PlanetSolar system
Mercury
The smallest planet and the closest to the Sun: a cratered, almost airless world of extremes, where a single day lasts longer than its year.
- Planet type
- Terrestrial
- Mean radius
- 2,439.7 km
- Distance from the Sun
- 0.387 AU · 57.9 million km
- Day length
- 58.6 Earth days
- Orbital period
- 88 Earth days
- Surface temperature
- About −180 to 430 °C
- Moons
- None known
- Atmosphere
- A thin, temporary exosphere
The swiftest planet
Mercury completes an orbit in only 88 Earth days, faster than any other planet. Its spin and orbit are locked in a 3:2 resonance: it rotates three times for every two orbits, so one solar day on Mercury lasts about 176 Earth days — longer than its year.
Its orbit is also noticeably elliptical. At perihelion Mercury comes within 0.31 AU of the Sun; at aphelion it reaches 0.47 AU. That changing distance, together with the lack of a substantial atmosphere, produces some of the most extreme temperature swings in the Solar System.
A cratered, airless world
Mercury’s surface is dominated by impact craters, including the vast Caloris basin, and by long lobate scarps that record the planet’s cooling and contraction. Without a real atmosphere, there is no weather, no wind erosion, and very little protection from solar radiation.
Water ice can survive in permanently shadowed polar craters, even though the sunlit surface can reach 430 °C. Mercury’s weak global magnetic field and its surprising volatile inventory remain central questions for the BepiColombo mission.
A real view of Mercury
NASA/JHUAPL · MESSENGER
BepiColombo and the next questions
Europe and Japan’s BepiColombo mission has been using Mercury flybys to shape its trajectory and test its instruments. Its two orbiters are designed to study the surface, interior, exosphere, and magnetosphere together.
- How did Mercury form with such a large iron-rich core?
- How do volatiles survive on a planet this close to the Sun?
- What drives Mercury’s weak but complex magnetic field?
- How has the surface changed over the last four billion years?
Recent publications
| Year | Paper | Journal | Link |
|---|---|---|---|
| 2026 | BepiColombo at Mercury: Three Flybys, Three Magnetospheres | Geophysical Research Letters | DOI |
| 2026 | Observations of Mercury’s Bow Shock Disappearance in Quiet Solar Wind | Astronomical Journal | DOI |
| 2025 | He+ ions in the vicinity of Mercury observed by MESSENGER and BepiColombo | Planetary and Space Science | DOI |
| 2024 | Understanding the Dust Environment at Mercury: From Surface to Exosphere | Planetary Science Journal | DOI |
| 2024 | Mercury Spectrophotometric Modeling Using MESSENGER Data | Planetary Science Journal | DOI |
Compiled 20 October 2026. Links resolve to the publisher DOI record.