Why do some believe that Jupiter is a failed star?
Jupiter has similar composition to a star.
Jupiter is the largest planet in the solar system. It is also mainly hydrogen with some helium and heavier elements. So, Jupiter has a similar composition as a star.
Given its size and composition Jupiter has been thought of as a failed star. The film 2010 showed Jupiter being turned into a star.
In fact Jupiter is far too small to be a star. The temperatures and pressures in Jupiter's core are not sufficient to start any form of fusion reaction.
The true failed stars are brown dwarfs. They have masses between 13 and 80 times that of Jupiter. They are able to start deuterium fusion.
A true star has to be over 80 times the mass of Jupiter. The temperatures and pressures in a star's core are sufficient to start hydrogen fusion.