Mercury is the smallest and innermost planet in the Solar System. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the planets in the Solar System. Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It has no natural satellites; besides Earth, it is the smallest of the eight planets. The planet takes 58.6 Earth days to complete one rotation. Mercury's axis has the smallest tilt of any of the Solar System planets (about 2⁄3 of a degree). Mercury is named after the Roman deity Mercury, the messenger of the gods. In Hebrew, Mercury is called "Kokhav HaKhamah" (כוכב החמה), which translates to "the planet of the sun." In Spanish, Mercury is called "Mercurio," and in French, it is referred to as "Mercure." Mercury's surface is heavily cratered and similar in appearance to Earth's Moon, indicating that it has been geologically inactive for billions of years. Mercury's thin atmosphere, or exosphere, is composed mostly of oxygen, sodium, hydrogen, helium, and potassium.