Size / / /

"Whether outwardly or inwardly, whether in space or time, the farther we penetrate the unknown, the vaster and more marvellous it becomes." --Charles Lindbergh, Autobiography of Voice.


Percival Lowell was a man obsessed with ghosts. His "ghosts" were not the souls of the dead, but phantoms that only he could see. His idea of canals on Mars was but one of those phantoms. He insisted that there was a network of canals on the red planet that was, in his mind, an irrigation system created by an alien intelligence that watered a dry desert world. His vision never materialized except through science fiction, beginning with a contemporary of his, H. G. Wells, in his science fiction classic, War of the Worlds.

NASA Photograph

His other "ghost" was the so-called Planet X. Lowell began his search in 1905 by calculating the irregularities in the motion of the planet Neptune; he believed that this would isolate the elusive Planet X. (Ironically, Neptune itself was discovered by studying the irregularities in the orbit of the planet Uranus, which was discovered in 1781 by Sir William Herschel.) Lowell even founded the Lowell Observatory in Flagstaff, Arizona, to see his "ghosts." Lowell funded three different searches for the Planet X, and it was only on the third search that Dr. Vesto Slipher, the observatory's director, hired a young man from Kansas named Clyde Tombaugh. On February 18, 1930, Tombaugh discovered Planet X. The announcement of the discovery was kept a secret until Lowell's birthday, March 13. Ironically, it was not Lowell's painstaking calculations of Neptune's orbital peculiarities that found Pluto; instead, it was Tombaugh's persistent examination of photographic plates taken in two week cycles that found the elusive planet. Lowell, though dead by this time -- he died in 1916 -- was at last vindicated in his search for Planet X. There was a planet there.

Tombaugh's new planet was dubbed Pluto for the Roman god of the underworld, perhaps because being at the edge of the Solar System, it is bathed in perpetual darkness. Or, it could be for the fact that the first two letters of the name Pluto are P and L, the initials of Percival Lowell. Lowell's efforts did not yield an answer to the Neptune question, but his persistence in the search did help to find a new planet.

Estimates of the new planet's size ranged from seven times that of the Earth to 1/400 of the size of the Earth -- quite a wide range. Though the planet had been found, its presence still did not explain the peculiarities of the orbit of Neptune. In a sense, Planet X was still a ghost. Only with the information garnered from the probes that have ventured into the outer solar system has Neptune's orbit been explained. Pluto was only a small piece in the entire puzzle.

A Double Planet?

Pluto and Charon
NASA Photograph

Just a point of light in the sky, Pluto was of little interest to astronomers for many years. Unlike some of the gas giants and inner planets, not much could be gathered from its observation. Pluto is at an average distance of 3.67 billion miles from the Sun. (It is so far away from that Sun that if you were able to stand on its surface and turn your attention sunward, you would see a star with about the brightness of a full moon on Earth.) Because of this distance, Pluto remained as isolated as ever. It was merely a yellowish point of light in the night sky, visible through only the larger telescopes.

However, in 1978, all that changed, and Pluto moved into a new realm of astronomical interest beyond that of a minor planet at the edge of the Solar System. In 1978, James Christy and Robert Harrington of the U.S. Naval Observatory discovered that Pluto had a moon. While studying plates of Pluto, they found that there was an irregular bulb attached to it. At first they thought that it was a smudge on the photograph, but when it eclipsed Pluto 6.4 days later and continued to do so on a regular basis, they realized that they had discovered more than a smudge. There was indeed another object orbiting Pluto. It is common for a satellite to travel in a synchronous orbit with its planet, but Pluto is the only planet known that rotates synchronously with its satellite, one more example of Pluto's oddities. The moon was named Charon, for the ferryman who transported the dead across the River Styx to Pluto, the god of the underworld.

This discovery opened up a whole new avenue of exploration. Charon and Pluto together have been classified by some as the only example of a double planet in our solar system. The initial estimates of the size of Pluto and Charon were based on the fact that the two objects eclipsed each other regularly. From 1985 to 1990, Earth was aligned so that such an eclipse could be observed once each Pluto day. Only with the Hubble Space Telescope in 1994 were very accurate results obtained. Pluto's diameter is about 2320 km and Charon's is 790 km. Charon lies at a distance of about 19000 km from its parent planet.

With the discovery of Charon, the actual mass of both Pluto and Charon could also be determined by observing the orbital effects that they have on each other. Pluto was found to weigh 1.3 X 1022 kg and Charon weighed 0.16 X 1022 kg. It was also possible to map the surface features using a method called occultation. Occultation is the blockage of radiation from a celestial body when another body moves in between it and the observer. Because Charon and Pluto eclipse each other, occultation is a good method to use for mapping their surfaces, using albedo. The albedo is the ratio between the amount of light that an object receives and the amount of light that it reflects; for example, an albedo of 0.5 means that half the light that the surface receives is reflected. By observing the changes in the albedo of Pluto when Pluto is covered by Charon, which happens every 6.4 Earth days, a map of the side of Pluto facing Charon could be made. Unfortunately, Pluto and Charon have a peculiar orbit called a tidal lock. Charon hangs motionless in Pluto's sky and vice versa as they move around each other. For this reason, only one face of Pluto could be mapped through the occultation studies that were done in the 1990s.

Science fiction rarely explores the possibility of double planets. One novel by Bob Shaw, The Ragged Astronauts, did look at the possibility, though in order to make human life possible, his example of double planets was far more habitable than the icy cold grip of Pluto and Charon.

Modern Observations

More recently, the Hubble Space Telescope has examined the high color contrasts on the surface of Pluto. There is speculation that the variations are caused by topographic features such as basins and impact craters, like those on our own Moon. However, there is another more probable and more interesting theory: the highly contrasting surface of Pluto is caused by a complex distribution of frosts of nitrogen, methane and some carbon monoxide that migrate across the surface of the planet in accordance with its seasons and orbital variations. In 1999, the Mauna Kea Observatory in Hawaii even revealed the presence of ethane ice on the surface of Pluto as well. Its origin may have something to do with the reaction of methane with sunlight.

Observation of Pluto has also demonstrated that it has an atmosphere as well. It truly is an astronomer's planet, in that everything that we know about this planet has been learned through direct observation on the various wavelengths of the electromagnetic spectrum. The atmosphere was discovered in June of 1988 when Pluto passed in front of a bright stellar field. The result of this was a backlighting of the atmosphere. Careful modeling based on these observations allowed researchers to determine that the planet's middle atmospheric temperature is about -173 degrees Celsius. A temperature inversion occurs at the surface of the planet, where the temperature has been determined by the spectra of nitrogen gas to be -233 degrees Celsius. With spectrographic analysis, we have learned that this thin atmosphere, which is about one hundred thousand times thinner than Earth's atmosphere, is made up of 90 percent nitrogen and ten percent methane complexes. Pluto's atmosphere is so close to its moon Charon that some of the atmosphere of Pluto may even cross the orbit of Charon.

Pluto has difficulty retaining its atmosphere because of the light gravitational pull. The mass of the planet is 1/455 that of the Earth. Since mass is related to gravity, your immediate assumption might be that the Pluto's gravity is 1/455 that of Earth's. However, the gravitational force of an object is also related to the square of the distance of the center of the mass of that object. If you were standing on the surface of Pluto, you would be 5.5 times closer to the center of its mass than you would be on Earth. Therefore, using Newton's famous equation for gravity, your weight on the surface of Pluto would not be 1/455 of your weight on Earth, but 1/15.

The interior of the planet is a matter of pure speculation. The best theories available suggest that the planet has an ice exterior with a rocky core. Density readings of the planet indicate that it is almost twice as dense as water. Charon, on the other hand, has a density that is barely above that of liquid water, indicating a far smaller concentration of rocky material.

Science fiction has not ignored the possibility of life on Pluto, in spite of the harsh conditions. In Robert Silverberg's novel World's Fair, an expedition to Pluto is launched as part of a publicity stunt. Small crustacean-like organisms, based on silicon/cobalt chemistry unlike our carbon/iron chemistry, are found there. Kim Stanley Robinson's novel Icehenge takes place in the 23rd century. On the north pole of Pluto (which, because of its unusual spin axis at 119.6 degrees is almost sideways, unlike our north pole which sits nearly at the top of the world,) explorers find ice slabs towering 200 feet up. One in the center has Sanskrit writing on it. Larry Niven's short story "Wait it Out" has Plutonian lifeforms that appear like amoebae.

Origins of Pluto and Charon

There are several theories as to why Pluto exists. One is that Pluto is an escaped moon of Neptune. There is also the matter of Neptune's moon Triton to consider when looking at this theory. Triton orbits in a retrograde fashion. The theory is that Pluto and Triton collided at some point in the distant past. Pluto was ejected by this collision into its present eccentric orbit whereas Triton's orbit was spun around after the gravitational effects of the collision dissipated. Charon could have been born out of the accumulated lighter materials, such as water ice, that resulted from that cataclysmic collision. Science fiction has explored this escaped moon hypothesis with Larry Niven's World of Ptavvs.

Charon could also have been an object captured by Pluto after it gained independence from Neptune. However, if there are objects as large as Charon for Pluto to catch, then Pluto may have been such a body too, and its origin may be unrelated to Neptune. Support for this theory was strengthened with the discovery of objects in the Kuiper Belt.

The Kuiper Belt is a primordial disk of rocky material beyond the orbit of Neptune. Some scientists believe that Pluto and Charon are actually the most visible objects in the Kuiper Belt. Recent discoveries of other sizeable objects in this outer region of the solar system lend credence to this theory. Still, the objects discovered in this belt are somewhat smaller than Pluto and less than 400 km across. Four hundred km is a crucial number, because the definition of a planet is that it must orbit a star and be large enough to pull itself together into a sphere; 400 km across appears to be this critical diameter. Neptune may only be a moderating influence on the highly eccentric orbit of Pluto.

Pluto's orbit is bizarre. In fact, it is not always the furthest planet from the Sun; sometimes Neptune is. Pluto's eccentric orbit sometimes causes the planet to cross over inside of Neptune's orbit. Will they ever collide? Eccentricity is a measure of how much an orbit deviates from a circle; the closer the eccentricity is to 0, the closer the shape is to that of a perfect circle. Pluto has an eccentricity of 0.249 (the next most eccentric in the solar system is Mercury, which has an eccentricity of 0.206) and an inclination of 17.14 degrees, more than twice that of any other planet. It is unlikely that Neptune and Pluto will ever collide, because Pluto's orbit is always above or below the plane of Neptune's orbit. The eccentricity of its orbit and its size have caused some researchers, led by Dr. Brian Marsden, to reclassify Pluto as a member of the family of objects that includes all icy bodies, ranging from comets to objects the size of Pluto. The debate over whether Pluto qualifies as a planet will continue until some consensus is reached in the astronomical community, but for now, Pluto remains classified as a planet.

There is another possibility for Charon's origin as well. This explanation is similar to the theory of the origin of Earth's Moon. A large object in the distant past may have struck Pluto, ripping away a chunk of matter that became Charon. However, the great differences in the densities of the two bodies seems to indicate that they had independent origins. As with many astronomical problems, the density readings from the Hubble Space Telescope are still being challenged by ground-based observations.

Science fiction authors have ventured into totally new realms for an explanation of the origin of Pluto: it is an artificial world. One good example of this is a short story by Clifford Simak. "Construction Shack" is about a manned expedition to Pluto that discovers that the planet is an artificial creation.

Further Explorations

USPS Pluto Stamp
NASA Photograph

We can see that Pluto, its moon, and the space beyond warrant further exploration because of the many peculiarities of the planet. The Kuiper Belt is assumed to be the leftovers of our planetary system. It is also the source of many short-term comets. If this assumption is true, then the Kuiper Belt may contain some of the least processed material in our solar system, offering many clues as to its origin.

A Pluto-Kuiper Belt Mission was originally planned to be launched in 2001 to reach Pluto and the Belt by 2006. The mission was to consist of a pair of small, fast (and cheap by NASA standards) spacecraft that were to pass within 15,000 km of Pluto and Charon. There are several reasons for the tight timeframe. One is that launching the mission at that particular time would allow us to maximize the use of Jupiter's gravity to create a slingshot effect for the spacecraft, thereby reducing both travel time and fuel needed. Another reason is that Pluto is moving away from the Sun and as it does so, it will get colder. The colder it gets, the more likely it is that the atmosphere will freeze out; scientists would like to study the atmosphere at its maximum density if possible. The last reason is that the longer that we wait to launch, the more the spacecraft's ability to take pictures using reflected light will be hampered, as Pluto moves further away from the Sun. Unfortunately, on September 13, 2000, NASA issued a stop work order on the project because of cost overruns.

NASA did not shut the door completely, but instead left it open after pressure was applied from various lobbying groups. In response, NASA announced a Pluto Mission Competition to the aerospace industry. Organizations were encouraged to make proposals, and the criteria were simple: the craft would have to reach Pluto by 2015 and the cost would have to be maintained at less than 500 million dollars.

Two teams were selected for feasibility studies from the proposals that were submitted. One was called New Horizons with Alan Stern as the principal investigator, and the other was POSSE (short for Pluto and Outer Solar System Explorer) with Larry Esposito as the principal investigator. In 2001, NASA officially chose the New Horizons proposal.

With the deadline of reaching Pluto by 2015 looming ever closer and funding beyond the 2002 budget year a great unknown, we can only hope that the political will is there to take advantage of the opportunity to venture into this still unknown realm of the solar system, our stepping stone to the beyond.


Reader Comments

Peter Jekel is the Director of Infectious Disease Prevention in one of the largest Health Department Districts in Ontario. He has lived in Bracebridge, Ontario with his family for the past 16 years. His previous publications in Strange Horizons can be found in our Archive.

Further Reading

EOA Scientific Systems Inc. 2001. The Outer Planets. EOA Scientific Systems.

Stern, Alan, and David S. (ed.). 1998. Pluto and Charon. University of Arizona Press.

Stern, Alan, Jacqueline Mitton and S. Alan Stern. 1997. Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System. John Wiley and Sons.

Peter Jekel runs the Infectious Diseases Program for one of the largest Health Department Districts in Ontario, Canada. He lives with his family, made up of wife, daughter, dog, 3 cats, 20 fish, and a rabbit. His previous publications for Strange Horizons can be found in our Archive.
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