Facts About Mercury, The Smallest Planet In The Solar System
Mercury is rather bright but still not easy to see: most of the time, it’s close to the Sun in the sky. Learn how to track Mercury’s movements and pick the best time to spot the planet with the astronomical app Star Walk 2!
Contents
- Mercury planet facts
- What’s Mercury like? Size, temperature, composition
- Mercury in the sky 2026: visibility guide
- Mercury F.A.Q.
- Facts about Mercury: to sum up
Mercury planet facts
- Planet type: terrestrial
- Radius: 2440 km (1516 miles)
- Mass: 3.3011×10^23 kg
- Aphelion: 69.8 million km (43.4 million miles)
- Perihelion: 46.0 million km (28.6 million miles)
- Average distance from the Earth: 77 million km (48 million miles)
- Surface temperature: -173°C to 427°C (−280 °F to 800 °F)
- Solar day length: 176 Earth days
- Sidereal day length: 59 Earth days
- Year length: 88 Earth days
- Age: 4.503 billion years
- Named after: Roman god of commerce
6 fun facts about Mercury we bet you didn’t know
- There are no seasons on Mercury because its axis has almost no tilt (only 2 degrees).
- Ancient astronomers believed Mercury was two different objects because it could appear in the evening western sky or rise in the morning eastern sky. By the way, they thought the same about Venus.
- Like the Moon and Venus, Mercury has phases, but you can see those only through a telescope.
- Mercury is almost certainly not habitable. Its rough temperatures and closeness to the Sun make the planet too extreme for living organisms.
- Mercury has a tail. Many people know comets have tails. But have you heard about a planet that has one? Mercury has a cometlike tail produced when solar radiation and the solar wind release sodium atoms from the planet’s surface. It’s not easy to see: you’ll have to take a long-exposure photo using a telescope and a special filter.
- In Sanskrit, Mercury is called Budha (not to be confused with Buddha, the founder of Buddhism). The Vedic texts call it Budh planet and refer to it as a deity, the son of Soma (or Chandra, the Moon god) and Tara (wife of Bṛhaspati, the god of Jupiter).
What’s Mercury like? Size, temperature, composition
How big is Mercury?
Mercury is the smallest planet in the Solar System in both mass and diameter. It’s 18 times less massive than the Earth, and its diameter barely reaches two-fifths of the Earth’s size. To give you a better understanding of Mercury’s size, let’s say that it’s only a third larger than the Moon.
Surprisingly enough, this small planet got even smaller over time. Cooling of its core has caused the entire planet to shrink, reducing Mercury’s radius by about 5-10 kilometers.
How hot is Mercury?
The temperature of Mercury’s surface varies more than on any other planet in our Solar System. It can range from −173 °C (−280 °F) at night to 427 °C (800 °F) during the day. This happens because the smallest planet has almost no atmosphere to trap heat, and it quickly loses the energy received from the Sun during the daytime.
Although Mercury is the closest planet to the Sun, it isn’t the hottest one (Venus is).
Do you know which object is the coldest, the biggest, or the fastest in the Solar System? Which planet hosts the highest mountain, and where is the largest ocean of liquid water (spoiler: not on the Earth)? Take this quiz and learn more about the space record holders!
How long is a day on Mercury?
A solar day – the amount of time it takes the Sun to rise, set, and rise to the same place again – on Mercury lasts about 176 Earth days. And here is another weird fact about Mercury: one year on this planet is twice shorter than a single day and takes 88 Earth days. How come?
The smallest planet spins slowly about its axis – one rotation takes 59 Earth days. However, its sunrises and sunsets are quite different from the Earth’s ones. Due to the planet’s unusual 3:2 rotation pattern (it rotates three times on its axis for every two orbits around the Sun) and its eccentric orbit, the Sun can appear to rise twice: once, shortly before setting, and then again from some parts of the surface. The same thing occurs in reverse at sunset. Thus it takes much longer for the Sun to appear in the same place again, and one solar day lasts almost twice as long as a year.
Even though it spins slowly, Mercury travels around the Sun faster than any other planet – at 47 km/s (29 mi/s). In comparison, the second-fastest planet in our Solar System, Venus, has a speed of 35 km/s (21 mi/s).
What is Mercury made of?
Since Mercury is a terrestrial planet, it’s mainly composed of iron, nickel, and silicate rock. It has a large iron core, which is about 61% of the planet’s volume (in comparison, the Earth’s core is only 16%), and a 400-kilometer (250 miles) thick outer shell. By the way, Mercury’s core is about the same size as our Moon. Another similarity with the Moon is that the planet’s dry and rocky surface is full of craters.
Craters are the results of impacts with asteroids or other space objects – the more craters a planet has, the older it is. Since Mercury’s surface is heavily cratered, it’s most likely ancient.
Mercury in the sky 2026: visibility guide
Here is a quick visibility guide for Mercury in the upcoming months of 2026:
- February: A very favorable evening visibility begins; Mercury reaches its greatest elongation on February 19, appearing farthest from the Sun in the sky. On February 25, Mercury begins retrograde motion.
- March: Mercury becomes unobservable as it reaches inferior conjunction on March 7. Later in the month, it appears in the morning sky. On March 19, Mercury ends retrograde motion.
- April: Mercury is a morning object, reaching its greatest elongation on April 4. This is the year’s greatest elongation of Mercury!
- May: The morning apparition ends in early May as Mercury disappears into the sunrise glow; it reaches its superior conjunction on May 14.
- June: Mercury appears in the evening sky, reaching its greatest elongation on June 15, best visible from the Southern Hemisphere. The planet begins retrograde motion on June 29.
- July: Mercury heads toward inferior conjunction on July 13, appearing lower in the evening sky; it switches from retrograde back to direct motion on July 23 and becomes a morning object by the end of the month.
- August: Mercury is visible in the morning sky, best-seen from the Northern Hemisphere. It reaches its greatest elongation on August 2.
- September: Mercury remains in the morning sky into mid-September; it reaches superior conjunction on September 27, getting lost in the Sun’s glare.
- October: Mercury returns to the sky as an evening object. On October 24, Mercury begins retrograde motion.
- November: Mercury reaches inferior conjunction on November 4 and disappears into the Sun’s glare. The planet ends retrograde motion on November 13, and reaches its greatest elongation on November 21, appearing in the morning sky.
- December: Mercury remains a morning object until mid-December, gradually slipping back into the sunrise glow.
Mercury events to see in 2026
Mercury likes to stay close to the Sun, so you can’t see it whenever you want, but when it does pop into view, it often comes with some great sky moments: close passes with the Moon, planets, or stars. To quickly check whether Mercury is visible now from your location (and exactly where to look), use the Star Walk 2 planetary guide. Just type the name of a planet in the search field, and the app will show you its position in the sky.
February 18: Mercury near the Moon, lunar occultation of Mercury
- Conjunction time: February 18, 23:03 GMT
- Conjunction distance: 0°08'
- Close approach time: February 18, 23:10 GMT
- Close approach distance: 0°07'
- Occultation start: February 18, 20:50 GMT
- Occultation end: February 19, 01:30 GMT
After sunset on February 18, look low in the west to spot Mercury near the horizon. The planet will sit very close to the Moon in Aquarius and shine at about magnitude -0.6, bright enough to see with the naked eye if the sky is clear and you have an unobstructed horizon. The Moon, however, will be only 2.5% illuminated (just after the New Moon phase), so it will likely be hard to notice without optics.
For observers in some regions, there’s an extra treat: a lunar occultation of Mercury, when the almost invisible lunar disc passes in front of the planet, and Mercury briefly disappears from view. This occultation will be visible from parts of the world, including the contiguous United States, Mexico, eastern Australia, and New Zealand.
February 19: Mercury at greatest eastern elongation
On February 19, at 16:59 GMT, Mercury (mag -0.5) will be at 18°07′ east of the Sun. The planet will be visible in the constellation Aquarius after sunset.
As a bonus, a thin waxing crescent Moon, Venus, and Saturn will gather in the same area of the sky. The Moon will be only 5% illuminated, so it may be hard to spot, especially if it’s low above the horizon, but Venus (mag -3.9) and Saturn (mag 1.0) will be easily visible without any optics. It’s a great chance to catch the Moon and three planets together in a small patch of sky!
February 25: Mercury goes retrograde
On February 25, Mercury starts its retrograde period, meaning it will begin to move backward (from east to west) in the sky relative to the background stars. The planet isn’t actually reversing course in space – it’s just an optical illusion that occurs because Earth and Mercury orbit the Sun at different speeds and distances. The retrograde motion of Mercury will end on March 19, 2026.
Retrograde motion happens to other planets too, but for some reason, only Mercury gets the reputation for causing all sorts of earthly chaos. Read our article on retrograde motion to ditch the superstitions – or, if you prefer, to find more retrograde planets to blame for your bad days.
February 26: Mercury near Venus
- Conjunction time: February 26, 23:09 GMT
- Conjunction distance: 4°44'
- Close approach time: February 27, 21:46 GMT
- Close approach distance: 4°30'
On February 26, look to the western sky after sunset to see Mercury (mag 1.2) close to brilliant Venus (mag -3.9). Both planets will sit low above the horizon, so an open view is important. Mercury will be in Pisces, while Venus will shine in nearby Aquarius. Saturn (mag 1.0) will also be in the area, forming a little planetary triangle with Mercury and Venus.
February 28: Mercury takes part in a 6-planet alignment
Around February 28, 2026, just after sunset, six planets – Jupiter, Uranus, Saturn, Neptune, Venus, and Mercury – will line up in the sky. Four of them (Jupiter, Saturn, Venus, and Mercury) will be easily visible to the naked eye, while Uranus and Neptune will require binoculars or a small telescope. It’s an excellent opportunity for stargazing or astrophotography – learn more about this planetary alignment in our dedicated article.
Mercury F.A.Q.
What color is Mercury?
Mercury is primarily dark gray. A planet’s color depends on the color of its surface and its reflective abilities. Unfortunately, Mercury’s rocky exterior covered with dust doesn’t give a wide variety of colors.
How many moons does Mercury have?
None. Without going into details, Mercury just cannot form, capture, or acquire a moon due to the planet’s small size, weak gravity, and proximity to the Sun.
How far is Mercury from the Sun?
Its average distance from the Sun is about 58 million km (36 million miles). Thus, among other planets in the Solar System, Mercury is the closest one to the star.
When was Mercury discovered?
We don’t know for sure – the first recorded observation was made in 265 BC. Because Mercury is so close to the Sun, it’s often hidden by its glare. The first people who observed Mercury through a telescope were Galileo Galilei and Thomas Harriot in the 17th century.
Why is Venus hotter than Mercury?
Despite Mercury being the closest planet to the Sun, Venus is hotter because of the thick Venusian atmosphere trapping heat. Mercury, in its turn, has no significant atmosphere and can’t hold the Sun’s energy.
What does the Sun look like from Mercury?
If you could stand on Mercury, at aphelion (the farthest Mercury can get from the Sun), you would see the Sun twice bigger than it appears from the Earth. At perihelion (the closest distance), the Sun would appear three times larger than it does on our planet.
Facts about Mercury: to sum up
Mercury is, without a doubt, one of the most extreme planets of all: this planet is the smallest and the closest to the Sun, and the difference between night and day temperatures here makes up about 610°C (1,100°F). Mercury has no moons and almost no atmosphere. This planet is also one of the hardest to observe as it’s near the Sun in the sky most of the time.
The good news is that Mercury still puts on some great sky shows – for example, on February 18, 2026, it appears extremely close to the Moon, and in some regions it’s even occulted (briefly hidden) by the lunar disk. To catch moments like this and find Mercury from your exact location, use the free Star Walk 2 app.
You can also watch the fun and educational cartoon about Mercury that explains the main facts about the planet in simple words.
Wishing you clear skies and happy observations!
