# Some people say that there is a planet behind the sun, which is in line with the earth's sun. Is it possible?

As early as 2000 years ago, Western Greek philosophers put forward a hypothetical model of the universe. They thought that the earth, the moon and an invisible "anti earth" revolved around the sun. The "anti earth" was always located on the other side of the sun and was in the same straight line with the sun and the earth. The "anti earth" could never be seen from the earth. Later, this view has been recognized and sought after by some people, and has become a rare theme in some science fiction and modern science fiction films. This "anti earth" is regarded as the "twin brother" of the earth in the solar system. So, does this hypothetical ninth planet really exist?

In modern times, this problem has become one of the hot topics of many astronomers and mathematicians. Especially in the 1860s, when the Swiss mathematician Euler proposed the "three body" problem of the motion of celestial bodies, he thought that if a small celestial body in the universe was affected by the gravity of the other two larger celestial bodies, it would be maintained at some special points in the orbit of the universe The first three solutions of the orbit equation of the small celestial body are obtained by calculation and deduction after the hypothesis is put forward by Euler. In the 1870s, the French mathematician Lagrange found the remaining two solutions. These five special points were later called "Lagrange points" by the scientific community.

Among these five points, L1, L2 and L3 are all in the same straight line with the other two large objects, where L2 and L3 are respectively outside the two large objects, and L1 is between the two large objects. L4 and L5 are located at the top of an equilateral triangle formed by the line between two large celestial bodies.

Later, through the precise calculation of scientists, it was considered that L1, L2 and L3 are unstable. Take the solar system as an example, if there is a "three body" system composed of the earth, the sun and another "anti earth", they will not only be restricted by the gravity of the other two celestial bodies, but also affected by the gravity of other celestial bodies, especially the mass For planets such as Jupiter, this kind of interference will obviously affect the "three body" system, making it very easy to change the state of the three body connection.