Is it possible for a star to become a planet?

Stars are celestial bodies composed primarily of hydrogen, with large amounts of helium and small amounts of other chemical elements, with a mass large enough to allow their cores to reach temperatures exceeding 4 million degrees Celsius (10 million degrees Fahrenheit), high enough to initiate the nuclear fusion of lighter atoms into heavier elements.

Planets, on the other hand, can be rocky or gaseous — like those found in our own Solar System — but do not have enough mass to reach temperatures sufficient to initiate nuclear fusion reactions in their cores.

If mass is the dominant factor in initiating nuclear reactions, might it be possible to remove the mass of a star to stop this process and cool it enough to become a planet?

At first glance, this seems unlikely, since there aren’t many things that could extract that much mass from something as compact as a star. However, not only does the Universe have a way of doing this, but we’ve already observed some cases where it happens. Here’s how.

When stars form, they do not result in solar systems like ours, where a central star is orbited by smaller bodies such as planets, moons, asteroids, and others. In fact, some solar systems form with similar properties to ours, but they represent only 50% of all stars formed in the Universe. The remaining 50% are associated with multiple star systems: binary systems, triple systems, and multiple systems with larger numbers of stars.

In general, single-star systems behave according to predictable and theoretical models of stellar evolution: The central star burns hydrogen fuel in its core as soon as nuclear fusion begins, and continues to do so until the hydrogen is exhausted. At this point, the fusion rate slows down and the external radiation pressure is no longer sufficient to hold the star’s core against the force of gravity.

What happens next is a series of significant events. Inside, the core begins to contract as the internal gravitational force begins to overcome the external radiation pressure. Just as a falling ball converts gravitational potential energy into kinetic energy, the contraction of the star’s core converts gravitational potential energy into kinetic energy, and collisions between particles in the core rapidly convert this kinetic energy into heat.

But as the core contracts, it heats up. This heat radiates outward from the star, causing the inner regions (where fusion occurs) to expand. As the core, now mostly helium, contracts and heats up, the thin shell-like layer of hydrogen around it begins to fuse into helium, injecting more heat into the star.

But the outermost layers begin to swell and expand. Over time, the star will become a subgiant, and the inner core will become hotter and hotter. Eventually, the inner core will reach a temperature high enough that helium can fuse into carbon, while the outer layers will become so dispersed that the star will become a red giant.

This is the fate of all single stars born with a mass at least 40% of our Sun. What happens next will depend on the amount of this mass: for stars with a starting mass of less than about 8 times the mass of our Sun. Our Sun, our Sun, will eventually shed its outer layers as its core contracts into a white dwarf. Stars with a starting mass above this mass threshold will undergo a series of additional fusion reactions, resulting in a catastrophic supernova. The end result of both steps will be a stellar remnant that is less massive than the star that came before, but more massive and much denser.

If this remnant is in a binary system, it could “cannibalize” its neighboring star, transferring enough mass to itself to cause the donor star to lose its stellar status. The transition from a star where nuclear fusion is the defining feature of the universe to an object that lacks sufficient mass to initiate and sustain fusion is a remarkable event.

Among the more than 5,000 exoplanets discovered, the list includes three old stars: ASASSN-16kr, ASASSN-17jf, and SSSJ0522-3505. These are objects whose outer layers have been sufficiently torn away and stolen by the remnants of a nearby star. All three are much more massive than Jupiter and represent the first known group of stars to have lost enough mass to be demoted to planetary status.