Abstract

What is a planet and how many planets are there?: A brief outlook of evolution of the concept of planet in astronomy and a very short summary of current status of extrasolar planet searches results are presented. It is shown that the concept evolved substantially between the ancient Greek,,wandering star’’ and current understanding of these celestial bodies and in every epoch reflected current status of the development of physical sciences. However, planets are difficult objects to define and even these days we see evolution of the concept as perfectly illustrated by the history of Pluton and the planet-Brown Dwarf vague border. Obviously the public appreciates the current searches for planets around other stars as possible sources of information concerning alien life. In this context it is important to notice that common understanding of planets as the only possible cradles of such life is not correct and life may be appear to be present on other bodies as well.