MENU  HOME PAGE  HISTORY OF COSMOLOGY  ASTRONOMY  ROCKETRY  SATELLITES & ORBITS  SPACE CRAFTS  SOLAR SYSTEM  SUN  MERCURY  VENUS  EARTH  MARS  JUPITER  SATURN  URANUS  NEPTUNE  PLUTO  KUIPER'S BELT AND OORT CLOUD  ASTEROID BELT  PLANET PROBES  STARS  GALAXIES  THE UNIVERSE  EXTRATERRESTRIAL LIFE  MODERN DEVELOPMENTS  STARGAZING  REFERENCES & LINKS  INTERACTIVE  ABOUT US  OTHER  DIFFERENT LANGUAGES

BACKGROUND INFO SYMBOL FOR PLUTO   Pluto, which was discovered in 1930, is but a dot of light in even the largest Earth-based telescopes. Pluto is 2/3 the size of Earth's moon but 1,200 times farther away, which makes viewing surface detail as difficult as trying to read the printing on a golf ball located thirty-three miles away. The adjacent movie made from recent Hubble Space Telescope computer enhanced images indicates that we are finally beginning to resolve some detail on the surface of this distant planet. GENERAL INFO   Pluto is on a highly elliptical orbit at an average separation of almost 40 A. U. from the Sun, with an orbital period of 248 years. Since the planet was only discovered in 1930, we have observed only a portion of its orbit so far. Further, the orbit is tilted by about 17 degrees relative to the plane of the ecliptic, much more than for any other planet. Its equatorial radius of 1150 km is only 20% of that of the Earth, and its mass is only 0.0025 that of the Earth. Thus, it is by far the smallest planet, either in mass or diameter. Its period of rotation appears to be almost 6 1/2 days. Because of the eccentricity of Pluto's orbit, its orbit lies inside that of Neptune for the period from 1979 to 1999. Thus, strictly, Pluto isn't even the most distant planet at present, as illustrated in the following diagram. Of course, we term it the 9th planet because its average separation from the Sun (length of semimajor axis) is greater than that for Neptune.   This, and other peculiar aspects of Pluto's orbit, have led to some speculation that Pluto is not really a planet but instead an escaped moon of one of the gas giant planets, most likely Neptune. Its composition, as inferred from its density of 2.1 g/cc, is largely ices. Thus, Pluto is more similar in structure to moons of the gas giant planets than it is to the terrestrial planets. However, other details may favor an origin of Pluto independent of Neptune, so this is an open question at present. BACK TO TOP   SURFACE, ATMOSPHERE, & INTERIOR   These pictures of Pluto were taken by the hubble space telescope. This is basically all that is known about the surface. Pluto isn't large enough to retain much of an atmosphere, but it has a thin one that appears to be mostly nitrogen with some methane. We know essentially nothing about Pluto's interior at this point. BACK TO TOP   PLUTO'S MOON   In 1978, careful Earth-based observation indicated that the image of Pluto had a slight bulge. This was interpreted as evidence for a previously unknown moon, named Charon. Early Hubble Space Telescope observations that show conclusively the existence of Charon. The orbit is show in the inset. The figure shown below shows a more recent Hubble Space Telescope image of Pluto and Charon.         With the presence of a moon, it was now possible to determine the mass of Pluto to much better precision than before because of the gravitational interaction between the moon and planet. This caused a drastic decrease in the previously assumed value for the mass of Pluto (previously the mass had been assumed to be as large as 10-100% of that of the Earth). BACK TO TOP