“Twenty years from now you will be more disappointed by the things that you didn’t do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.”
- Mark Twain
because
"A ‘planet’ is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit."
- from IAU Resolution 5A
The IAU came up with the definition to resolve astronomers’ long-running debate over whether or not Pluto was a planet. In early August 2006, a group of astronomers appointed by the IAU to determine the definition of a planet decided on this definition as well as the definition of a dwarf planet. On August 24, 2006, the IAU passed one resolution that declared this definition of a planet and defined all other objects that orbit the sun as dwarf planets or small solar system bodies. The second resolution declares Pluto a dwarf planet, based on the definitions in Resolution 5A.
source: http://blog.calendars.com/2012/08/pluto-demoted/
further reading :
http://earthsky.org/space/this-date-in-science-pluto-gets-its-name
http://curiosity.discovery.com/question/why-is-pluto-no-longer
