Assignment 4: The Changing Pluto

Pluto is a small celestial body known as a dwarf planet. Its diameter is roughly 20 percent of Earth's and its mass only 2 one thousandths [4]. Not only is Pluto small compared to the Earth, but it is also smaller than a number of moons in our Solar System, including Ganymede and Europa [8]. Given that its orbital distance is quite far relative to the solar system, at 39.3 AU and its general lack of atmosphere, Pluto is also incredibly cold. At minus 230 degrees Celsius, Pluto's surface temperature is only about 40 Kelvin above absolute zero, the coldest possible temperature [4].

Fig. 3: Interior of Pluto

Although Pluto usually does not have much of an atmosphere, it grows much more substantial as Pluto moves towards its perihelion [5], and across its path, it experiences changes in apparent brightness ranging from about 15-13.5 on the magnitude scale [6]. As far as composition goes, spectroscopy reveals that Pluto's icy surface is comprised predominantly of nitrogen, which freezes under the immense cold [7]. Underneath Pluto's frozen skin, lies its core of solid rock [5], furthermore, estimates involving Pluto's density place the ratio of rock to ice between 1:1 and 3:7. In other words the rocky components of Pluto contribute to over half of its mass; under the [reasonable] assumption that the majority of the rock is contained in the core. This suggests that Pluto has a substantially large rocky core, with a thinner icy surface [7]. Though it is the only dwarf planet to have ever been considered a planet, Pluto may not be the largest celestial object of its kind in our Solar System, Eris is a close contender, so close in fact that our current technologies cannot determine which is larger [4]. It is known however, that Eris is more massive than Pluto by almost 30 percent [9].

Fig. 4: Percival Lowell

The discovery of Pluto began with the discovery of Neptune. By observing the perturbations in the orbit of Uranus, an astronomer by the name of Urbain Le Verrier was able to mathematically predict the existence of Neptune; his prediction was incredibly accurate, placing Neptune within one degree of its exact location. Using his prediction, astronomers managed to finally observe the planet less than a year after Le Verrier's final prediction [10]. This idea of using perturbations to find a planet sparked a sudden interest in the astronomical community. Many individuals began searching for another planet who's gravity was acting on Neptune and Uranus, at the time, this mystery planet was dubbed "Planet-X" [11]. It was not until 1906 that any significant progress in this idea would be made, at which point a man named Percival Lowell initiated the search for "Planet-X" that would eventually lead to its discovery. Using a 23 cm telescope, Lowell managed to plot several probable locations for Pluto. Though he never managed to successfully identify it, a photograph containing Pluto was taken at his observatory nearly 15 years before it was observed by the man credited with its discovery [12]. 

Fig. 5: Tracking Perturbations in Orbits

Fig. 1: Clyde Tombaugh

The actual discovery of Pluto is credited to an avid stargazer by the name of Clyde Tombaugh. Born to a family of farmers in 1906, Illinois, Tombaugh grew up in relative poverty. Though unable to afford a formal higher education, Tombaugh studied rigorously on his own and used his finely honed knowledge of geometry to build his first telescope at the age of 20 [1]. Dissatisfied with his own initial handiwork, as well as the available telescopes on the market, Tombaugh continued to improve his designs. Eventually, his work with telescopes would bear him much fruit; notably, the 23 cm reflector telescope he built in 1928 got him his job at the Lowell Observatory [2]. He had based his design for the telescope on the details listed in an issue of Popular Astronomy, that

Fig. 2: Tombaugh's 1928 Telescope

being said, Tombaugh was not just hired for his design, he was hired for his observations with his telescopes. While working at the observatory, Tombaugh would accomplish much, his first major breakthrough being the discovery of Pluto. Using a telescope almost 50% larger than his design in 1928, he tracked the motion of celestial bodies [3], and in 1930, discovered the ex-ninth planet near one of the locations predicted by Lowell [12].  The next 5 years would be eventful for Tombaugh, during that time he began his formal university education in astronomy at the University of Kansas and married his wife Patricia Edson. By 1939, Tombaugh had graduated with both his bachelors and masters degrees in astronomy from UofK and had two children with Patricia, all the while working on and off with the observatory [2]. Tombaugh spent  nearly the next entire decade after his graduation noting down the motions of over 30,000 celestial bodies as his last contribution to the Lowell Observatory, after which he decided to travel to New Mexico [3]. Tombaugh spent the rest of his working days at the State University there, teaching lectures and raising funding for students, he died in the comfort of his own home after 9 decades of life [1].

At the time of its discovery, Pluto was believed to be a planet with mass approximately equivalent to that of Earth, however, this belief was founded solely on the perturbations of Neptune's and Uranus' orbit [13]. Once Charon, a moon of Pluto, was discovered by James Christy in 1978 [upon noticing a bulge towards one side of a photo of Pluto] [17], more precise measurements of Pluto's mass could be made by observing Charon's orbit [18].

Fig. 6: Discovery of Charon

Fig. 7: Orbit of Charon

These new calculations yielded a surprising result, that Pluto's mass was over 500 times smaller than astronomers initially believed [4]. This new discovery began to instigate doubt among many, as to whether Pluto should be considered a planet at all, and ensured that Pluto could not be the cause of the perturbations in Uranus' orbit [13]. Then, in 2006 Pluto was officially demoted from the rank of planet due to the fact that it failed to possess one of the three defining properties of planets as proposed by the International Astronomical Union; that every planet must orbit the sun, have enough mass to gain a spherical shape and have enough gravitational influence to absorb all competitive bodies in its vicinity. It is clear that Pluto satisfies the first two of these requirements, earning it the title of dwarf planet, as it is spherical and orbits the sun, however Pluto is not massive enough to gravitationally overpower its similarly sized neighbors and as such, cannot be considered a planet [14].


Among these similarly sized neighbors to Pluto, are the Plutinos, objects belonging to the Kuiper belt at a very specific location in space. All Plutinos have an orbital distance roughly equivalent to Pluto's, in addition to a whole number

Fig. 5: Kuiper Belt

resonance with Neptune [15], and are named as such because Pluto was the first Plutino. Some other notable Plutinos are Orcus and Ixion, as they rank among the brightest of currently known Plutinos [16]. Ultimately, Plutnios differ from Pluto and other Plutoids, bodies similar to Pluto, in orbital motion and size, with Pluto having a 3:2 resonance with Neptune while other Plutinos  have resonances ranging from 3:2 to 2:1 [19]. Furthermore, Plutoids (other than Pluto) tend to be smaller than Pluto by quite a bit and are even occasionally thrown out of resonance under the influence of Pluto's gravity [15].



Works Cited:

1. http://www.achievement.org/autodoc/page/tom0bio-1 
2. http://www.space.com/19824-clyde-tombaugh.html
3. http://www.britannica.com/EBchecked/topic/598927/Clyde-W-Tombaugh
4. http://space-facts.com/pluto/
5. http://solarsystem.nasa.gov/planets/profile.cfm?Object=Pluto
6. http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html
7. http://www.space.com/43-pluto-the-ninth-planet-that-was-a-dwarf.html
8. http://www.mikebrownsplanets.com/2010/11/how-big-is-pluto-anyway.html 
9. http://en.wikipedia.org/wiki/Eris_%28dwarf_planet%29 
10. http://www.universetoday.com/21621/who-discovered-neptune/
11. http://en.wikipedia.org/wiki/Pluto#cite_note-Tombaugh1946-40 
12. http://en.wikipedia.org/wiki/Percival_Lowell#Pluto
13. http://en.wikipedia.org/wiki/Pluto#Demise_of_Planet_X
14. http://www.loc.gov/rr/scitech/mysteries/pluto.html
15. http://scienceworld.wolfram.com/astronomy/Plutino.html
16. http://en.wikipedia.org/wiki/Plutino
17. http://jameschristy.weebly.com/james-w-christy.html
18. http://lasp.colorado.edu/~bagenal/1010/SESSIONS/17.PlutoCharon.html
19. http://cseligman.com/text/planets/plutoid.htm

Figures: 

1. http://www.achievement.org/achievers/tom0/photos/tom0-006a.gif
2. http://upload.wikimedia.org/wikipedia/commons/4/43/ClydeTombaugh2.gif 
3. http://www2.astro.psu.edu/users/niel/astro1/slideshows/class39/035-pluto-interior.jpg 
4. http://media.web.britannica.com/eb-media/63/133163-004-13A32A74.jpg 
5. http://i.space.com/images/i/000/018/506/i02/kuiper-belt.jpg?1339696229
6. https://solarsystem.nasa.gov/multimedia/gallery/Charon_Disc.jpg
7. http://spaceplace.nasa.gov/review/ice-dwarf/pluto_orbit.en.gif