Do you know about Mercury? Learn more about Mercury by reading Mercury Facts. Mercury is the smallest planet in our Solar System. It orbits the Sun every 87.97 days, the shortest of all the planets orbiting the Sun. Learn more about Mercury's amazing qualities. Become a astronomer and learn more about Mercury. Listed below are some Mercury Facts. Know about Mercury's composition. Mercury is a chemical element, which means that it contains two elements: hydrogen and helium. Mercury's atmosphere is thin and does not protect the planet. It consists primarily of hydrogen, sodium, and oxygen. This thin atmosphere constantly loses atoms to space. Thankfully, solar wind and radioactive decay replenish the planet's atmosphere. However, it does not protect Mercury from solar radiation. That's why scientists believe it is a planet of extremes. It is so extreme, in fact, that its atmosphere is almost entirely absent. Scientists use the names of the planets to better understand the formation of the Solar System. Mercury doesn't have rings or moons, and is therefore the only planet in the Solar System without any. If you ever saw it in the night sky, you can thank a renaissance astronomer. He was the first person to observe Mercury's motions. Then, over a century later, he rediscovered it with a telescope.