Pluto

Pluto is one of several million objects lying in an outer layer of small icy rocks on the edge of our Solar System, in the Kuiper Belt region. Most of these Kuiper Belt Objects (KBOs) have solid silicate cores with several layers of ice near and at the surface. Pluto may contain ammonia and methane ice as well. [1] Recent observations of Pluto also suspect levels of nitrogen and carbon monoxide, and possibly a gaseous, albeit thin, atmospheric layer. [2]

Pluto and Mercury, as the smallest planets* and as the furthest and closest planets to the Sun, respectively, have orbits with the least approximate circular motion. As we have come to understand in class on the laws of motion pertaining to astronomical bodies, Pluto's revolutions are some of the slowest in the Solar System, being one of the furthest objects from the Sun, taking nearly 250 (Earth) years to complete a single orbit. [3] What is interesting about Pluto here is that it rotates sideways to other planets (as does Uranus). [4]

Pluto has three moons. The largest of these, Charon, was discovered in 1978 and is large enough (about half the size of Pluto) that Pluto-Charon was the first double planet in the Solar System. [5] 

The following table gives some basic information on Pluto's characteristics [6]:

	Pluto	Earth	Relativity (Pluto-Earth)
Diameter (mean)	2280 km	12 740 km	0.179
Distance from Sun	5.9 billion km	149.6 million km	39.5
Surface temperature (Kelvin)	50	260-310	N/A
Mass	0.078 x 10²⁴ kg	5.98 x 10²⁴ kg	0.013
Mean density	2048 kg m⁻³	5520 kg m⁻³	0.371

American astronomer Clyde Tombaugh discovered Pluto in 1930, basing his search for the ninth planet on previous predictions established by Percival Lowell. Lowell had assumed that the gravitational motions of Uranus and Neptune were being affected by a massive object that lay beyond the two planets. That 'massive object' was thought to be Pluto. It was only after its discovery about thirty years later at the observatory which bore his name - the Lowell Observatory in Arizona - that it was realised that due to its small size, Pluto could not have had such a dramatic effect on Uranus's and Neptune's motions. In retrospect, Lowell's prediction was baseless, and can best be explained by faulty measurements. [7] It did, however, establish the beginning of a search for what lay on the outer reaches of our Solar System.

Continuous exploration and scientific discoveries in this part of the Milky Way have led to changing theories and debates regarding Pluto's status. In 2006, the International Astronomical Union (IAU) recategorised Pluto as a dwarf-planet, demoting it from the other eight planets in the Solar System. This ruling deemed that a planet must be spherical and orbit the Sun - two classifications which Pluto fits - but also be in a space free of cosmic objects. As stated above, Pluto is within the Kuiper Belt - a large swath of objects similar to an asteroid belt. Therefore, Pluto was considered unfit to hold full planetary status. In 2014, a debate amongst three of the most established astronomers in the world, Owen Gingerich, Dimitar Sasselov, and Gareth Williams, showcased a public decision at the Harvard-Smithsonian Center, in which a popular majority 'voted' to re-instate Pluto's planetary status to full membership. [8]

"Found in the inner parts of the Kuiper Belt, plutinos are Kuiper Belt objects {KBOs) that have orbital periods very similar to that of Pluto. Their orbits are stabilised against gravitational perturbations by the 3:2 mean-motion resonance with Neptune, meaning that they orbit the Sun twice for every three orbits of Neptune." [9]

This summer, NASA's New Horizons mission will reach Pluto - the farthest any object from Earth has been - and is sure to teach us new things about what lies on the peripheries of our Solar System.


End Notes:

[1] Roger Smith (Editor). (1998). The Solar System (Vol. 3). Pasadena, CA: Salem Press, 887.

[2] "Pluto". (n.d.). In Royal Museums Greenwich - National Maritime Museum: http://www.nmm.ac.uk/explore/astronomy-and-time/astronomy-facts/solar-system/pluto.

*- The planetary nature of Pluto has been disputed, as will be examined further in this post. From this source, Pluto is still considered a planet in our Solar System.

[3] Keith Holliday. (1999). Introductory Astronomy. Chichester, England: John Wiley & Sons, 90-1.

[4] Holliday, Introductory Astronomy, 65-6.

[5] "New Horizons: The First Mission to Pluto and the Kuiper Belt: Exploring Frontier Worlds." (January 2006). NASA: http://www.nasa.gov/pdf/139889main_PressKit12_05.pdf.

[6] Holliday, Introductory Astronomy, 67 [Table 5.2; calculations for Pluto are my own].

[7] Blondel, Philippe, and John W. Mason (Editors). (2006). Solar System Update. Chichester, England: Praxis Publishing, 267; "Pluto", National Maritime Museum; "Celestial Mechanics - Planetary Perturbations." (n.d.). Retrieved from http://science.jrank.org/pages/1295/Celestial-Mechanics-Planetary-perturbations.html.

[8] "Pluto", National Maritime Museum; Jennifer Hackett. (February 13, 2015). "Pluto's ongoing identity crisis stirs planet definition debate." In Scienceline: http://scienceline.org/2015/02/plutos-ongoing-identity-crisis-stirs-planet-definition-debate/.

[9] "Plutinos." (n.d.). In Cosmos - The SAO Encyclopedia of Astronomy. Swinburne University of Technology: http://astronomy.swin.edu.au/cosmos/P/Plutinos.


References:

"Celestial Mechanics - Planetary Perturbations." {n.d.). Retrieved from http://science.jrank.org/pages/1295/Celestial-Mechanics-Planetary-perturbations.html.

"New Horizons: The First Mission to Pluto and the Kuiper Belt: Exploring Frontier Worlds." (January 2006). NASA: http://www.nasa.gov/pdf/139889main_PressKit12_05.pdf.

"Plutinos." (n.d.). In Cosmos - The SAO Encyclopedia of Astronomy. Swinburne University of Technology: http://astronomy.swin.edu.au/cosmos/P/Plutinos.

"Pluto". (n.d.). In Royal Museums Greenwich - National Maritime Museum: http://www.nmm.ac.uk/explore/astronomy-and-time/astronomy-facts/solar-system/pluto.

Blondel, P., & Mason, J.W. (Editors). (2006). Solar System Update. Chichester, England: Praxis Pubishing.

Hackett, Jennifer. (2015, February 13). "Pluto's ongoing identity crisis stirs planet definition debate." In Scienceline: http://scienceline.org/2015/02/plutos-ongoing-identity-crisis-stirs-planet-definition-debate.

Holliday, Keith. (1999). Introductory Astronomy. Chichester, England: John Wiley & Sons.

Smith, Roger (Editor). (1998). The Solar System (Vol. 3). Pasadena, CA: Salem Press.