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Pluto

Pluto in True Color
Discovery
Discovered by	Clyde W. Tombaugh
Discovery date	February 18, 1930
Designations
Designation	134340 Pluto
Minor planet category	dwarf planet
Adjectives	Plutonian
Symbol
Orbital characteristics
Epoch J2000
Aphelion	7,375,927,931 km 49.305 032 87 AU
Perihelion	4,436,824,613 km 29.658 340 67 AU
Semi-major axis	5,906,376,272 km 39.481 686 77 AU
Eccentricity	0.248 807 66
Orbital period (synodic)	366.73 d
Average orbital speed	4.666 km/s
Inclination	17.141 75° (11.88° to Sun's equator)
Longitude of ascending node	110.303 47°
Argument of perihelion	113.763 29°
Known satellites	3
Physical characteristics
Mean radius	1195 km (19% of Earth, or 742.5 mi)
Surface area	1.795×10 km² (0.033 Earths)
Volume	7.15×10 km³ (0.0066 Earths)
Mass	(1.305±0.007)×10 kg (0.0021 Earths)
Mean density	2.03±0.06 g/cm³
Surface gravity	0.58 m/s² (0.059 gee)
Escape velocity	1.2 km/s
Sidereal rotation period	−6.387230 d (6 d 9 h 17 m 36 s)
Equatorial rotation velocity	47.18 km/h (at the equator)
Axial tilt	119.59° (to orbit) 112.78° (to the ecliptic)
North pole right ascension	133.045±0.02° (8 h 52 min 11 s)
North pole declination	-6.145±0.02°
Albedo	0.49–0.66 (varies by 35%)
Surface temp. min mean max Kelvin 33 K 44 K 55 K
Atmosphere
Surface pressure	0.30 pascals (summer maximum)
Composition by volume	nitrogen, methane

Pluto (IPA: /ˈpluːtəʊ/), 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 celestial pair 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 currently 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 Solar System's ninth planet. 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. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto, which was then reclassified under the new category of dwarf planet along with Eris and Ceres. 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.

Headline text

PLUTO IS A FUCKING PLANET AND YOU KNOW IT BITCHES! DWARF PLANETS ARE GAY!

PLUTO IS A PLANET!

Moons

Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005.

Charon

The Pluto-Charon system is noteworthy for being the largest of the solar system's few binary systems, i.e. whose barycenter lies above the primary's surface (617 Patroclus is a smaller example). This and the large size of Charon relative to Pluto lead some astronomers to call it a dwarf double planet. The system is also unusual among planetary systems in that they are both tidally locked to each other: Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon.

Nix and Hydra

Two additional moons of Pluto were imaged by astronomers working with the Hubble Space Telescope on May 15 2005, and received provisional designations of S/2005 P 1 and S/2005 P 2. The International Astronomical Union officially christened Pluto's newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra (Pluto III, the outer moon, formerly P 1), on June 21, 2006.

These small moons orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometers from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon, and are very close to (but not in) 4:1 and 6:1 mean motion orbital resonances with Charon.

Observations of Nix and Hydra are ongoing to determine individual characteristics. Hydra is sometimes brighter than Nix, speculating that it either is larger in dimension or different parts of its surface may vary in brightness. Sizes are estimated from albedos. The moons' spectral similarity with Charon suggests a 35% albedo similar to Charon's; this results in diameter estimates of 46 kilometers for Nix and 61 kilometers for brighter Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km respectively. At the larger end of this range, the inferred masses are less than 0.3% of Charon's mass, or 0.03% of Pluto's.

With the discovery of the two small moons, Pluto may possess a variable ring system. Small body impacts can create debris that can form into a ring system. Data from a deep optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope suggests that no ring system is present. If such a system exists, it is either tenuous like the Rings of Jupiter, or it is tightly confined to less than 1000km in width.

Distribution

The distribution of Plutonian moons is highly unusual compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere radius (stable gravitational zone of influence) of 6.0 million kilometers. In simple terms, an imaginary sphere is drawn around an object to represent the potential of an object to have other objects orbit it stably. For example, Psamathe orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty."

Additional moons?

In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km (or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometers.

Exploration of Pluto

Pluto presents significant challenges for space craft because of its small mass and great distance from Earth. Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, which resulted in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto. In 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.

The first spacecraft to visit Pluto will be NASA's New Horizons, launched on January 19, 2006. The craft will benefit from a gravity assist from Jupiter, and the closest approach to Pluto will be on July 14, 2015. Scientific observations of Pluto will begin 5 months prior to closest approach and will continue for at least a month after the encounter. New Horizons captured its first images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI). The images, taken from a distance of approximately 4.2 billion kilometres (2.6 billion miles), confirm the spacecraft's ability to track distant targets, critical for manoeuvring toward Pluto and other Kuiper Belt objects.

New Horizons will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and characterize Pluto's neutral atmosphere and its escape rate. New Horizons will also photograph the surfaces of Pluto and Charon. Some of the ashes of Pluto's discoverer, Clyde W. Tombaugh, are aboard the spacecraft.

Discovery of moons Nix and Hydra may present unforeseen challenges for the probe. With the relatively low escape velocity of Nix and Hydra, collisions with Kuiper belt debris may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for micrometeorite damage that could damage or disable the probe.

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.

Omission from museum models

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the solar system. Some omissions were intentional; the Hayden Planetarium reopened after renovation in 2000 with a model of 8 planets without Pluto. The controversy made headlines in the media at the time.

Commemoration as a planet

Pluto is shown as a planet on the Pioneer plaque, an inscription on the space probes Pioneer 10 and Pioneer 11, launched in the early 1970s. The plaque, intended to give information about the origin of the probes to any alien civilization that might in the future encounter the vehicles, includes a diagram of our solar system, showing nine planets. Similarly, an analog image contained within the Voyager Golden Record included on the probes Voyager 1 and Voyager 2 (also launched in the 1970s) includes data regarding Pluto and again shows it as the ninth planet.

Elements 92, 93, and 94 are named uranium, neptunium, and plutonium respectively after Uranus, Neptune, and Pluto. The Disney character Pluto, introduced in 1930, was also named in honour of the planet.

New discoveries ignite debate

Continuing advances in telescope technology allowed for further discoveries of Trans-Neptunian objects in the 21st century, some of comparable size to that of Pluto. In 2002, 50000 Quaoar was discovered, with a diameter of 1,280 kilometers, about half that 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.

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, however, he does not deny planetary classification to objects like Pluto & Ceres that have not "cleared their neighborhood".. Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitoners 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."

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.

The words "plutoed" and its variant "to pluto" were coined in the aftermath of the decision. In January 2007, the American Dialect Society chose "plutoed" as its 2006 Word of the Year, defining "to pluto" as "to demote or devalue someone or something", an example being "as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."

See also

• Pluto in fiction
• Pluto in astrology
• Solar eclipses on Pluto
• Aspects of Pluto
• Definition of planet
• Pluto prototype
• Plutino
• New Horizons

References

1. Akwagyiram, Alexis (2005-08-02). "Farewell Pluto?". BBC News. Retrieved 2006-03-05. {{cite web}}: Check date values in: |date= (help)
2. "The Final IAU Resolution on the definition of "planet" ready for voting". IAU. 2006-08-24. Retrieved 2006-08-24. {{cite web}}: Check date values in: |date= (help)
3. Kole, William J. (2006-08-24). "Dinky Pluto loses its status as planet". AP. Retrieved 2006-10-22. {{cite web}}: Check date values in: |date= (help)
4. Spahr, Timothy B. (2006-09-07). "MPEC 2006-R19 : EDITORIAL NOTICE". Minor Planet Center. Retrieved 2006-09-07. {{cite web}}: Check date values in: |date= (help)
5. Shiga, David (2006-09-07). "Pluto added to official "minor planet" list". NewScientist. Retrieved 2006-09-08. {{cite web}}: Check date values in: |date= (help)
6. Gugliotta, Guy. "Possible New Moons for Pluto." Washington Post. November 1, 2005. Retrieved on October 10, 2006.
7. H. A. Weaver, S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young Discovery of two new satellites of Pluto. Nature 439, 943–945 (23 February 2006) Final preprint on ArXiv
8. ^ Steffl, Andrew J. "First Constraints on Rings in the Pluto System". astro-ph/0608036. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
9. S.A. Stern, H.A. Weaver, A.J. Steffl, M.J. Mutchler, W.J. Merline, M.W. Buie, E.F. Young, L.A. Young, J.R. Spencer Characteristics and Origin of the Quadruple System at Pluto. Nature 439, 946–948 (23 February 2006) Final preprint on ArXiv
10. A.J. Steffl (2006). "New Constraints on Additional Satellites of the Pluto System". The Astronomical Journal. 132: 614–619. {{cite journal}}: Cite has empty unknown parameter: |1= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)(Final preprint)
11. "Voyager Frequently Asked Questions". Jet Propulsion Laboratory. January 14, 2003. Retrieved 2006-09-08.
12. New Horizons, Not Quite to Jupiter, Makes First Pluto Sighting
13. Tyson, Neil deGrasse (2 February 2001). "Astronomer Responds to Pluto-Not-a-Planet Claim". Space.com. Retrieved 2006-09-08.
14. "IAU 2006 General Assembly: Result of the IAU Resolution votes". IAU. 24 August 2006.
15. Britt, Robert Roy (24 August 2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". Space.com. Retrieved 2006-09-08.
16. Ruibal, Sal (January 6, 1999). "Astronomers question if Pluto is real planet". USA Today. {{cite news}}: Check date values in: |date= (help)
17. Britt, Robert Roy (21 November 2006). "Why Planets Will Never Be Defined". Space.com. Retrieved 2006-12-01. {{cite news}}: Check date values in: |date= (help)
18. Britt, Robert Roy (Aug 24, 2006). "Scientists decide Pluto's no longer a planet". MSNBC. Retrieved 2006-09-08.
19. Shiga, David (25 August 2006). "New planet definition sparks furore". NewScientist.com. Retrieved 2006-09-08.
20. Stern, S. Alan (2002). "Regarding the criteria for planethood and proposed planetary classification schemes" (PDF). Highlights of Astronomy. 12: 205–213, as presented at the XXIVth General Assembly of the IAU - 2000 . {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
21. http://www.lowell.edu/users/buie/pluto/iauresponse.html
22. Pluto Is Demoted to ‘Dwarf Planet’, New York Times
23. DeVore, Edna (7 September 2006). "Planetary Politics: Protecting Pluto". Space.com. Retrieved 2006-09-08.
24. ">Sapa (21 October 2006). "Pluto's still the same Pluto". IOL.co.za. Retrieved 2006-11-01.
25. ""Plutoed" Voted 2006 Word of the Year" (PDF). American Dialect Society. 5 January 2007. Retrieved 2007-01-07.

Further reading

• Henderson, Mark (Oct. 30, 2005). "Pluto may lose status of planet". New Straits Times, p. F17.
• Kaufmann III, William J., "Universe", 2nd Edition, pp.302–303
• Pasachoff, Jay M., and Alex Filippenko, 2007, "The Cosmos: Astronomy in the New Millennium," 3rd Edition.
• Chhabra et al., "Prediction of Pluto by Ketakar", Indian Journal of History of Science, 19(1), pp.18–26, 1984

External links

• Astronomy Education Review, Vol. 5 No. 2: Teaching What a Planet Is: A Roundtable on the Educational Implications of the New Definition of a Planet
• NASA's Pluto fact sheet
• Lowell Observatory History, Mission, Important Discoveries
• David H. Freedman, "When is a Planet Not a Planet?", Atlantic Monthly, February 1998.
• "Why Pluto Had To Go" – An opinion piece by astronomer Bryan Gaensler in Cosmos Magazine.* NASA's Pluto-Kuiper Belt Mission – launching in early 2006.
• Pluto in Science Fiction – Bibliography of science fiction which is set on Pluto.
• Windows to the universe – Pluto
• Space.com article on second and third moons
• NASA's Hubble Reveals Possible New Moons Around Pluto – Hubble Press Release
• Elliot et al., The recent expansion of Pluto's atmosphere
• Pasachoff et al., "The Structure of Pluto's Atmosphere from the 2002 August 21 Stellar Occultation," Astron. J., 129, 1718–1723.
• Planets – Pluto A kid's guide to Pluto.
• A simulation of Pluto's orbit, showing its resonance with Neptune.
• NPR article on IAU's anticipated recommendation of Pluto's status as a planet.
• NPR segment discussing the linguistic and cultural aspects of Pluto's planethood.
• Experts meet to decide Pluto fate - BBC News 14th August 2006
• Geologists fight for their plutons
• Pluto died much as it lived -- famous, but an oddball
• IAU General Secretary, Dr Luboš Perek's view of the decision

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• Dwarf planets
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