The smallest and closest planet to the Sun in the Solar System. Mercury, which belongs to the Earth group of planets, is the fastest orbiting planet around the Sun and completes one revolution every 88 days. 1 day on Mercury is equal to 116 days on Earth. Temperatures near Mercury's equator reach 427 °C during the day, but can drop to -173 °C at night. Temperatures in the polar regions of Mercury are always below −93 °C.