Can a star turn into a planet?
Category: Space Published: February 24, 2019
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Yes, a star can turn into a planet, but this transformation only happens for a very particular type of star known as a brown dwarf. Some scientists do not consider brown dwarfs to be true stars because they do not have enough mass to ignite the nuclear fusion of ordinary hydrogen. At the same time, some scientists do not consider brown dwarfs to be true planets either because they typically sit at the center of a solar system, just like a star. A brown dwarf is a strange object with a mass that is greater than the biggest regular planets (i.e. above 13 times the mass of Jupiter) and less than the smallest regular stars (i.e. below 80 times the mass of Jupiter). Although a brown dwarf does not have enough inward gravitational pressure to ignite the nuclear fusion of regular hydrogen, it does have enough to ignite the nuclear fusion of heavy hydrogen (deuterium). Early in the life of a brown dwarf, the nuclear fusion of its heavy hydrogen releases large amounts of light and heat. As a result, a young brown dwarf glows like a regular star. Despite its name, a brown dwarf that is still glowing does not appear brown. Rather, it appears magenta or reddish orange. Despite beginning life as a star, a brown dwarf quickly uses up its heavy hydrogen, goes dark, cools, and spends the rest of its life as a planet.
An atom of heavy hydrogen is just like an atom of regular hydrogen except that it has a neutron in its nucleus in addition to its proton. This extra particle in the nucleus makes it heavier. This neutron also acts like additional nuclear glue, making it easier to fuse together two atoms of heavy hydrogen. Additionally, heavy hydrogen is much rarer in the universe and in stars than regular hydrogen. Therefore, a brown dwarf cannot burn its regular hydrogen and quickly burns up its heavy hydrogen (because there is 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 more like a planet such as Jupiter. Despite the fact that a typical brown dwarf remains positioned at the center of a solar system like a regular star - it spends the rest of its life looking and acting like a planet. In such a solar system, the end result is a collection of planets orbiting a large central planet with no star to be found anywhere. Such a solar system ends up very cold and dark. (Note that some brown dwarfs orbit regular stars.)
As of 2018, over 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 hard to detect. This makes sense when you remember that through most of its life, a brown dwarf is dark like a planet. A recent statistical analysis estimates that there are about as many brown dwarfs in our galaxy as there are regular stars.