1. Concepts 2. Solar System Origin 3. Planetary Processes 4. Earth Processes 5. Meteorites
6. Our Moon 7. Remote Sensing 8. Mercury 9. Mars 10. Venus, Our Twin
11. Jupiter & Jovian Moons 12. Saturn, Rings & Moons 13. Uranus 14. Neptune 15. Pluto, Charon & Comets

Pluto & Charon . . . and beyond . . .   symbol

by Kari Hetcher and Mary Hodges

Please read Chapters 21, 23, 24 of your text, paying special attention to the section of Pluto and Charon.


Pluto was originally considered the ninth planet from the Sun. For years it's status fell into question and in 2005 the International Astronomical Union (IAU) defined a planet formally and the following year Pluto was excluded as a planet, but placed in the "dwarf planet" category.

Notice in the diagram that Pluto's orbit sometimes places it as the 9th object from the Sun and sometimes the 8th object from the Sun. On February 11, 1999, Pluto regained it's position as the ninth object. Pluto's orbit is so strange that sometimes it is only 4.5 billion km from the Sun, while at others, it is 7.4 billion km away! The orbit is also inclined at about 20 degrees from the plane of the rest of the solar system.

Percival Lowell noticed irregularities in Neptune's orbit as early as 1905, and began a search for another planet beyond all previously studied that would be influencing its orbit. Lowell

But Pluto is so small and distant, however, that it was not discovered until 1930 by Lowell's assistant, Clyde Tombaugh. Even after its discovery, little was known about this far away world, including its diameter, density, and whether or not it had any satellites. In 1978 Charon (pronounced KEHR-on), was detected and since then more have been discovered.

Moons of Pluto

Pluto's Moons | Fice Satellites of Pluto

Pluto's Moons Seen in Highest Detail Yet

Most of the information that follows needs to be updated. Pluto has had many discoveries since this module was written.


Though discovered early in the century - before the space age of Sputnik, Apollo, and Voyager - the planet Pluto remains as dim and remote as its mythological namesake, the Roman god of the underworld.

Super-Comet or Ice Dwarf?

Pluto is so unique it almost defies classification. Though it orbits the Sun, Pluto neither qualifies as a terrestrial nor as a gas giant planet. Though it behaves like a comet by periodically warming and losing its atmosphere into space, Pluto is far too large for that category. Pluto may be the last survivor of a lost population of objects called ice dwarfs that inhabited the primeval solar system. Neptune's moon Triton might be a distant cousin, and other relatives may dwell in the Kuiper Belt, a disk of ice debris left behind from the solar system's birth. Pluto and Triton survive because they have found gravitational niches in the solar system where they remain in stable orbits. Pluto is in a resonance orbit with Neptune (Neptune circles the Sun three times for every two orbits of Pluto), which means that Pluto never gets close enough to Neptune to be thrown out the solar system. Triton was gravitationally captured by Neptune and was therefore prevented from being ejected from the planetary region. It is believed that all of the other ice dwarfs formed inside 50 AU were ejected by gravitational interactions due to the giant planets in the ancient past.

Once accurate measurements were determined for Pluto's size and density, hypotheses about its internal structure were developed. With such a high density when compared to the gas giants, Pluto is thought to consist of between 70 and 80% rock. The outer shell of the planet is probably made up of ices of methane, nitrogen, and water.

The Double Planet

Pluto and Charon is the best example of a double planet, which occurs when two bodies are reasonably close in mass and so orbit around a common center of gravity - or barycenter (analogous to two children balancing on a teeter-totter). One theory is that Charon may have been born through a head-on collision between Pluto and another large ice body, in much the same way as the Earth-Moon system is believed to have formed. Charon has a low density (1.3 g/cm3) when compared to that of Pluto (2.02 g/cm3), which suggests it may be just a mass of fractured ice and rock. A large collision is supported by the fact that Pluto is inclined nearly 122 degrees from its orbital plane. Therefore, similar to Uranus, Pluto keeps its poles toward the Sun. How did Pluto form in the first place? To determine this answer we must first examine the composition and atmospheric conditions of this tiny, cold planet.According to computer models, some of the debris from this giant impact on Pluto went into orbit around Pluto and coalesced to form Charon.

The last time Pluto was this close to the Sun (and Earth!), George Washington was a boy!

Source: NASA Spacelink is a service of the Education Division of the National Aeronautics and Space Administration. http://spacelink.msfc.nasa.gov/Instructional.Materials/Curriculum.Support/Space.Science/Our.Solar.System/Pluto/Pluto-The.Solar.Systems.Final.Frontier

Now the Hubble Space Telescope has finally given us better pictures and information about this bizarre planet/moon system. NASA launched a mission to Pluto on January 19,2006 called the New Horizons or Pluto-Kuiper Express. The probe will get a gravity assist from Jupiter in February 2007, take an interplanetary cruise until June 2015 and reach the Pluto-Charon system in July 2012.pict

Pluto has a diameter of only 2300 km, similar to Neptune's largest moon, Triton. Charon is over half the size of Pluto, with a diameter of nearly 1200 km. It also orbits very close to Pluto, at a distance of only 20,000 km. Charon might have been formed during a violent impact on Pluto, which ejected material to form the moon. Until the discovery of Charon, it was believed that the Earth's moon was the only satellite planetary body formed by this mechanism.

Charon keeps the same face toward Pluto and, uniquely, Pluto keeps the same face toward Charon. This means that someone living on the far side of Pluto would never even know they had a moon!

A Dynamic, Unique Atmosphere

Despite its small size and remote location in the solar system, Pluto undergoes dramatic seasonal changes driven largely by Pluto's highly elliptical orbit, which carries it as close as 2.8 billion miles to the Sun (inside Neptune's orbit) and as far as 4.6 billion miles from the Sun. As Pluto recedes from the Sun, much of its atmosphere is believed to freeze out onto the surface. This explains the abundance of fresh white ice on the surface. Pluto essentially "launders" its surface by evaporating dirty, old ice in the summer, and depositing a fresh new layer of ice each 248-year orbit.

Pluto passed its closest point to the Sun in late 1989. As a result, it presently enjoys a relatively "balmy" surface temperature near -350 degrees Fahrenheit in the dark areas, and a cooler -380 degrees in the ice areas. It is likely that this sets up tremendous pressure differences at the surface, creating high wind speeds in Pluto's tenuous atmosphere. For astronomers it's a rare and ideal time for viewing Pluto and studying these changes.

Pluto's tenuous atmosphere of N2 and CO gas probably gives the planet a pink, wispy appearance. There may be seasonal changes within Pluto's atmosphere as it orbits around the Sun. At the closest point of its 248 year "year", the atmosphere may become thick with N2 gas, but at aphelion, the atmosphere may re-freeze and fall to the surface. Charon is a dusky grey color and might have a surface made up of water ice. A bizarre atmospheric transfer between Pluto and Charon causes this difference between the two. During Pluto's summer, methane ice changes to gas on both Pluto and Charon. Charon is so small, however, it does not have the gravitational pull to retain the methane gas and loses it to Pluto.

Get to know this distant planetary body -- it really is considered by many the last frontier of our Solar System. Visit the Nine Planets website to understand more about the Pluto-Charon pair. Learn about how Pluto was discovered by accident, and named as the god of the underworld, or perhaps the "PL" in Pluto are simply the initials of Percival Lowell.

Pluto's moon Charon remains an enigma -- at least until the New Horizons spacecraft encounters the pair and we learn more about it's composition and, hence, it's origin. Charon was only recently discovered in 1978 by Jim Christy, but still little is known about the size other than it is the largest moon relative to it's primary planet in the Solar System. Charon might be similar in composition to Rhea or other icy moons of Saturn. Read about some of the open issues concerning the Pluto-Charon system (especially the on-going debate over whether Pluto really is a planet at all -- why or why not?).

Websites to visit:

Prof. Fran Bagenal is a co-investigator with the New Horizons mission. She is on the faculty at the Department of Astrophysical and Planetary Sciences at the University of Colorado at Boulder.

The Solarviews Pluto website by Rosanna Hamilton has much information on the physical properties.

NASA New Horizons Images of Pluto.

Phil Plait's Bad Astronomy: Beyond Pluto - Bite-size astronomy lessons.

* Assignments for Pluto-Charon *


The appearance of a bright comet in the night sky filled our ancestors with awe and dread. Change in the unchanging sky portended plagues and death at the very least. And indeed, in 1066, Halley's comet accompanied the bloody beginning of Norman rule in England, an event recorded for us by the needles of the women who embroidered the Bayeux Tapestry.  Halley's Comet as depicted on the Bayeux Tapestry, which commemorates the Norman Conquest of England. In the upper left corner of the image.tapistry

Perhaps the most important thing to learn about comets is where they live most of the time. Study the Kuiper Belt and the Oort Cloud and learn what they are and where they are relative to other parts of the Solar System.

The Oort Cloud

Comets are a rather familiar part of our solar system. The most famous, Halley's Comet, orbits the Sun once every 76 years, spending most of its time in the far outer reaches of the solar system. Cometary bodies are thought to populate a region of the solar system known as the Oort Cloud, named after the astronomer Jan Oort who first proposed the existence of such a cloud of comets. Several astronomers have theorized that the Oort Cloud may be the source of gamma-ray bursts.

The Oort Cloud is centered on the Sun, not the Earth. At first glance, this scenario would seem to immediately violate the angular isotropy observed in the burst distribution, because the Earth is offset from the center of the Oort Cloud by nearly 93,000,000 miles. However, the extent of the Oort Cloud is extremely large compared to the distance between the Earth and the Sun. The cloud of cometary bodies is believed to extend to distances 100,000 times the distance from the Earth to the Sun. The Earth is offset from the center of the cloud, but only by a small amount relative to the extent of the cloud itself. Consequently, it still appears as if the Earth is at the center of this distribution of objects. Consider again standing at the exact center of a large open building like the Astrodome. The building completely surrounds you, and the distance to any exit is roughly the same. Now, if you move your position just a few inches or perhaps a foot, you still have the perception that you are standing in the center of the building, although you actually are slightly offset. The amount of offset from the center, however, is very small compared to the dimensions of the building, so it still appears to you that you are standing in the middle.

The strange orbit of Pluto/Charon, its small size, and its similarities to Triton raise questions about the actual origin of Pluto. Over half a dozen other small bodies, including Chiron, an asteroid/comet, have been discovered within the Uranus-Pluto region. If more of these bodies are found, eventually Pluto may lose its planet status and become known as an interplanetary body.

Oort Cloud - - https://en.wikipedia.org/wiki/Oort_cloud

Be sure to read about the importance of comet Shoemaker-Levy 9! Recall that this comet encountered the Roche Limit of Jupiter, broke into 21 fragments, and impacted Jupiter while people on Earth watched!

Today we know that these icy visitors arrive and depart as dictated by physical laws. They do bring material from the outer reaches of the solar system to the planets. Read this PDF document by NASA "Facts: Asteroids, Comets, and NASA Research" (you probably will want to open this in a new window)

-- or visit the "Deep Space 1" site which is all about new missions to study comets in greater detail - http://nmp.jpl.nasa.gov/ds1/

In 1950, Jan Oort made a statistical analysis of the orbits of long-period comets.

Read this PDF document discussing Oort cloud.

You can also visit "Hubble Identified a Long Sought Population of Comets Beyond Neptune" - http://www.jpl.nasa.gov/sl9/news65.html

After studying comets and their origins, we really do begin to understand how the Solar System was formed and what components are important in forming planets, moons, asteroids and comets. Treat this exercise as one the begins your extended learning experience and think about this learning process when New Horizons makes news in 2015.

* Assignments for Oort Cloud and Kuiper Belt *


End Of The Module
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