Re: What is the difference between a planet and a star?

This is a question that would once have been easy to answer and has become more difficult as we've discovered more! The simplest answer is that a star produces its own light and heat through nuclear fusion and planets only have reflected light.

In the centre of our Sun, and all the other stars atoms are being combined to form new elements. Usually this takes the form of two atoms of hydrogen forming one of helium - this produces a huge amount of energy and so the Sun shines. In order for this to happen, the pressure at the centre of the star must be huge (scientists would LOVE to be able to use this method of producing energy on Earth but the necessary conditions are just too extreme). Large gas giant planets like Jupiter in our Solar System are simply failed stars that never grew big enough to begin nuclear fusion. As for small rocky planets like our Earth, they never even got close; we're just the leftovers from the Sun....

So why is this getting harder to answer? In the last few years astronomers have begun to discover planets around other stars - and lots of these are much bigger than Jupiter and it's difficult to tell whether they are giving off their own light or not. Until we get much more information, we won't be sure which of the categories they fit into...