Pluto/Charon - Double Dwarf Planet

Above is a Hubble Space Telescope image of the actual planet Pluto. Below is an idea of what Pluto might be feeling emotionally after being summarily demoted from the status of planet to that of lowly dwarf planet following the August, 2006 meeting of the International Astronomy Union. I thought this form of bullying was no longer tolerated, but tell that to "Poor Pluto."

Introduction to Pluto

Pluto was the most distant planet in our solar system on average. Neptune now holds the title of most distant planet. Pluto comes closer to the Sun every few hundred years, due to the high eccentricity of Pluto's orbit. Pluto used to be designated as the ninth planet until August 24, 2006 when the Internation Astronomy Union officially demoted Pluto to the status of "dwarf planet." In fact, the relative size of Pluto's moon Charon caused astronomers to designate the pair of mutually orbiting objects as a double dwarf planet system. Pluto is also the least understood of all the larger solar system objects because no satellite mission has ever visited it, and it is too distant for detailed telescopic observations. Additionally, Pluto is so small that many considered Pluto to be more like a large comet than a planet, and debate among astronomers continued until the recent downgrade of Pluto from a more lofty "planet" level. One of the most interesting aspect of Pluto is the history of its discovery, which you will find below.

Planetary data

Mass (kg), and mass relative to Earth

6.6x10^21 kg = .0022 earths

Equatorial diameter (km)


Mean density (gm/cm^3)


Acceleration of gravity (m/s^2)


Velocity of escape (km/s)


Period of rotation

6.3874 days

Period of revolution

247.698 years

Aphelion (AU)


Aphelion (km)


Perihelion (AU)


Perihelion (km)


Mean orbital distance from the sun (AU)


Mean orbital distance from the sun (km)


Orbital velocity (km/s)






Inclination to the ecliptic

17.148 degrees

Inclination of the equator to the orbit

122.46 degrees

Number of natural satellites


Names of natural satellites

Charon, Styx, Nix, Hydra, Kereberos

More Information on the Pluto from the Nine Planets Website

Much of the information below is direct from the Nine Planets Website. Some material has been altered by me for this course, while other items and comments are directly copied. I hope to maintain a continuous update of this material to keep up with the findings from space satellites and telescopes.

Pluto is the ninth-most distant object from the Sun (usually) and by far the smallest. Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)
diameter: 2274 km
mass: 1.27e22 kg




In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. In Disney, Pluto is really a dog, and when were visiting Disney World as a family in February, 2004, we happened so see Pluto wandering around and therefore we stopped to get a picture of him with our daughters. All three are seen below.




Discovery of the Pluto

In the early 1900's, Percival Lowell predicted a ninth planet was orbiting somewhere beyond Neptune. Lowell had noticed minor perturbations in the orbit of Neptune, indicating another object was gravitationally disturbing its path. He calculated where this ninth planet ought to be, but was unable to find it himself even after two vigorous searches. Following his death, Vesto Slipher was hired as the director of the Lowell Observatory and a third search for this "Planet X" was initiated.







Clyde W. Tombaugh was one of the young astronomers working at Lowell's Flatstaff observatory, and he was given the task of hunting down the ninth planet. He took a photograph of a place in the night sky, and then two weeks later, took another photograph of the same area. Using an instrument known then as a "Blink Comparator," Tombaugh compared the two photographs and looked for "stars" that were not in the same location in both photos. He discovered several asteroids in this fashion, and then in February 18, 1930, he hit the planetary jackpot. Turns out, the discovery was serendipitous. The actual mass of the Pluto-Charon has since been determined to be insufficient to perturb Neptune to the extend that Lowell calculated. Also, Neptune's orbit does not show the perturbations that Lowell was claiming. It just happened that Tombaugh was looking in the right place at the right time. The two photographs of Pluto's discovery are shown below.





The next issue for the Astronomy community was the naming of the planet. Tombaugh wanted to name the ninth planet "Lowell" after the man who predicted its existence. However, the International Astronomy Union (IAU) would not permit a planet to be named after a human, since all the others were named after mythological gods. I think Lowell's drawings of Martian canals, belief in Martians, and his miscalculation of Neptune's orbital nuances deserve some sort of mythological status, but alas, his name was not to be on this little planet. An international contest was held for the naming of the planet, and a 12 year-old English girl suggested Pluto. Here is the god of the underworld ... cold and dark. Here too is a wondrous nicety that so rarely occurs in science. The Greek symbol for Pluto is a "P connected to L." These just happen to be Lowell's initials. EVERYONE WAS HAPPY! Well, they were all happy until August 24, 2006 when Pluto was no longer deemed worthy of the title planet.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no tenth planet. Some predicted the existence of Planet Vulcan ... orbiting exactly opposite Earth, relative to the Sun, but this too was proved false.


Pluto has never been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). However, the Pluto Express mission







Pluto's radius is not well known. The JPL's value of 1137 is given with an error of +/-8, almost one percent.

Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the center of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won't get really accurate data until a spacecraft is sent.

Pluto is the second most contrasty body in the Solar System (after Iapetus), as evidenced in the image below. Exploring the origin of that contrast is one of the high-priority goals for the proposed Pluto Express mission.

Pluto in True Color
Credit: Eliot Young (SwRI) et al., NASA
Explanation: Pluto is mostly brown. This picture (left) captures the true colors of Pluto as well as the highest surface resolution so far recovered. The above map was created by tracking brightness changes from Earth of Pluto during times when it was being partially eclipsed by its moon Charon. The map therefore shows the hemisphere of Pluto that faces Charon. Pluto's brown color is thought dominated by frozen methane deposits metamorphosed by faint but energetic sunlight. The dark band below Pluto's equator is seen to have rather complex coloring, however, indicating that some unknown mechanisms may have affected Pluto's surface.



The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths.

Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets. Pluto crossed Neptune's orbit January 21, 1979, made its closest approach September 5, 1989, and will remain within the orbit of Neptune until February 11, 1999. This will not occur again until September 2226.

Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide. (Here is a more detailed explanation.)

Unlike most planets, but similar to Uranus, Pluto rotates with its poles almost in its orbital plane. Pluto's rotational axis is tipped 122 degrees. When Pluto was first discovered, its relatively bright south polar region was the view seen from the Earth. Pluto appeared to grow dim as our viewpoint gradually shifted from nearly pole-on in 1954 to nearly equator-on in 1973. Pluto's equator is now the view seen from Earth.

During the period from 1985 through 1990, Earth was aligned with the orbit of Charon around Pluto such that an eclipse could be observed every Pluto day. This provided opportunity to collect significant data which led to albedo maps defining surface reflectivity, and to the first accurate determination of the sizes of Pluto and Charon, including all the numbers that could be calculated therefrom.


The first eclipses (mutual events) began blocking the north polar region. Later eclipses blocked the equatorial region, and final eclipses blocked Pluto's south polar region. By carefully measuring the brightness over time, it was possible to determine surface features. It was found that Pluto has a highly reflective south polar cap, a dimmer north polar cap, and both bright and dark features in the equatorial region. Pluto's geometric albedo is 0.49 to 0.66, which is much brighter than Charon. Charon's albedo ranges from 0.36 to 0.39.

The eclipses lasted as much as four hours and by carefully timing their beginning and ending, measurements for their diameters were taken. The diameters can also be measured directly to within about 1 percent by more recent images provided by the Hubble Space Telescope. These images resolve the objects to clearly show two separate disks. The improved optics allow us to measure Pluto's diameter as 2,274 kilometers (1413 miles) and Charon's diameter as 1,172 kilometers (728 miles), just over half the size of Pluto. Their average separation is 19,640 km (12,200 miles). That's roughly eight Pluto diameters.

Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays. The extreme cold temperatures of Pluto and Charon, as well as that of Triton are such that a bad human mannerism would take on a more interesting aspect. If you were to venture outside of your heated station on Pluto's surface and pass gas, it would instantly turn into small blue flakes of methane snow and accumulate in a little pile on the ground. No more would you be able to point the finger at someone else in blame for the proof of your mishap would lie beneath you!


Little is known about Pluto's atmosphere, but a rare occultation by Pluto took place in June, 1988. As Pluto covered a distant star, astronomers were able to watch the effect of Pluto's atmosphere on the star's light just before the star disappeared behind the planet. As that starlight was shining briefly through Pluto's atmosphere, its light would be spectrscopically affected by the gases in a measurable manner which would allow chemists to determine just what those gases were. The atmosphere of Pluto it thin, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. The summary of the surface methane discovery (right), and the atmosphere occultation (left) are shown below.

The Pluto Express mission planners wanted to arrive at Pluto while the atmosphere is unfrozen. Otherwise the gases presently in the thin atmosphere will freeze and snow back to the surface, covering it up and prevent planetary geologists from determining what Pluto is composed of. Unfortunately, as you may have discovered when you clicked on the Pluto Express mission site, the mission has been canceled due to budget cutbacks. However, a New Pluto Mission has been proposed, and accepted. This mission is called New Horizons, and it launched January 19, 2006 and will arrive and then fly by Pluto in the summer of 2015.

The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body.

There are some who think Pluto would be better classified as a large asteroid or comet rather than as a planet. Some consider it to be the largest of the Kuiper Belt objects (also known as Trans-Neptunian Objects). There is considerable merit to the latter position, but historically Pluto has been classified as a planet and it is very likely to remain so.

Pluto can be seen with an amateur telescope but it is not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several months will be required to actually find it. Suitable charts can be created with many planetarium programs such as Starry Night.


Charon ( "KAIR en" ) is Pluto's only known satellite:
orbit: 19,640 km from Pluto
diameter: 1172 km
mass: 1.90e21 kg

Charon is named for the mythological figure who ferried the dead across the River Acheron into Hades (the underworld).
(Though officially named for the mythological figure, Charon's discoverer James Christy, was also naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with the first syllable sounding like 'shard' ("SHAHR en").

Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. The images taken by Christy are shown to your left, and demonstrate the presence of an orbiting moon in the left photo, compared to that same moon being either in front or behind the planet.

Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.

Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.

Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.)

Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto. Pluto's average density lies between 1.8 and 2.1 grams per cubic centimeter. It is concluded that Pluto is 50% to 75% rock mixed with ices. Charon's density is 1.2 to 1.3 g/cm3, indicating it contains little rock. The differences in density tell us that Pluto and Charon formed independently, although Charon's numbers derived from HST data are still being challenged by ground based observations. Pluto and Charon's origin remains in the realm of theory.

Unlike Pluto, Charon does not have the same type of large albedo features, though it may have smaller ones that are just now being resolved. Additionally, when the eclipses of Pluto and Charon occured between 1985 and 1990, it was discovered that Pluto was made of brighter material than Charon, causing some speculation that Charon may not have formed simultaneously with Pluto, but was somehow captured.





Below is a "map" of Charon's surface, with a resolution of 500 km. I find it interesting to look at this image and try to understand what I am seeing relative to the preceding paragraph that describes Charon as lacking large albedo features. That information is from a public website, and perhaps the new images from the HST or from atop Mauna Kea have revealed that there is more to Charon than previously thought. Or, perhaps the entire moon, that is made of darker material than Pluto is, has fewer bright regions and is thus lacking in large scale features that are distinctly different in albedo relative to others.

It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon.

It is doubtful that Charon has a significant atmosphere, but it is possible that Charon and Pluto may if not share a common atmosphere, at least swap gas particles because they orbit so closely to each other.

Pluto newly discovered moons. In late 2005, a team using the Hubble Space Telescope discovered two additional tiny moons orbiting Pluto. Provisionally designated S/2005 P1 and S/2005 P2, they are now known as Nix and Hydra. They are estimated to be between 60 and 200 kilometers in diameter. The Hubble Image is seen below. Click on the picture to learn a little more.

Great websites for more study of Pluto and Charon:

JPL Webpage on Pluto

Pluto Home

Pluto from Solarviews

You have just completed a tour of the planets of the Solar System. While there is a quiz on this planet, it cannot be completed without learning about the minor members of the Sun's family ... comets and asteroids. That quiz is called the Pluto, Comets, and Asteroids Quiz and can be found at the end of the Asteroid page. However, there is a lab exercise that comes now regarding Pluto.

Please move ahead now to the Pluto-Diameter page where you will have an opportunity to see numbers from a series of 1985 occultations of Pluto and Charon that were used by the professionals to measure the diameters of Pluto and Charon. Now, you get to see how you fare with the same data.

Or you could move ahead to the Comet Page because Pluto is so much like a comet, and later do the Pluto-Diameter lab. Or you can go back to Netpune or Triton and make comparisons, or most sadly you could return to the Syllabus.

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