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Pluto discovered in 1930. It is small and frozen. It has one satellite. It turns around the sun every 249 years. Pluto is smaller than 7 of the moons in the Solar System. Because it is so small many scientists don't consider it a planet at all.
Source:http://www.wsc.ma.edu/personalpages/draker/edcom/final/webprojects/fa06/ultra/u8.html
Because this planet lies so close to the Sun, and as a result somewhat near to the Earth, it is visible to observers on Earth in the night time sky.
Source:http://en.wikiversity.org/wiki/Solar_System/Earth/Solar_System_overview/Mercury
Source: http://newswiseblog.blogspot.com/2009_05_01_archive.html