Pluto - The Last Planet

PLUTO - The Last Planet
Spurred by the success of the discovery of Neptune, astronomers searched for evidence for even more dis­tant planets. Percival Lowell (1855-1916), fou nder of the observatory bearing his name in Flagstaff, Arizona, was convinced by his calculations begun in 1905 that minute discrepancies still complicated the orbit of Uranus. (Neptune had not been observed long enough to provide useful data.) He concluded that the irregularities might be caused by a planet beyond Neptune.
Several years of intermittent and unproductive search passed. Then in January 1929, the Lowell Ob­servatory acquired a 13-inch photographic refractor and put a young assistant, Clyde Tombaugh, to work on a new search. After a year of photographing star fields along the ecliptic and later allover the sky, Tombaugh made the historic find on photographs taken in January 1930.
Pluto's great distance from the sun (a semimajor axis of about 40 AU) and its very long sidereal period of almost 248 years have made it a difficult planet to study. At the time of its discovery it was estimated that Pluto was approximately earthlike in size and mean density. Since then, with a longer period for studying the planet, estimates of its size and mass have de­creased.
It is by means of Pluto's gravitational attraction for Neptune and-to a lesser extent-Uranus that astron­omers have tried to estimate its mass. This means that they must know its orbit, which turns out to be the oddball among the nine planets. Pluto's large eccen­tricity carries it as close to the sun as about 30 AU and as far away as almost 50 AU, a variation of almost 20 AU, or about 3 billion kilometers. Thus during a por­tion of its orbit it is closer to the sun than Neptune is. In fact Pluto has been closer to the sun than Neptune since the winter of 1978 and 1979 and will remain so until the spring of 1999. Pluto now moving along that portion of its orbit that is north of the plane of the ecliptic (on the right-hand side). Pluto reaches aphelion while it is south of the ecliptic plane (on the left-hand side). At that point it will be about 14 AU below the plane of the ecliptic, which is a greater distance than Saturn is from the sun. Pluto's crossing of Neptune's orbital plane is done well above or below it so that there is no chance for the two planets to collide.
Pluto's brightness varies slightly, presumably be­cause sunlight is reflected unevenly from its surface­perhaps because of light and dark areas. Photoelectric observations of these variations reveal that the period of Pluto's rotation is 6.4 days. The nature of the bright­ness variation also suggests that Pluto's axis of rotation is inclined relative to its orbital plane by an angle in excess of 50°. There is a good chance that Pluto's sat­ellite orbits in the equatorial plane of the planet. If so, then Pluto's axis of rotation is almost in its orbital plane like that of Uranus.
The planet's small disk, measured with difficulty even by a large telescope, is less than one·fourth the earth's diameter. Pluto's mass is at most a few percent that of the earth's, which leads to a best guess for the mean density of about 1 gram per cubic centimeter. This prompts the belief by astronomers that Pluto is composed primarily of water in a solid form.
Recent infrared observations suggested a surface composition dominated by ices, primarily solid meth­ane. Pluto's low surface gravity and extremely low temperature mean that its atmosphere is a tenuous one of methane and possibly neon; indeed, recent infrared observations have spectroscopically detected methane in Pluto's atmosphere. However, its ob­served reflectivity is not consistent with a prediction of methane ice, or frost, alone. That is, the dark patches are possibly a rocky, sil icate material.
Pluto appears to have a satellite, Charon, which was taken in june, 1978. (The discovery was made during the examination of photo­graphs taken as part of the routine task of refining data on the planet's orbital motion.) In Figure 6.12 the im­age of Pluto is elongated, with a bulge that is hardly a mountain on the surface. Although the photographs do not clearly resolve the satellite, its estimated di­ameter is around 900 kilometers.
If Pluto and its satellite have equal mean densities, the satellite is about 5 to 10 percent of the mass of Pluto, which would make it by far the largest satellite in the solar system in comparison with its parent planet (the moon is about 1.2 percent of the earth's mass). The satellite orbits Pluto in an approximately circular orbit at an estimated distance of 20,000 kilo­meters from the center of the planet. The orbital pe· riod is 6.4 days and appears to be inclined to Pluto's orbital plane.
Is it possible that even more remote planets lie beyond Pluto? There is no physical reason that other objects like Pluto can­not exist. In fact for 13 years after the discovery of Pluto Tombaugh continued to search for more trans­Neptunian planets. His and subsequent efforts have shown that, if one or several are out there, they are extremely faint, and it is unlikely that they could be anything like the size of jupiter.