The sun is a nuclear reactor at the hub of Earth's solar system that loses 4 million tons of mass each second.

Of the nine planets in Earth's solar system, Mercury is the closest planet to the sun. Baked and irradiated, Mercury is a cratered world, a record of the impacts that once rained from space during the early solar system.

Venus could once have been Earth's twin with oceans and continents, even simple life, but now it is a lifeless planet, home to a dense and choking atmosphere with temperatures hot enough to melt lead.

Earth is the largest inner planet and the first with a moon, at just the right distance from the sun, where life has evolved in the oceans and green plants have flourished to to produce breathable air for humankind.

The moon was most likely formed when a body the size of Mars hit early Earth twice; the first collision was a glancing blow, the second, two days later, was a major impact that threw enough material into orbit to form the moon.

A total eclipse of the sun is one of the greatest spectacles in the solar system. It happens when the moon, which is 400 times smaller, completely obscures the sun, which is 400 times farther from Earth than the moon.