Does a planet with a diameter of 1 light year exist in the universe? One of the more common thoughts on this question is that it is not impossible to form a planet with a diameter of 1 light year if there is enough matter, yet according to the laws of the universe as we know them, such a thing is simply not possible.

Gravity is one of the forces that keep a planet stable. However, in addition to bringing the matter that makes up a planet together, gravity also causes the planet to shrink inward as a whole. Collapse.

Since the magnitude of gravity is proportional to mass and is a long-range force with only 'attractive' forces and no 'repulsive' forces, the more massive a planet is, the stronger its tendency to shrink inwards due to gravity, and the stronger it needs to be to resist it. Otherwise, it will collapse gravitationally and be unable to maintain its stability.

In the known universe, the most powerful force inside a planet that can resist gravity is nuclear fusion. If a planet does not have nuclear fusion inside it, then once its mass exceeds the oppenheimer limit (generally considered to be 3.2 solar masses), there is no force that can prevent its own gravitational collapse, in which case it will eventually evolve. In this case, it will eventually evolve into a black hole, and we will not be able to call it a "Planet".

It is for this reason that the planets in the universe that exceed the oppenheimer limit are invariably "Burning" Stars, with two powerful forces inside them: Gravity, which is directed inwards, and nuclear fusion inside the planet. The other is the energy released by nuclear fusion inside the planet, which is directed outwards and can be described as 'radiation pressure' for convenience.

The reason why nuclear fusion occurs inside a star is that the gravitational collapse of the star creates a high temperature and pressure environment in its core. The more intense the nuclear fusion reactions within the star, the stronger the "Radiation pressure" Generated.

For a stable star, there is a dynamic balance between these two forces inside the star, which is expressed by the fact that when the gravitational force is strong, the star will contract, and when the star contracts, the temperature and pressure of its core will increase, so that the nuclear fusion reaction will be more intense, thus producing a stronger "Radiation pressure", and then the star will the star expands, the temperature and pressure in its core decreases, the fusion reaction weakens, gravity takes over again, and the star contracts again, and so on.

Incidentally, because the reaction rate of nuclear fusion is so sensitive to changes in temperature, the change in volume of a star in the main sequence phase, such as the sun, is so subtle that we can barely observe it.

The energy of nuclear fusion is derived from the strong nuclear force within the nucleus, the strongest of the four fundamental forces in the universe, compared to the weakest gravitational force, so that as the mass of a star increases, there is not a one-to-one relationship between gravitational force and 'radiation pressure', and when the mass of a star exceeds a limit known as the "When the mass of the star exceeds a critical value known as the eddington limit, the "Radiation pressure" Inside the star exceeds the gravitational force.

In this case, the extra "Radiation pressure" Will continue to "Blow" Away some of the material in the outer layers of the star, thus continuing to reduce the mass of the star, and as the mass of the star continues to decrease, the fusion reaction of the star will weaken, and the "Radiation pressure" Will be reduced. As the mass of the star decreases, the nuclear fusion reaction decreases and the "Radiation pressure" Becomes less and less, and when it reaches a new equilibrium with gravity, the star will no longer lose mass.

This means that the mass of a star cannot grow indefinitely, but only up to the "Eddington limit", beyond which it will soon "Eject" The extra matter.

 It follows from the above that the largest planets in the universe can only be stars, and that the mass of a stable star can only reach the "Eddington limit" At most. The value of the "Eddington limit" Depends on the internal conditions of the star, but theoretically it is at most a few hundred solar masses.

Okay, now let's look at the mass of a planet with a diameter of 1 light year.

As we all know, the mass of a planet is equal to the product of its volume and its average density. Now that the volume is determined, we still need to set an average density for it, and the smaller the average density is for the same mass, the larger the planet will be, so this density is of course as small as possible, but not too small, after all, the density is too small to form a planet.

About 20,000 light-years away in the direction of the constellation shield, there is a red giant known as stevenson 2-18, which is the largest known star, about 10 billion times the size of the sun, but only 12 to 16 times the mass of the sun, which means that its lowest estimated density is only about 0.0000000012 times the sun's 0.0000000012 times the density.

This is the lowest density of any known planet, so let us take this as a reference, a planet with a diameter of 1 light year could have a mass of up to 376.7 billion times the mass of the sun, which is far beyond the eddington limit. This is why we can be sure that there can be no planet in the universe with a diameter of 1 light year.