Mercury is the closest planet to the Sun and the smallest planet in the solar system. This rocky world is only slightly larger than Earth’s Moon and completes an orbit in just 88 Earth days. Its heavily cratered surface, extreme temperature changes and unusually large iron core make Mercury far more complex than its Moon-like appearance suggests.
The Closest and Smallest Planet
Mercury is the first planet from the Sun, orbiting at an average distance of about 36 million miles (58 million kilometers). Its elliptical orbit carries it between approximately 29 million and 43 million miles (47 million and 70 million kilometers) from the Sun.
At 3,032 miles (4,880 kilometers) across, Mercury is only about 40% wider than Earth’s Moon and less than half the diameter of Earth. Although it has only about 5.5% of Earth’s mass, its surface gravity is approximately 38% as strong as Earth’s—almost the same as the gravity on Mars. Mercury has no moons or rings.
Mercury’s Orbit and Unusual Rotation
Mercury travels around the Sun faster than any other planet, moving at an average speed of about 29 miles (47 kilometers) per second. It completes one orbit in just 88 Earth days, giving it the shortest year in the solar system.
The planet rotates much more slowly, turning once on its axis every 58.6 Earth days. Mercury completes exactly three rotations for every two orbits of the Sun, a relationship known as a 3:2 spin–orbit resonance. Because its rotation and orbital motion work together, one complete solar day—from one sunrise to the next—lasts 176 Earth days, or twice as long as a Mercurian year.
A Heavily Cratered Surface
Mercury’s gray, rocky surface resembles the Moon because both worlds preserve the scars of billions of years of impacts. Its largest named feature is the Caloris Basin, an immense impact basin about 960 miles (1,550 kilometers) across. Mercury also has broad plains created by ancient volcanic eruptions, showing that it was once geologically active.
Long, curved cliffs known as lobate scarps cross many parts of the planet. Some extend for hundreds of miles and rise more than half a mile above the surrounding terrain. They formed as Mercury’s interior cooled and contracted, causing its crust to buckle and break. Evidence from MESSENGER suggests that this gradual shrinking has continued much more recently than scientists once believed.
An Enormous Iron Core
Mercury has the largest core relative to its size of any planet in the solar system. Its metallic core extends through approximately 85% of the planet’s radius, leaving only a comparatively thin rocky mantle and crust. This unusually large concentration of iron helps explain why Mercury is the second-densest planet after Earth despite being much smaller.
Early theories suggested that a giant impact stripped away much of Mercury’s original rocky exterior. However, measurements of volatile elements in its surface material indicate that its formation was more complicated than simple heat loss or a single collision. Mercury cannot therefore be explained merely as a larger planet that was stripped down to its iron core.
Mercury Has a Magnetic Field
Mercury is the only rocky planet other than Earth with a global magnetic field generated inside the planet. Its field is only about 1% as strong as Earth’s at the surface, but it is powerful enough to create a small magnetosphere around Mercury.
The field is thought to be produced by the movement of electrically conducting material within Mercury’s partly molten outer core. It is also noticeably offset toward the planet’s north pole, making the magnetic field much stronger in the northern hemisphere than in the south. This small magnetic shield is highly dynamic because Mercury orbits so close to the Sun.
Almost No Atmosphere
Mercury does not have a substantial atmosphere capable of trapping heat or producing weather. Instead, it is surrounded by an extremely thin exosphere made of atoms released from its surface. These include oxygen, sodium, hydrogen, helium and potassium.
Solar radiation, the solar wind and tiny meteoroid impacts continually knock particles from the ground, while Mercury’s weak gravity allows many of them to escape into space. The exosphere must therefore be constantly replenished. Sunlight and the solar wind also push some sodium atoms away from the planet, creating a faint tail that stretches millions of miles behind Mercury.
Extreme Temperatures and Polar Ice
With almost no atmosphere to redistribute heat, Mercury experiences the greatest day-to-night temperature range of any planet. Temperatures near the equator can reach about 800°F (430°C) during the day before falling to around −290°F (−180°C) at night. Mercury is therefore not the hottest planet overall—that distinction belongs to Venus, whose dense atmosphere retains heat.
Despite Mercury’s proximity to the Sun, deposits of water ice exist inside permanently shadowed craters near its poles. Because the planet’s axis is barely tilted, sunlight never reaches the floors of some deep polar craters. Radar observations first indicated reflective material in these regions, and MESSENGER later confirmed that the deposits contain substantial quantities of water ice.
Known Since Ancient Times
Mercury has no known discoverer because it is visible without a telescope and has been observed since antiquity. Babylonian astronomers recorded its movements nearly 3,700 years ago, while the ancient Greeks eventually recognized that its appearances before sunrise and after sunset belonged to the same object.
The Romans named the planet after Mercury, their swift messenger god, because of its rapid movement through the sky. His Greek counterpart was Hermes, and both gods were associated with travel, communication and commerce.
Spacecraft Missions to Mercury
Mercury has been explored by three spacecraft missions. NASA’s Mariner 10 made three flybys between 1974 and 1975, photographing about 45% of the surface and discovering that the planet has a magnetic field. It was the first spacecraft to use a gravity-assist maneuver to travel from one planet to another.
NASA’s MESSENGER became the first spacecraft to orbit Mercury in 2011. During more than four years in orbit, it mapped the entire planet and transformed our understanding of Mercury’s composition, internal structure and volcanic history. The mission ended in 2015 when the spacecraft was deliberately allowed to strike Mercury’s surface.
BepiColombo, a joint mission from the European Space Agency and the Japan Aerospace Exploration Agency, completed its sixth Mercury flyby in January 2025. It is scheduled to enter orbit on November 21, 2026, before separating into two orbiters that will study Mercury’s surface, interior, exosphere and magnetic environment.
How to See Mercury
Mercury is visible without a telescope, but it can be difficult to find because it always remains close to the Sun in our sky. At greatest elongation it can appear no more than about 28 degrees from the Sun, so it is seen low in the east shortly before sunrise or low in the west shortly after sunset. A clear, unobstructed horizon is essential.
The best evening appearances for Northern Hemisphere observers generally occur around spring, while the most favorable morning appearances tend to occur in autumn. An astronomy app or current sky chart can provide Mercury’s position for a particular date and location. To use binoculars safely, wait until the Sun has completely set during an evening appearance; never sweep the sky near the Sun with binoculars or a telescope.
To the naked eye, Mercury resembles a bright, steady point of light. A small telescope can reveal its crescent and gibbous phases as the planet moves around the Sun. Its small apparent size and low position in turbulent air make surface details extremely difficult to observe.