Mercury is the nearest known planet to the Sun.
It is three times closer to the Sun than Earth. Mercury is hot. Its rocky, barren, and cratered surface is 800° Fahrenheit (F) during the day, while at night the temperature falls to –280°F. Mercury has been baked by the Sun for 4.5 billion years. Not long after Mercury first formed, it may have collided with a giant asteroid that blasted off most of the planet’s outer covering. This means that today Mercury is mostly a ball of hot iron with a thin, rocky outer crust.
