In the solar system, Mercury is the smallest planet. It was discovered as a planet in 1643 by a model published by Copernicus. Out of all of the other planets in the solar system, it has the shortest orbital system around the Sun, only taking 87 days. This makes Mercury closer to the Sun than the others. It is also the second densest planet, after Earth.
Its density is due to the fact that it is composed of metals and rock. Along with being dense, Mercury is the second hottest planet, behind Venus. Its high temperature is because there is no regulation of temperature due to the world having no atmosphere.