Mercury is the planet closest to the Sun and therefore not always visible due to the sun's glare. It is classified as a terrestrial or inner planet due to its proximity with the Sun. It is the smallest planet in our solar system and mostly made up of rocks. It has no atmosphere and therefore, cannot sustain life. Mercury can be seen as a bright spot in the sky during specific times of the year. In the months of September and October, it can be seen just before sunrise and in the months of March and April it can be seen just after sunset.