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Mercury is the closest planet to the Sun and the eighth largest. The planet Mercury is very difficult to study from the Earth because it is always so close to the Sun and has no substantial atmosphere. These factors contribute to the fact that the surface of Mercury has the greatest temperature range of any planet or natural satellite in our solar system. The surface temperature on the side of Mercury closest to the Sun reaches 427 degrees Celsius, a temperature hot enough to melt tin.

It is the second smallest planet (it was believed to be the smallest until the discovery that Pluto is actually much smaller than originally thought), and also the fastest in its orbit since it is the innermost planet. In fact, the name Mercury derives from its speed in moving around its orbit. Mercury is only about one-third the size of the Earth. Scientists theorize that Mercury's field is due to an iron-bearing core or possibly to the solar winds. Mercury's atmosphere is very thin and is composed of helium and sodium.

Mercury is much like the Earth on the inside and much like the Moon on the outside. The orbits of Venus and Earth appear to be circles on this scale, but the orbit of Mercury is clearly not circular. As a result of its highly elliptical path the distance between Mercury and the Sun varies a large amount, from 46 million kilometres at perihelion to 70 million kilometres at aphelion.