17/05/2012
Dr Michio Kaku, theoretical physicist and co-founder of the String Theory proposes three types of extraterrestial civilisations. There are types I, II and III.
Type III civilisations are those extraterrestials capable of harnessing the power of a galaxy, that is they are able to harness Galactic forms of energy for instance black holes or supernorvae.
Type II civilisations are those extra terrestials that are capable of harnessing stellar forms of energy e.g. energy of a star.
Type I civilisations are those beings who can harness the energy of a planet. They are planetary civilisations.
And, following closely (not even worth mentioning) are
Type 0 civilisations. These are civilisations that are not capable of harnessing the power of a planet (in which they live). Humans fall in this category, with all our sophistry and technology we are not even a Type I civilisation.
(See first comment for a summary of Physics behind it).