A planet of 1 light year

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, however, according to the laws of the universe as we know it, such a thing is simply not possible.

A planet is a large spherical object formed by various substances in the universe. Gravity is one of the forces that maintain the stability of a planet, but in addition to bringing the various substances that make up a planet together, gravity also causes the planet as a whole to tend to shrink inward, and if there is no strong enough force inside the planet to resist gravity, then the size of the planet will shrink, which is also known as gravitational collapse.

Since the magnitude of gravity is proportional to the mass and is a long-range force with only "attraction" and no "repulsion", the larger the mass of a planet, the stronger its tendency to shrink inward due to gravity, and the stronger it needs to be to resist the gravitational force. Otherwise, it will collapse gravitationally and will not be able to maintain its stability.

In the known universe, the most powerful force inside a planet that can resist gravity is nuclear fusion. If there is no nuclear fusion inside a planet, then when its mass exceeds the Oppenheimer limit (generally considered to be 3.2 solar masses), there is no force that can stop its gravitational collapse, in which case it will eventually evolve. In this case, it will eventually evolve into a black hole, and we can no longer call it a "planet".

It is for this reason that the planets in the universe whose masses exceed the Oppenheimer limit are invariably "burning" stars, and there are two powerful forces inside these planets, one gravity, which is directed inward, and the other nuclear fusion inside the planet. One is the gravitational force, which is directed inward, and the other is the energy released by the nuclear fusion inside the planet, which is directed outward and can be described as "radiation pressure".

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 larger the mass of a star, the stronger its tendency to contract inward due to gravity, and the higher the temperature and pressure in its core, accordingly, the more intense the nuclear fusion reaction inside the star, and the stronger the "radiation pressure" it generates. The stronger the "radiation pressure" will be.

For a stable star, these two internal forces maintain a dynamic balance, specifically, the gravitational force is strong, the star will contract, the star contracted, its core temperature and pressure will increase, so the nuclear fusion reaction will be more intense, and thus produce stronger "radiation pressure", and then the star will The star expands, the temperature and pressure of its core decrease, the fusion reaction weakens, gravity prevails again, and the star contracts again, and so on.

By the way, because the reaction rate of nuclear fusion is very sensitive to changes in temperature, the change in the 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 originates from the strong nuclear force in the nucleus, which is the strongest of the four fundamental forces in the universe, compared to the weakest gravitational force, so that as the mass of the star increases, the gravitational force and the "radiation pressure" do not correspond to each other. When the mass of the star exceeds a critical value known as the "Eddington limit", the "radiation pressure" inside the star will exceed the gravitational force.

In this case, the extra "radiation pressure" will continue to "blow" away part of the outer material of the star, so that the mass of the star continues to decrease, and as the mass of the star continues to decrease, the fusion reaction of the star will be followed by weakening, and the "radiation pressure" will be reduced. The "radiation pressure" will gradually become smaller, and when it reaches a new equilibrium with the gravitational force, 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.

To sum up, it can be concluded that the largest planet in the universe can only be a star, and the mass of a stable star can only reach the "Eddington limit" at most. As for the value of the "Eddington limit", it depends on the specific internal conditions of the star, and 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 its volume will be, so this density is, of course, the smaller the better, but it can't be too small, after all, the density is too small to form a planet.

In the direction of the shield about 20,000 light years away from us, there is a red special supergiant called "Stevenson 2-18", which is the largest known star, its volume is about 10 billion times the Sun, but only 12 to 16 times the mass of the Sun, that is, its lowest estimated density is only about the Sun 0.0000000012 times the density.

This density is considered the lowest of all known planets, so we may take this as a reference, according to this density, a planet with a diameter of 1 light year, its mass can be as high as 376.7 billion times the mass of the Sun, as you can see, this is far beyond the "Eddington limit", it is because of this that we can be sure that there can be no planet in the universe with a diameter of 1 light year.