All 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 Sky Tonight!
Contents
- Mercury planet facts
- What’s Mercury like? Size, temperature, composition
- Observing Mercury: a quick guide
- F.A.Q.
- Planet 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 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 by the solar wind that pushes sodium atoms off 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 volume by about 5-10 kilometers in radius.
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 eccentric orbit, the Sun appears 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.
Observing Mercury: a quick guide
Is Mercury visible tonight?
From January through early February 2025, the planet will be observed against the background of the dawn. During this period, the planet will move through the constellations of Ophiuchus, Sagittarius, and Capricornus. On February 9, the planet will reach its superior solar conjunction and will be unobservable until the second half of February. Then, it will appear in the evening sky and reach its best visibility on March 8.
Since the planet is very close to the Sun, there are not many opportunities during a year to observe it. The easiest way to find Mercury in the sky is to use the planetary guide Star Walk 2. Just type the name of a planet in the search field, and the app will show you its position in the sky.
Mercury events 2025
Mercury is not easy to see. The planet often gets close to the Sun in the sky, so the sunlight outshines it. Learn when to see the elusive planet with the Sky Tonight app. Tap the magnifier icon in the lower part of the main screen, enter the planet’s name in the search field, and choose the corresponding result. Then, go to the Events tab and see the list of all Mercury-related astronomical events. A quick tip: the planet is best visible at greatest elongation when its apparent distance from the Sun is the largest.
May 30: Mercury at superior solar conjunction
On May 30, 2025, at 02:44 GMT, Mercury will pass behind the Sun, as seen from the Earth. The apparent distance between the two celestial bodies will be 0°33'. The event is called superior solar conjunction and occurs once in 116 days when the planet forms a straight line with the Sun and the Earth. For a few weeks, Mercury will be lost in the Sun’s glare. After that, it will reappear in the sky as an evening object. Avoid observing Mercury while it’s close to the Sun: it may result in permanent blindness.
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.
Planet 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. Share this article with your friends and keep learning more about astronomy with Star Walk 2 and Sky Tonight. 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!
