For most of us, our first experience with astronomy is looking up at the stars. Whether it is a prehistoric humanoid or a child living in a major metropolis, at some point we have all looked at the night sky and wondered at the distant point of light. As a result you would think that we know everything that there is to know about stars, but we do not.

Recently the theoretical physics community has started discussing a new type, called a Planck Star. At the moment it is a very theoretical and speculative idea, but some astronomers have started searching through observations in various energy ranges seeking evidence of these odd new objects. 

In spite of the name, Planck stars are actually closer in nature to black holes. If they exist, then they are actually the final state of cosmological black holes.

Since the theory of general relativity was first published, physicists have been intrigued by the properties of very dense objects. When a very massive star reaches the end of its life, its core can collapse into an object so dense that its gravitational force stops even light from leaving its surface. And since the theory of relativity states that nothing can travel faster than light, that means that anything that falls into this object is trapped forever and will never again be seen. This object is called a black hole, and we now believe that they are common in the Universe.

Except that they might not be as permanent as we used to believe...

In the 1970s Hawking (and others) showed that quantum mechanics, when applied to the surface of black holes, allows a small flux of particles to escape from the horizon. This Hawking radiation causes black holes to evaporate, but it is a very slow process. For the supermassive black holes that are believed to exist in the center of galaxies such as our own, this process takes longer than the expected lifespan of the Universe.

However we do not actually know how to combine quantum mechanics with general relativity. These are the two most precise theories of nature, and both have been extensively studied in experiments and proven to be correct in their respective areas. But they are not compatible with each other, and after nearly a century of study no one has yet been able to produce a valid theory of quantum gravity. And unfortunately black holes, being high energy objects with strong gravitational fields, cannot be adequately described without quantum gravity. We just don't know what happens in a real black hole.

However we can use our knowledge of both theories to speculate on what a quantum black hole would look like. This is where the idea of Planck stars enters the picture.

One of the fundamental ideas of quantum mechanics is that there is a limit to how precise the location of a particle can be determined. At some point the particle would be just a smeared out cloud, rather than a definite point particle. For this reason (and other more technical reasons beyond the scope of this article) many physicists believe that space and time at their smallest scales are actually discrete. We can talk about distance of a one meter,  one centimeter, one nanometer and so on down the scales, but there will be a distance which cannot be divided any further. This is called the Planck Length.

If this is true, and we have every reason to believe that it is, then this creates a conflict in the formation and life of the black hole.  When the core of the massive star is collapsing, at some point gravity will be pulling particles together that are already a single Planck length apart. Gravity will be exerting an enormous force on these particles, and the discrete nature of spacetime will be pushing back just as strong. It is at this point that the collapsing core forms a Planck Star.

Our limited knowledge of quantum gravity restricts how much we can predict the properties of this new object. Some researchers have suggested that the effect is strong enough to prevent black holes from ever forming. The core collapses to a certain point, but then this quantum force pushes it back out again and forms a cosmic explosion similar to a supernova. Other researchers have suggested that the black hole will form, and perhaps even remain stable for a very long span of time, but then eventually as the black hole absorbs more matter it will reach a density at which the quantum repulsion is strong enough to cause the black hole to explode, sending forth matter and radiation into its surrounding region of space.

In either case, these cosmic explosions would create a signal that could be detected by experiments on Earth and in orbit. By searching through astronomical data, it is possible that we could detect the high energy radiation produced by Planck stars and with careful analysis separate them from the more common supernovae signals. And if we can do that, then Planck stars could be our first experimental evidence of quantum gravity.

For now it is just an interesting theoretical concept, with no strong evidence for or against it. But in a year when we have seen the first gravitational waves detected, and studied neutron star collisions, it is not beyond belief that in the next few years we could be detecting even more exotic objects.

It will be interesting to see where this idea takes us in the coming years.