Assignment 4

The changing Pluto

Introduction:

Pluto was once considered a major planet of the solar system mainly made up of ice, orbiting the Sun beyond Neptune. However, it was classified as a dwarf planet in 2006 [1], considered to be a large member of the Kuiper belt of objects. The surface temperature of Pluto is 375 to 400 degrees [2] below zero. It is on average more than 5.8 billion kilometers away from the Sun (about 40 times as far from the Sun as the Earth). It is only 2300 kilometers [2] wide and is slightly smaller than our moon. The period of Pluto's revolution is about 248 years. Pluto has three noticeable moons, the largest being Charon, and the other two, Nix and Hydra (discovered in 2005 by NASA's Hubble Space Telescope). The mass of Pluto is very low so that its gravity is about one-fifteenth [2] that of Earth. This means that a person who weighs 100 pounds on Earth would weigh only 7 pounds on Pluto.  

Figure 1: Artistic Rendering of Pluto's surface with the view of its big moon, Charon (1)

The atmosphere in Pluto is mainly composed on nitrogen [3] , carbon monoxide and methane. The perihelion (nearest point to Sun) and aphelion (farthest point to Sun) vary quite a lot for Pluto, 30 to 50 AU, because of its highly eccentric orbit. This affects the dwarf planet's atmosphere largely. At perihelion, thin layers of gases/ices are formed in the atmosphere, and near its aphelion, these gas molecules solidify. The atmosphere also is thought to have clouds and winds, but the confirmation has to wait until we have more detailed observations from space crafts.

Discovery, an "accident":

Percival Lowell [1] , an American astronomer observed deviations from the predicted orbits of Neptune and Uranus which led him to think there could be the gravity of another object that could be causing it. In 1915, he predicted such an object, but didn't find it. However, Clyde Tombaugh used these predictions, to discover Pluto in 1930 at the Lowell Observatory in Flagstaff of Arizona. The discovery was an accident as the calculations later turned out to predict a much more massive planet X [4] beyond Neptune and Uranus to account for the observed motions of the two gas giants, Neptune and Uranus. We now know there is no such planet X but there are a large number of small asteroid-type objects (Kuiper belt) beyond the orbit of Neptune.

Figure 2: Clyde Tombaugh, the discoverer of Pluto, looking through a telescope at the Lowell Observatory (2)

Clyde William Tombaugh discovered the first objects of Kuiper belt.  He was born in Streator, Illinois. A hailstorm destroyed his family's farms which hindered his college start. So in 1926 [5], he started making telescopes and started a job at Lowell Observatory in Arizona where he worked until 1945. He graduated from the University of Kansas in 1938 with a master's degree. After working in the observatory, he was part of the World War II working at the White Sands Missile Range after which from 1955 until his retirement in 1973, he was an instructor of astronomy at New Mexico State University. 

As the sub-heading suggests, Pluto wasn't the predicted planet and there were systematic differences [6] between the observations and predictions of the planet X. The predictions of these planets can be tied back to the "law" of Titius-Bode which had reasonably good predictions for the planets close to the Sun which was the motivation to extend it beyond Jupiter and other gas giants as time progressed. It was probably not a clear prediction from the laws of celestial mechanics which would have not caused the systematic differences between observations and the theoretical predictions.

The sliding "status" of Pluto:

On the episode named "The Pluto Flies" [7] in the series NOVA, Dr. Neil Degrasse Tyson provides an interesting story of the status of Pluto, how it was considered to be one of the planets in solar system and how it was sharply demoted into one of the many objects in solar system. He narrates that for more than 75 years, our solar system was considered to contain nine planets (including Pluto), but at the start of this 21st century, a visitor to the Hayden Planetarium (where Neil worked), could take a look at all the then planets of solar system, except for Pluto.  This turned out onto the front page of newspapers, one headline read, "Pluto's Not a Planet? Only in New York" [7]. Following this, Neil had received unhappy responses on this subject, "The Pluto Flies". The scientific responses to it were that Pluto is shaped close to a sphere unlike other bodies comparable to its size. The arguments for the demotion of Pluto consisted of (i) Pluto's orbit crossed that of Neptune due to its highly ecliptic orbit, (ii)Charon, one of Pluto's moon is as big as Pluto itself, so it would be equally reasonable to call Charon a planet and Pluto its moon. (iii) There are many other bodies (some moons and asteroids) as massive as Pluto that would need to be considered a planet.

IAU, the International Astronomical Union is the official group which has the power to decide on the names of the extra terrestrial objects based on the regulations set by the union. The union agreed that the Solar system would consist of only eight planets and that Pluto was classified into a new class of objects, "dwarf planets" which are different from planets. Ceres and Eris were also grouped with Pluto. In spite of the demotion, the dwarf planet Pluto was considered to be an important object in the category of Trans-Neptunian objects (those that cross the orbit of Neptune) perhaps one of the factors could have been the history and controversy of the status of Pluto. In light of this, the union also named these Trans-Neptunian objects as plutoids [8].  In the present, Pluto is still considered as a dwarf planet.

Plutinos:

Figure 3: Some objects that fall under the class of plutinos (3)

Plutinos are a sub-class (largest) of Trans-Neptunian objects that are in 2:3 orbital mean-motion resonance with Neptune of which Pluto itself is one of them. This class refers only to this resonance [9] and in no way relates to the physical characteristics of the object with Pluto or Neptune. Some of the Kuiper belt objects also fall under this category. The first plutino, other than Pluto itself, was discovered on September 16, 1993 [9] was named (385185) 1993 RO. Some of the other objects that are classified as Plutinos are: 2005 TV 189 , 2007 JH 43 and 2002 VX 130 . These three objects are unusual for their ecliptic and highly inclined orbits unlike most other plutinos which have Pluto-like orbital inclination and eccentricities. The unusual plutinos because of their close approaches to Pluto resonate with Pluto which puts off the plutino's long term stability.

Interesting Fact [10] : 2015 is the year of Pluto, as NASA's New Horizons spacecraft is approaching the edge of our Solar system to Pluto.

References:

Text:

[1] http://www.space.com/43-pluto-the-ninth-planet-that-was-a-dwarf.html

[2] http://www.nasa.gov/audience/forstudents/k-4/stories/what-is-pluto-k4.html#.VP0nUPnF-jk

[3] http://www.space.com/18564-pluto-atmosphere.html

[4] http://nineplanets.org/pluto.html

[5] http://en.wikipedia.org/wiki/Clyde_Tombaugh

[6] Celestial Mechanics 43 (1988), 55-68, Kluwer Academics Publishers. 

[7] http://www.pbs.org/wgbh/nova/space/pluto-files.html

[8] https://www.iau.org/public/themes/pluto/

[9] http://en.wikipedia.org/wiki/Plutino

[10] http://lowell.edu/in-depth/pluto/

Figures:

(1) http://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&IM_ID=16383

(2) http://www.space.com/19824-clyde-tombaugh.html

(3) http://en.wikipedia.org/wiki/Plutino#mediaviewer/File:ThePlutinos_Size_Albedo_Color2.svg