In the solar system, there are currently eight known planets. Astronomers have been trying to find more planets in the solar system, but have never found anything. So, could there be a planet hiding behind the sun all the time? Can we confirm the existence of such planets?
If there is a planet that always hides behind the sun, it must share the same orbit as the Earth, and it is distributed at opposite ends of the same orbit as the Earth. If this hidden planet is not in Earth's orbit, it will not always hide behind the sun due to the difference in the angular velocity of the orbit around the sun, and sooner or later we will be able to find the planet in the night sky.
If such a hidden planet existed, humans would not have known about its existence for a long time. Because it's always blocked by the sun, we simply can't observe the planet on Earth.
However, since humans entered the space age, humans have the ability to send probes into space. As long as the speed of the probe reaches the second cosmic speed, it can get rid of the shackles of the Earth's gravity and enter the heliocentric orbit, thus becoming a satellite orbiting the sun.
Humans have launched a range of solar probes, for example, NASA launched Helios 1 in 1974 and the Parker Solar Probe in 2018. These solar probes orbit the sun, and their orbit is different from Earth's, so the angular velocity is different, which means they are able to fly to the back of the sun. If there were hidden planets on the back of the sun, they would have been discovered by solar probes a long time ago, but they are not in reality.
On the other hand, from an astrophysical point of view, hidden planets that share orbits with Earth will not exist either. Theoretically, the location of the hidden planet is one of the five Lagrangian points of the Sun and Earth, which is called the third Lagrange point (L3).
If the celestial body is to operate stably on L3, its mass needs to be much smaller than that of Earth, and it cannot be planetary. Otherwise, the object will be deorbited by gravitational disturbances and may even collide with the Earth. Or conversely, the Earth, which is on L3, which hides the planet and the Sun, will de-orbit, and the Earth will not be able to orbit in a stable orbit for a long time.
In short, it is impossible for the sun to hide a planet that has not yet been discovered. If there were a ninth planet and more in the solar system, they would not appear in the inner part of Mercury's orbit, they would all appear outside neptune's orbit, very far from the sun. Because these possible planets are far from the sun, they appear very dark and move extremely slowly in the sky, making them difficult to observe.
Currently, astronomers search for the ninth planet indirectly. Previous studies have shown that the orbits of several small bodies outside the orbit of Neptune have been abnormal, and astronomers believe that only the existence of the ninth planet can explain this anomaly. There is reason to believe this, because when astronomers discovered that there was an anomaly in the orbit of Uranus, they predicted the existence of Neptune through theoretical calculations, and finally found the eighth planet.
According to rough estimates, the ninth planet that could exist would be an ice giant, slightly smaller than Neptune. In addition, the planet's semi-major axis of orbit is equivalent to 700 times the distance between the Sun and the Earth, and it takes at least 10,000 years to orbit the Sun.