Question: is it true that stars are really planets

Jen Gupta answered on 14 Jun 2011:

No, that’s not true!

A star is a ball of gas that is emitting energy through nuclear fusion. Nuclear fusion is a process where two atoms join together, this forms a new atom and, as long as the new atom is lighter than iron, emits energy. In most stars like our Sun, hydrogen atoms are being fused together to make helium atoms. However in older stars, all the hydrogen has already been used up so heavier elements are being fused together. Eventually there will be no more atoms left for nuclear fusion to happen and the star dies. The definition of a star that’s important here is that it is emitting energy because of nuclear fusion.

According to the International Astronomical Union, there are three conditions that a body in space has to meet in order to be a planet:
(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
(c) has cleared the neighbourhood around its orbit.

However, things aren’t quite that simple because astronomers keep on finding planets orbiting around other stars (we call these extra-solar planets or exoplanets) and obviously these aren’t going around the Sun so don’t fulfil part a of the planet definition! The IAU also has a “working definition” to try to say what a planet is. The important part of this definition basically says that a planet has be orbiting a star and be be small enough that nuclear fusion doesn’t happen. This means that planets have to be below 13 times the mass of Jupiter (if they are made of the same stuff as the Sun). Anything larger than this is called a brown dwarf.

So basically in stars nuclear fusion means that they emit energy (and therefore light) while planets orbit around a star and we only see them because they reflect light from their star.