Pluto, The Ninth "Planet"?

I remember when the news first came out about Pluto no longer being a planet in our solar system I felt a little bad for Pluto. No longer a planet…? But why!?! Well, to this day I never actually bothered to read about the specific details on this new definition of a “planet” in our solar system. I also didn't really know much about this icy, now, dwarf planet. Below is a summary of interesting facts about Pluto and the new definitions of celestial bodies orbiting our sun and what it really means to be a major planet in our solar system.

History & Discovery of Pluto

On February 18^th, 1930, astronomer Clyde W. Tombaugh (with the help of William H. Pickering) discovered the planet Pluto at the Lowell Observatory in Flagstaff, Arizona. However, he was not the first one to observe this planet. In 1906, Percival Lowell, the founder of the Lowell Observatory, began an intense hunt to search for this “planet X.” Although he had predicted its location, the search to find the planet continued even after his passing. Before being discovered, images of Pluto had been captured, although they were not recognized as such; some even as early as August 20^th, 1909, by the Yerkes Observatory. The search dissipated around the time of Lowell’s death. It wasn't until 1929 when that search resumed. The hunt was then handed over to Tombaugh, who was twenty-three years old at the time. His responsibility was to image the night sky, using pairs of photographs taken weeks apart, and then to determine whether any objects shifted position. After almost a year, on February 18^th, 1930, Tombaugh discovered a possible moving object. After these images were confirmed with more photographs, on March 13^th, 1930, the discovery was telegraphed to the Harvard College Observatory.

This discovery of “planet X” made major headlines. Since they discovered it, the Lowell Observatory had the right to name the planet. They received several suggestions, but settled on the name “Pluto” (mythological Roman god of the underworld), which was initially proposed by an eleven year old schoolgirl from Oxford, England named Venetia Burney. She suggested it to her grandfather, who then passed the suggestion to Professor Herbert Hall Turner, who then sent the suggestion to the United States. The planet’s name became Pluto officially on March 24^th, 1930 and was announced on May 1^st, 1930. As a reward Venetia received five pounds.

Pluto Characteristics 

Pluto is located about 39.5 astronomical units (AU) away from the sun; in other units, 3.67 billion miles (mi), or 5.91 billion kilometers (km) away! Its diameter measures about 1485 mi (or 2390 km). That is virtually less than half the size of small Mercury, slightly smaller than our known moons, and about half the width of the US! Pluto’s gravitational force is about one-fifteenth of that on Earth. This means that a 150 pound person on Earth would weigh only 10 pounds on Pluto. So skinny! Pluto’s average surface temperature is around 50 Kelvin (K), which is about -223° Celsius, or -369° Fahrenheit. Its surface is believed to be rocky, covered with frost and ice and composed of methane and nitrogen. Pluto orbits the sun in 248 Earth years. It completes a complete rotation (1 Pluto day) in 6.375 Earth days. And interestingly enough, its rotational orbit is retrograde, that is, it rotates from east to west. Pluto orbits the sun in an ellipse, just as the other planets; however, its orbital plane is inclined from the others by about 17.16 degrees. Its orbit is so highly eccentric that it can reach distances as far as about 50 AU from the sun, and for 20 years of its orbital period, it gets closer to the sun than Neptune. Pluto is located in the Kuiper Belt, which is an immense disc-like band, located beyond Neptune, that contains small, icy object or “debris” that orbit the sun.

Pluto’s Moons  

In 1978, Pluto’s moon, Charon, was the first to be discovered, at the United States Naval Observatory Flagstaff Station (NOFS). Named after the Greek mythological demon who ferried souls to the underworld, Charon measures about 737 mi (or 1186 km) in diameter. That’s practically half the size of Pluto! Basically making it a plutonian co-equal. In fact, because of its big size (relative to Pluto), many scientists refer to Pluto and Charon as a double dwarf planet or binary system. Their distance from each other is only 12200 mi (or 19640 km). Charon also has the same rotational period as Pluto (6.375 days) – in fact, the two are “tidally locked  in a synchronous orbit” which means that they are continuously facing each other, hovering the same spot on one another.

Pluto and its five moons.
               P4 is Kerberos and P5 is Styx.

In 2005, the Hubble Space Telescope helped discover two other moons: Nix and Hydra. Nix has a mean diameter between 29-85 mi (or 46-137 km), while Hydra between 38-104 mi (or 61-167 km) in diameter. In 2011 and 2012, Kerberos and Styx  were discovered, respectively. Tiny Kerberos measures between 8-21 mi (or 13-34 km) in diameter, and Styx even smaller between 6-16 mi (or 10-25 km).

What does it means to be a “planet"?

Starting in 2000, celestial bodies similar to Pluto’s size and orbit were discovered. For example, in 2005 Eris was discovered by Mike Brown.Eris is located beyond Pluto and is larger in size. It measures 1445 mi (or 2326 km) in diameter and is 27% more massive than Pluto. The fact that Eris is more massive than Pluto and has a similar orbit makes it clear that it, too, deserves to have the title of “planet” as well. However, since Eris is one of the many objects that are as large or larger than Pluto also with similar orbits, this means that the total number of major planets in our solar system would expand. Realizing that Pluto had to be reclassified, discussion was underway about the politically correct definition of a planet in our solar system. The International Astronomical Union (IAU), which is the group responsible for naming objects in space, decided that Pluto and other objects like it were not truly planets. Their formal decision on the definition of celestial bodies was approved on Thursday, August 24^th, 2006 at its General Assembly in Prague, and is as follows: 

IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:
(1) A "planet"¹ is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape², (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects³, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
¹The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
²An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
³These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
RESOLUTION 5B
Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading:
(1) A classical "planet"¹ is a celestial body . . .
and
¹The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
IAU Resolution: Pluto
RESOLUTION 6A
The IAU further resolves:
Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
RESOLUTION 6B
The following sentence is added to Resolution 6A:
This category is to be called "plutonian objects."

Under the IAU definition, Pluto was no longer a “classical planet,” but was now a “dwarf planet” since it does not meet the third requirement (to clear the neighborhood around its orbit). However, Pluto is not alone. Obviously many other objects also fell into that category, such as Eris, Ceres, Haumea and Makemake. Along with several other bodies in the Kuiper belt.

Artist's portrayal of New Horizons.

 New Horizons

On January 19^th, 2006, NASA launched New Horizons, the first mission aimed for Pluto. The spacecraft is set to fly out to the edge of our solar system, to observe Pluto, its moons, and other objects in the Kuiper belt. Five months will be dedicated to studying Pluto and its moons and afterward it will study other objects in the Kuiper belt. It is scheduled to reach Pluto on July 14^th, 2015. Next year! The data that will be collected will be much more than we’ve had about objects that far out in space.

If still functioning, in December 2038, it would reach 100 AU from the Sun! The space capsule is about the size of a grand piano and is rumored to carry some ashes of Pluto’s discoverer, Clyde Tombaugh! Hmm… Gross or sweet? Who’s to say?

Works Cited

Nasa:   http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html
           http://www.nasa.gov/audience/forstudents/5-8/features/what-is-pluto-58.html#.Uuc3XdLTnnA
IAU:    http://www.iau.org/public_press/news/detail/iau0602/
Time:   http://newsfeed.time.com/2012/07/13/why-pluto-now-has-five-moons-but-its-still-not-a-planet/
NBC News:     http://www.nbcnews.com/id/14498750/#.Uuc0ktLTnnB
National Geographic:     http://news.nationalgeographic.com/news/2006/08/060824-pluto-planet.html
Library of Commerce:    http://www.loc.gov/rr/scitech/mysteries/pluto.html

Space:     http://www.space.com/43-pluto-the-ninth-planet-that-was-a-dwarf.html
Seasky:   www.seasky.org/solar-system/pluto-charon.html
Wikipedia:        http://en.wikipedia.org/wiki/Pluto
                        http://en.wikipedia.org/wiki/New_Horizons