<text><span class="style10">he Outer Planets (4 of 6)</span><span class="style7"></span><span class="style10">Pluto</span><span class="style7">Although Pluto is normally the most distant member of the Solar System, it has a highly eccentric orbit that sometimes carries it inside that of Neptune. Thus for 20 years from 1979 to 1999 Neptune is temporarily the most distant planet.Pluto is a very small, icy planet. Its satellite, Charon, is very large by comparison, having 10% of Pluto's mass - compared to the Moon, which has only 1.2% of the Earth's mass. Together, Pluto and Charon virtually form a twin-planet system. Little is known about the nature of Pluto, but evidence of a thin methane atmosphere has been detected.</span><span class="style10">Planetary rings</span><span class="style7">The gas planets Jupiter, Saturn, Uranus and Neptune all possess systems of rings. Of these, Saturn's is by far the most spectacular.Rings are made from millions of small particles of ice and dust. Rings can have diameters of thousands of kilometers, but they are typically less than one kilometer thick. The mechanics of ring systems are not fully understood, but, especially in the case of Saturn, the ring particles appear to be kept in place by tiny `shepherd' satellites.GE</span></text>
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<text><span class="style10">riton</span><span class="style7">, Neptune's largest moon, photographed by Voyager 2. At the bottom is the south polar cap.</span></text>
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<text>ΓÇó THE SUN AND THE SOLAR SYSTEMΓÇó THE INNER PLANETSΓÇó THE HISTORY OF ASTRONOMYΓÇó SPACE EXPLORATION</text>
