The planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and finally (perhaps) Pluto. There are two groups of planets - the gas giants and the rocky ones.  The rocky ones are the first four and they're much smaller and denser than the others.  The other four are instead a lot larger and have very dense atmospheres with small cores.  The reason for this state of affairs is that when the Solar System was born, most of the gasses near the Sun were either vaporised or blown away by the Solar winds so that only the heavy, metal & rock materials were left to form the inner planets.  Further away though there was plenty of gas and these joined together to form behemoths the size of Saturn and Jupiter.

Many probes, satellites and rockets have been sent out to different planets and it is hoped (by me) that soon there will be manned missions to the Moon and Mars. The only manned landings so far have been in the late 60's (69) and 70's with the Apollo missions.  The first man in space was the Russian Cosmonaut Yuri Gagarin in 1961 and the first satellite was the Russian Sputnik in 1958.  The first man to have a 'spacewalk' was Cosmonaut Alexi Leonov.  There have been no disasters involving the deaths of human lives in space, though in 1970 Apollo 13 experienced a few difficulties.

At the moment however, the main goal the big space agencies are working towards is the International Space Station which is at present orbiting the Earth with a crew always on board.