Which planet is closest to the Sun?

Mercury is the closest planet to the Sun. It is the innermost and smallest planet in the solar system, orbiting the Sun once every 88 days. It ranges in brightness from about −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°. The temperature on the side of the planet facing the Sun is as high as 430 C. The shaded side of Mercury which faces away from the Sun is bitterly cold at -180 C. It can only be seen in morning or evening twilight.

Mercury has almost no atmosphere because it has been burned off by the Sun. The planet consists of bare rock, pitted and scarred by the impact of meteorites. It has extremely steep cliffs that are hundreds of kilometers long. These were formed when the planet cooled from its original molten state millions of years ago.

Comparatively little is known about the planet: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40%–45% of the planet’s surface.