Hugo's Solar System
Planet
Mercury
The smallest planet, racing round the Sun in just 88 days
A rocky old pebble, all dusty and grey,
"I'm closest to the Sun, it's toasty all day."From Hugo's Journey Through Space
Key facts
- Diameter
- About 3,030 miles / 4,880 km (a little over a third of Earth's)
- Distance from the Sun
- 0.39 AU (about 36 million miles / 58 million km on average)
- One orbit
- 88 Earth days
- One day (rotation)
- 59 Earth days to spin once; sunrise to sunrise takes 176 Earth days
- Average temperature
- About 430°C by day and about -180°C at night
- Known moons
- 0 (as of September 2026)
- Rings
- None
- Named after
- Mercury, the swift Roman messenger god
- Discovered
- Known since ancient times
- Visited by
- Mariner 10 (1974-75), MESSENGER (2011-15), BepiColombo (orbit due 21 November 2026)
- Magnetic field
- Yes, about 1% as strong as Earth's
About Mercury
Mercury is the smallest planet. It is the closest planet to the Sun. Mercury is a small, grey, rocky ball. It is only a bit bigger than our Moon. It has no moons of its own. The Sun looks huge from Mercury. Daytime there is hotter than an oven. Night time is colder than ice. Mercury has lots of bumpy craters. It zooms round the Sun very fast. One trip takes just 88 days. Can you see it in the sky? Look just after sunset or before sunrise.
Mercury is the smallest planet and the closest one to the Sun. It is a rocky sphere about 4,880 kilometres across. That makes it only a little wider than our Moon. Mercury orbits the Sun at an average distance of about 58 million kilometres.
Because it is so close, it moves faster than any other planet. One orbit takes just 88 Earth days, so its year is very short. Mercury rotates slowly on its axis, spinning once every 59 Earth days. Because of this, one full day and night lasts 176 Earth days.
Mercury has almost no atmosphere to hold heat in or keep it out. The sunny side can reach about 430 degrees Celsius, hot enough to melt lead. The dark side drops to about minus 180 degrees Celsius. Surprisingly, there is frozen water ice hidden inside deep craters at the poles.
Sunlight never reaches the bottom of those craters, so the ice stays cold. Mercury's surface is covered in craters, a bit like our Moon. The biggest one, called Caloris, is about 1,550 kilometres wide. Mercury has no moons of its own and no rings.
Three spacecraft have studied it closely: Mariner 10, MESSENGER and BepiColombo.
Mercury is the innermost and smallest of the eight planets, with a diameter of about 4,880 km, roughly a third of Earth's and only slightly larger than the Moon. It orbits the Sun at an average distance of 58 million km (0.39 AU) on an elliptical path ranging from 47 million km to 70 million km. It completes an orbit every 88 days at about 47 km per second, faster than any other planet.
Its rotation is unusual. Mercury spins once every 59 Earth days, locked in a 3:2 resonance with its orbit, so it rotates exactly three times for every two trips around the Sun. A full solar day, sunrise to sunrise, therefore lasts 176 Earth days, twice the Mercurian year. Its axis is barely tilted, so it has no seasons.
A common misconception is that Mercury, being closest to the Sun, must be the hottest planet. It is not. Mercury has only a wisp of an exosphere (oxygen, sodium, hydrogen, helium and potassium knocked off the surface by the solar wind), so it cannot trap heat. Daytime temperatures reach about 430°C, but the night side falls to about -180°C. Venus, wrapped in a thick carbon dioxide atmosphere, is hotter around the clock. Mercury's near-zero tilt also means the floors of some polar craters never see sunlight, and NASA's MESSENGER spacecraft confirmed water ice preserved there.
Mercury is the second densest planet after Earth. Its iron-rich core has a radius of about 2,074 km, about 85% of the planet's radius, leaving a rocky shell only about 400 km thick. The core generates a weak global magnetic field, about 1% the strength of Earth's. The heavily cratered surface includes the Caloris basin, an impact structure about 1,550 km across, and long cliffs formed as the planet cooled and shrank.
Three spacecraft have studied Mercury. Mariner 10 made three flybys in 1974 and 1975, imaging nearly half the surface and detecting the magnetic field. MESSENGER, launched in 2004, orbited from March 2011 until it was deliberately crashed into the surface on 30 April 2015, returning more than 200,000 images. BepiColombo, a joint ESA and JAXA mission launched on 20 October 2018, has completed six Mercury flybys and is scheduled to enter orbit on 21 November 2026 with two separate orbiters. Mercury has no moons and no rings. Known since ancient times, it is named after the swift Roman messenger god.
Did you know?
- Mercury is not the hottest planet, even though it is closest to the Sun. Venus is hotter because its thick atmosphere traps heat.
- One day on Mercury, from sunrise to sunrise, lasts 176 Earth days. That is twice as long as its year of 88 days.
- There is frozen water ice hiding in craters at Mercury's poles, where sunlight never reaches the ground.
Moons of Mercury
Mercury has no moons.
Questions children ask
Why is Mercury not the hottest planet if it is closest to the Sun?
Mercury has almost no atmosphere, so it cannot hold on to heat. The day side reaches about 430°C, but the night side drops to about -180°C. Venus has a thick blanket of carbon dioxide that traps heat all the time, so Venus is hotter.
Does Mercury have any moons?
No. Mercury has no moons and no rings, as of August 2026. Venus is the only other planet in the solar system with no moon.
How long is a day on Mercury?
It depends what you mean by a day. Mercury spins once on its axis every 59 Earth days. But because it also moves round the Sun, the time from one sunrise to the next is 176 Earth days, longer than its 88-day year.
Which spacecraft have been to Mercury?
Three so far. Mariner 10 flew past three times in 1974 and 1975. MESSENGER orbited Mercury from 2011 to 2015 and found water ice at the poles. BepiColombo, built by Europe and Japan, launched in 2018 and is due to enter orbit on 21 November 2026.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- How long does Mercury take to go once round the Sun?
- Which planet is hotter than Mercury?
- What did the MESSENGER spacecraft find hiding in craters at Mercury's poles?
Venus
Earth
The Moon