Mercury is the smallest planet in the Solar System. It is the closest planet to the sun. It makes one trip around the Sun once every 87.969 days. Mercury is bright when it is visible from Earth, ranging from −2.0 to 5.5 in apparent magnitude. It cannot be easily seen as it is usually too close to the Sun. Because Mercury is normally lost in the glare of the Sun, Mercury can only be seen in the morning or evening twilight or during a solar eclipse.
0.466 697 AU
0.307 499 AU
0.387 098 AU
|87.969 1 d|
(0.240 846 a)
Average orbital speed
|Inclination||7.005° to Ecliptic|
3.38° to Sun’s equator
6.34° to Invariable plane
|Known satellites||None diameter = 4,880 km|
|2,439.7 ± 1.0 km|
Sidereal rotation period
Equatorial rotation velocity
|10.892 km/h (3.026 m/s)|
|2.11′ ± 0.1′|
North pole right ascension
|18 h 44 min 2 s|
North pole declination
|up to −1.9|
|4.5" – 13"|
|Composition by volume||42% Molecular oxygen|
Trace amounts of argon, nitrogen, carbon dioxide, water vapor, xenon, krypton, & neon
Less is known about Mercury than about other planets of our Solar System. Telescopes on the Earth show only a small, bright crescent, and putting a satellite in orbit around it is difficult. 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 finished mapping the planet in March 2013.
Mercury looks like Earth's Moon. It has many craters and areas of smooth plains, no moons around it and no atmosphere as we know it. However, Mercury does have an extremely thin atmosphere, known as an exosphere. Unlike Earth's Moon, Mercury has a large iron core, which gives off a magnetic field about 1% as strong as that of the Earth. It is a very dense planet due to the large size of its core. Surface temperatures can be anywhere from about 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
Known sightings of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers thought that Mercury was two different objects: one able to be seen only at sunrise, which they called Apollo; the other that was only able to be seen at sunset, which they called Hermes. The English name for the planet is from the Romans, who named it after the Roman god Mercury, which they thought to be the same as the Greek god Hermes. The symbol for Mercury is based on Hermes' staff.
Even though Mercury is the closest planet to the Sun, it is not the warmest. This is because it has no greenhouse effect, so any heat that the Sun gives to it quickly escapes into space. The hottest planet is Venus.
Mercury is one of four inner planets in the Solar System, and has a rocky body like the Earth. It is the smallest planet in the Solar System, with a radius of 2,439.7 km (1,516.0 mi). Mercury is even smaller than some of the largest moons in the solar system, such as Ganymede and Titan. However, it has a greater mass than the largest moons in the solar system. Mercury is made of about 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only a little bit less than Earth’s.
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- Pluto was once thought to be the smallest, but, as of 2006, Pluto is now known as a dwarf planet.
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The symbol for Mercury represents the Caduceus, a wand with two serpents twined around it, which was carried by the messenger of the gods.
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