Mercury is the closest planet to the sun (only 57 910 000 kilometres away or 0.38 AU ) and has a diameter of 4,880 kilometres, making it the second smallest planet in our Solar System.
Although Mercury is not the hottest planet, the temperature variations there are probably the most extreme. Temperatures on Mercury range from 90 Kelvin to 700 Kelvin. Weighing only 3.30e23 kg, it is also the second lightest planet in our Solar System.
Density wise, Mercury is the second densest. The densest 'major body' in the solar system is Earth. But Earth's density is partially due to gravitation compression, so if not for this gravitational compression, Mercury would prove to be denser than Earth. This is because of Mercury's large, dense iron core. The radius of Mercury's core is 1800 to 1900 kilometres meaning that Mercury only has a relatively thin silicate mantle and crust. Mercury also has a small magnetic field whose strength is approximately 1% of Earth's.
Mercury has no known satellites and has only been visited by one spacecraft. This spacecraft was the Mariner 10, which flew by three times in 1974 and 1975. Unfortunately, it was only able to map 45% of Mercury's surface and because it is located too close to the sun it can not be mapped by the Hubble Space Telescope either.
Mercury's orbit is also highly eccentric. At perihelion it is only 46 million kilometres from the Sun, but at aphelion it is 70 million kilometres away. This is compared to other planets, which have a slightly rounder and centred orbit.