A planet, is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.
Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. The Solar System also contains at least five dwarf planets: Ceres, Pluto (originally classified as the Solar System's ninth planet), Makemake, Haumea and Eris. With the exception of Mercury, Venus, Ceres and Makemake, all of these are orbited by one or more natural satellites.
Mercury
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about the axis for every two orbits. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century; a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity. Mercury is bright when viewed from Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse, Mercury can only be viewed in morning or evening twilight.
Comparatively little is known about Mercury; ground-based telescopes reveal only an illuminated crescent with limited detail. The first of two spacecraft to visit the planet was Mariner 10, which mapped only about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER Spacecraft, which mapped another 30% during its flyby of January 14, 2008. MESSENGER will make one more pass by Mercury in 2009, followed by orbital insertion in 2011, and will then survey and map the entire planet.
Recorder observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes.
