Can a star turn into a planet?

 Can a star turn into a planet?

Yes, a star can transform into a planet, but this transformation only happens for a special type of star called a brown dwarf.  Some scientists do not consider brown dwarfs to be true stars because they do not have enough mass to ignite nuclear fusion of ordinary hydrogen.  Like stars, they are at the center of the solar system. Also, brown dwarfs are strange objects that have more mass than the largest regular planets.  (i.e. 13 times the mass of Jupiter) and less than the smallest regular stars (i.e. 80 times the mass of Jupiter).  However, a brown dwarf does not have enough internal gravitational pressure to ignite regular hydrogen nuclear fusion.  This is sufficient to ignite nuclear fusion of heavy hydrogen (deuterium).  Early in the life of a brown dwarf, nuclear fusion of its heavy hydrogen releases large amounts of light and heat.  As a result, a young brown dwarf shines like a regular star and does not appear brown. Rather, it appears magenta and reddish orange at its inception.  It then rapidly uses up its heavy hydrogen, begins to dim and cool, and spends the rest of its life as a planet.

A heavy hydrogen atom is like a regular hydrogen atom except that in addition to its protons, its nucleus also contains a neutron.  This extra particle in the nucleus makes it heavier.  This neutron also acts as additional nuclear glue, making it easier for two heavy hydrogen atoms to join together.  Moreover, heavy hydrogen is much less abundant than regular hydrogen in the universe and stars.  So a brown dwarf cannot burn its regular hydrogen and burns its heavier hydrogen more quickly (because it has so little of it).  As a result, a brown dwarf stops emitting light and heat early in its life.  It then steadily cools and dims until it behaves like a planet like Jupiter.  Despite the fact that a typical brown dwarf remains at the center of the solar system like a regular star, it spends the rest of its life looking and acting like a planet.  In such a solar system, a collection of planets revolves around a large central planet, with no stars to be found anywhere.  Such a solar system is very cold and dark.

As of 2018, more than 3000 different brown dwarfs have been identified by telescopes.  This does not mean that brown dwarfs are rare.  It just means that brown dwarfs are harder to detect.

By the way, most of our life is also dark like a brown dwarf planet.  A recent statistical analysis has estimated that there are almost as many brown dwarfs in our galaxy as there are regular stars.