media coverage.
Some have accepted the reclassification, while some are seeking to overturn the
decision, with online petitions urging the IAU to consider reinstatement. A resolution
introduced by some members of the California state assembly light-heartedly
denounces the IAU for "scientific heresy," among other crimes. Others reject the
change for sentimental reasons, citing that they have always known Pluto as a planet
and will continue to do so regardless of the IAU decision.