What is the smallest planet?
Mercury is the smallest planet in our solar system. It is about 4,880 kilometers wide, only slightly larger than Earth's Moon, and closest to the Sun.
Mercury became the smallest planet in 2006, when Pluto was reclassified as a dwarf planet. Two moons, Jupiter's Ganymede and Saturn's Titan, are actually larger than Mercury, although they are less massive.
Mercury has almost no atmosphere, so it cannot hold heat. Daytime temperatures reach about 430°C, while nights fall to about −180°C. Its surface is covered in craters and looks a lot like the Moon.
Fast orbit, slow day
Mercury races around the Sun in just 88 Earth days, faster than any other planet. But it spins slowly, so one full day from sunrise to sunrise lasts about 176 Earth days, two of its years. NASA's MESSENGER orbited Mercury from 2011 to 2015, and the European-Japanese BepiColombo mission is on its way there.
Did You Know?
Despite being so close to the Sun, Mercury has water ice inside deep craters near its poles, where sunlight never reaches.