Mercury
Mercury is the smallest planet in our solar system and the closest to the Sun. It orbits the Sun faster than any other planet, completing a full orbit in just 88 Earth days. Despite its proximity to the Sun, Mercury is not the hottest planet — that title belongs to Venus.
Think you know the planets? Test your solar system knowledge.
Play Solar System gamesKey Data
- Diameter
- 4,879 km
- Mass
- 0.055 Earths
- Distance from Sun
- 0.387 AU
- Surface gravity
- 3.7 m/s²
- Average temperature
- 167°C
- Known moons
- 0
- Orbital period
- 88.0 Earth days
- Rotation period
- 58.6 Earth days
- Ring system
- No
Source: NASA Planetary Fact Sheets (nssdc.gsfc.nasa.gov)
Atmosphere
Trace exosphere of oxygen, sodium, hydrogen, helium, and potassium. Too small and hot to retain a true atmosphere.
In Depth
Mercury is a world of extremes. As the innermost planet, it whips around the Sun at an average speed of 47 km/s, earning its name from the Roman messenger god. Its surface resembles our Moon — heavily cratered, airless, and geologically quiet. The Caloris Basin, formed by a massive ancient impact, spans 1,550 km and is surrounded by mountains nearly 2 km high. Mercury's oversized iron core takes up about 75% of the planet's radius, giving it the highest density of any planet after accounting for compression. The planet has virtually no atmosphere, just a thin exosphere constantly stripped away by the solar wind. This means there is no weather, no wind, and no protection from the Sun's radiation. Mercury's slow rotation combined with its fast orbit creates an unusual situation: a single solar day on Mercury lasts 176 Earth days — twice as long as its year. Water ice exists in permanently shadowed craters near the poles, confirmed by NASA's MESSENGER mission.
Notable Features
- 01
Caloris Basin — one of the largest impact craters in the solar system at 1,550 km across
- 02
Extreme temperature swings from -180°C at night to 430°C during the day
- 03
A massive iron core making up about 75% of the planet's radius
- 04
Scarps (cliffs) hundreds of kilometers long formed as the planet cooled and shrank
Exploration & Missions
Mercury has been visited by only two spacecraft. Mariner 10 made three flybys in 1974-1975, mapping about 45% of the surface. NASA's MESSENGER orbited Mercury from 2011 to 2015, mapping the entire planet in detail and confirming water ice in polar craters. The ESA/JAXA BepiColombo mission launched in 2018 and entered Mercury orbit in 2025 to study the planet's magnetic field, surface composition, and interior structure in unprecedented detail.
Fun Facts
Mercury is shrinking — its circumference has decreased by more than 14 km as its iron core slowly cools and contracts.
You could fit nearly 18 Mercurys inside Earth, but Mercury is denser than Mars.
Despite being closest to the Sun, Mercury has ice in permanently shadowed craters at its poles.
A 68 kg person would weigh only 25 kg on Mercury's surface.
Frequently Asked Questions
Why is Mercury not the hottest planet if it's closest to the Sun?▾
Mercury lacks a substantial atmosphere to trap heat. Venus, though farther from the Sun, has a thick carbon dioxide atmosphere that creates a runaway greenhouse effect, pushing surface temperatures to 464°C — nearly 300°C hotter than Mercury's average.
How long is a day on Mercury?▾
Mercury's sidereal rotation period is about 59 Earth days, but because of its orbital motion, a full solar day (sunrise to sunrise) lasts 176 Earth days. This means Mercury completes two full orbits around the Sun in a single solar day.
Does Mercury have any moons?▾
No, Mercury has no moons. Its proximity to the Sun means any moon would likely be pulled away by the Sun's powerful gravity. Mercury and Venus are the only two planets in our solar system without natural satellites.
Learn More
The 8 Planets in Order: An Interactive Guide for Students — an in-depth article covering all 8 planets, with size comparisons and learning activities.
Data source: NASA Planetary Fact Sheets
Play Solar System games →
Scale Detective, Planet ID, Gravity Slingshot, and more — powered by real NASA data.