The Solar System was formed 4.5 billion years ago. A small amount of hydrogen atoms attracted each other, forming a small "ball" of hydrogen. As time went by, this ball became larger and larger. We now call this "ball" of hot gas, the Sun. The Sun managed to attract towards it, small pieces of dust. These pieces became larger and denser until they formed the 9 planets. In the Solar System, there's an area called, the Habitable Zone. The temperature in this zone allows creatures to live in it. Two planets are found in this zone, the Earth and Mars. However, life only exists on Earth.
Most scientists believe that there are 9 planets in the Solar System. Why can't there be a tenth planet? I am not talking about Planet X, which scientists are looking for and which is believed to be further than Pluto. Why can't there be a planet "behind" the Sun? A planet which has approximately the same mass as the Earth and the same distance from the Sun. We will never be able to see this planet for 2 reasons. The first reason is that the planet is "behind" the Sun, so we will never be able to see it with the most powerful telescope on Earth. The second reason is that the other planet takes the same time to orbit the Sun. That means, there will never be a collision between them; when one planet reaches one side of the Sun, the other planet will reach the other side, so they will never meet.
If this planet has the same distance from the Sun as the Earth, than it will be in the Habitable Zone. If the necessary chemicals are found on this planet, then life will exist on its surface. Maybe, the living organisms might not have evolved with oxygen or carbon dioxide. They could've evolved with any other gas, such as methane or sulphur dioxide. Maybe, the living things don't need any gas for respiration! The living organisms might have evolved billions of years ago, or maybe a few months ago. They might be simple organisms, such as bacteria, or complicated organisms like humans. There might be organisms which are very stupid or which have advanced technology. They might walk on two, three or four legs. Their eyes might detect light, or other forms of radiation, like gamma rays. These aliens might have developed a constitution. Their lives might be similar to communism or fascism. Maybe, they have different countries with different ways of life. There might be different countries with different creatures or there might be some dominant creatures.
If humans want to visit this planet, they will have to invent a rocket which moves 2 times faster than Earth (about 212,000 km/h). The speed of the fastest rockets don't exceed 40,000 km/h. If this planet does exist, it will not be discovered in this century.