Uranus was the first planet discovered in modern history. It has an apparent magnitude of <1> and can just be seen with the naked eye, although it looks much like a faint star. The rings of Uranus were discovered in 1977 by <2>. The most interesting characteristic of Uranus is its obliquity of approximately <3>. The best theory for explaining this obliquity is that the entire Uranian system was knocked on its side by a large collision. Uranus was visited in 1986 by the Voyager spacecraft which noted its placid atmosphere and bluish-green appearance due to the presence of <4>.
Astronomers expected Neptune to be a peaceful world and were surprised when Voyager observed a very dynamic atmosphere with very high <5>, clouds, and giant storms appearing as dark spots. This vibrant weather is thought to be due to the interaction of convection currents due to <6> and rapid rotation. Neptune's large moon Triton was also surprising because of its <7>.
Many astronomers no longer classify Pluto as a planet. It never fit well with the other planets because of its large <8>, large inclination, and density in between that of the Terrestrial and Jovian planets. Its large moon <9> is so close to Pluto that the pair really form a binary object. When other large iceballs were discovered in the outer solar system, it rapidly became apparent that Pluto was simply the largest of many Kuiper Belt Objects.@ qu.1.1.blank.1=6,11,16,1@ qu.1.1.blank.2=stellar occultation,direct observation,the voyager spacecraft,thermal emission@ qu.1.1.blank.3=90 degrees,0 degrees,45 degrees,180 degrees,135 degrees@ qu.1.1.blank.4=methane,hydrogen,helium,carbon dioxide@ qu.1.1.blank.5=winds,temperature,sunlight,moisture@ qu.1.1.blank.6=interal heat being released,solar heating,heating from radioactive decays,heating from meteoritic bombardment@ qu.1.1.blank.7=active geysers,lava flows,giant scarps,highly elliptical orbit@ qu.1.1.blank.8=eccentricity,radius,albedo,metallicity@ qu.1.1.blank.9=Charon,Triton,Titan,Ganymeded,Fred@ qu.1.1.grader.1=menu@ qu.1.1.grader.2=menu@ qu.1.1.grader.3=menu@ qu.1.1.grader.4=menu@ qu.1.1.grader.5=menu@ qu.1.1.grader.6=menu@ qu.1.1.grader.7=menu@ qu.1.1.grader.8=menu@ qu.1.1.grader.9=menu@