Pluto Printable version INTRO:        Pluto, being one of the two planets that cannot be seen without a telescope, is the only planet discovered in the 20th century. It is also the most distant and smallest planet in the solar system. Just how small is Pluto? Seven moons in the solar system are bigger than Pluto- our own Moon, Io, Europa, Ganymede, Callisto, Titan, and Triton. Pluto is so far away from the sun that the sun is barely less than a minute of an arc on Pluto, looking more like a bright star or distant street light than the Sun. The brightness of the sunlight on Pluto is about .15 the brightness on Earth. Not much is known about Pluto because of its distance away from Earth, but scientists believe that it has a diameter of approximately 1,430 miles, or 2,300 kilometers and combined with its moon, Charon, have a mass of approximately 1.27 x 10e22 kilograms, about one fifth of Earth's size. Pluto's mass, excluding Charon, is hard to confirm. Temperatures on the surface of the planet range from -387° Fahrenheit to -369° Fahrenheit, about -233° Celsius to -223° Celsius. Because those are some of the coldest temperatures in the solar system, scientists believe that Pluto consists of mainly of ice or frozen methane, with is concentrated at the poles. THE SURFACE:        The surface of Pluto is mostly icy. With a density between 1.8 and 2.1 g/cc, Pluto is believed to be 50% to 75% rock; the rest composed of ice. Nitrogen (98%), solid methane and carbon monoxide make up the surface. Because of the solid methane, scientists believe that the surface temperature of Pluto never reaches above 70 Kelvin. Pluto's temperature depends greatly on its orbit. Pluto comes as close as 30 A.U. to the Sun and as far as 50 A.U. away from the Sun. As Pluto's orbit takes it away from the Sun, its atmosphere actually freezes and falls to the ground. Only when Pluto is in perihelion is the atmosphere in gas form. The bright areas of Pluto's surface are believed to be ices of nitrogen and smaller amounts of methane, ethane, and carbon monoxide. The dark areas may be due to primordial organic material or photochemical reactions. Obviously, mankind has never reached Pluto. However, in 2001, NASA plans to launch the Pluto-Kuiper Express to study Pluto. Pluto’s surface is not uniformly dark or bright, comprising of many different materials with different chemical compositions and reflectivity. In fact, Pluto’s brightness can vary depending on its rotation. The bright areas on Pluto’s surface are believed to be a smorgasbord of solid nitrogen and other molecules like methane and carbon monoxide. Nitrogen tends to form crystals in laboratories on Earth. Because of this, scientists believe that the nitrogen on Pluto’s surface must "anneal and sinter" into large semi-transparent chunks (in more scientific terms- expanses). Because nitrogen has a transition temperature (meaning it changes phases at this temperature) of 35.6 K, the nitrogen ice may sometimes change into liquid state when Pluto reaches its perihelion epoch. The temperature when this usually occurs is around 40 K. This change in phase causes physical and optic disruptions in Pluto’s nitrogen ice surface. Underneath all these chunks of nitrogen, it is believed that there is some methane, and possibly carbon monoxide, trapped. Because of Pluto’s diurnal and seasonal cycles, the distribution of this smorgasbord varies. The darker region of Pluto, with a lower albedo and red color, has a higher temperature. These darker areas tend to be near the equator of Pluto. The composition of these dark areas are not known, but are believed to be made up of refractory organic solids produced by photochemistry of the ice molecules and atmosphere and material from outside sources and bombardment of cosmic rays. The polar caps appear to be made up of nitrogen ice. Scientists also believe patches of pure methane may exist on Pluto’s surface where the temperature is higher. Pluto has the second largest surface contrast (The first is Iapetus). Pluto’s brightness varies about .35 mag. When Charon started shading Pluto during the mutual event season in the 1980s, scientists were able to finally map the locations of the albedo markings on Pluto’s surface (at least on one side of Pluto’s surface because the same faces of Pluto and Charon face each other all the time.) The Hubble Space Telescope allowed for a more accurate map to be created. Although the telescope did not offer as high a resolution as the mutual event, it did allow scientists to view the other side of Pluto, providing global coverage. The telescope was the first to show polar caps and large spots around the equator. Pluto’s surface reflectivity varies from the 40 to 60 percent. Unfortunately, the Hubble Space Telescope was not able to get clear pictures of Charon. Using mutual event data to map one hemisphere and rotational lightcurve (don’t ask) information to guess the other hemisphere, scientists believe Charon has less than 0.1 mag variation on its surface. Charon’s surface is, therefore, more uniform. Charon reflectivity is around 40 percent. COMPOSITION: The composition of Pluto is not completely known. Instead, scientists use the density, radius, and information about the rotation of Pluto and Charon to hypothesize the internal structure of Pluto and Charon. Scientists have made many models of the composition of Pluto. Two models are based on the fact that there is a high cosmochemical abundance of water ice and silicates on Pluto. Model 1 (from McKinnon in 1995) This model is a low-density model of Pluto. This shows that around half of Pluto is made up of water and ice. Model 2 (also from McKinnon in 1995) This model is the organic-rich model of Pluto. This shows that, instead of ice, Pluto is made up of organics. (Models to come later. They’ll be pics. Working on them!) Both these models would support the idea that Charon was formed as a result of an impact on Pluto. If Charon did form as a result of an impact on Pluto, Pluto’s interior would be very hot. This would lead the softening of ice and separation of rock from the core. Both models are not accurate; they are only guesses. Until a more definite radius can be measured, Pluto’s exact composition will be hard to tell. Scientists do believe that Pluto has a higher rock-to-ice ratio that first believed. This could be the result of loss of water early in its history, perhaps when a major collision occurred and caused the formation of Charon (which would explain why Charon is more icy that rocky.) BRIEF HISTORY:        An American astronomer named Percival Lowell first documented the idea of a distant ninth planet in 1905. Based on erroneous calculations (although, Lowell did not know they were incorrect at the time), Lowell predicted a planet beyond Neptune that was affecting the movements of Neptune and Uranus. He proceeded to continue his search for his mysterious planet, going so far as predicting a possible area the planet could be found. However, it asn't until the year 1930 that the planet was finally discovered by Clyde W. Tombaugh. In honor of Lowell, who had passed away in 1916 without fulfilling his dream, Tombaugh named the planet after the Roman god of the dead. However, this planet's mass was too small to be the one causing the discrepancies. The search for Planet X continued until Voyager 2 determined a new, correct mass for Neptune. Using the new mass of Neptune, there was no longer a discrepancy in calculations. In 1978, James Christy, an astronomer at the U.S. Naval Observatory in Flagstaff, Arizona, discovered a moon for Pluto. Named Charon, the moon had a diameter of only 740 miles (1,190 kilometers). SOMETHING COOL:       Pluto's orbit takes it 249 years to complete and has a 3:2 resonance with Neptune. This means its orbital period is 1.5 times longer than Neptune's is. Usually, its orbit puts it as the farthest planet from the Sun. However, for 20 years of its trip around the Sun, its orbit takes it inside Neptune's orbit. For these 20 years, Pluto is the eighth planet from the Sun. The most recent orbit crossing took place on January 21, 1979 when Pluto came inside Neptune's orbit. They switched back on February 11, 1999. This will not occur again until September 2226. Pluto and Neptune never collide as their orbits cross because as Pluto reaches perihelion, the closest it ever comes to the Sun, it is at its maximum degree of inclination. When the two planets pass by each other, Pluto is usually well below or above the plane Neptune's orbit is on. The closest the planets come together is approximately 18 A.U. NOTE: You may have noticed the shortage of images of Pluto on this page. That is because humans have not yet reached Pluto for an upclose and personal look. The best images we have so far are from the Hubble Space Telescope and other satellites that have visited some of the other outer planets. This will all change soon. NASAhas begun planning a mission to Pluto called the Pluto-Kuiper Express. To learn more about this mission, click here!