What is the Smallest Planet in Our Solar System? Mercury Facts for Kids

What is the smallest planet in the Solar System? This is one of the questions that might come to the minds of kids, and one which they usually ask their parents about to try and get an answer for. The right answer for this question is that the smallest planet in our Solar System is Mercury.

Introduction to Mercury: The Smallest Planet

Mercury is one of the eight planets that orbit the Sun in our Solar System. It is the smallest and the closest planet to the Sun. The average distance from Mercury to the Sun is about 58 million kilometers. As a result, Mercury travels around the Sun faster than any other planet.

Structure and Surface

Mercury is one of the rocky planets in our Solar System (the others are Venus, Earth and Mars). Its surface looks like the surface of the Moon in that it is solid and covered with craters. The craters form when chunks of rock or metal called meteorites hit Mercury. Some parts of Mercury’s surface have many craters, while other parts are very smooth and flat plains. Mercury is very heavy for its size. This is because it has a huge core made of iron.

Planets for Kids
Planets for Kids

Temperature and Atmosphere

Even though it is the closest planet to the Sun, temperatures on Mercury are both hot and cold. The average surface temperatures of Mercury can reach up to 800 degrees Fahrenheit / 430 degrees Celsius during the day. However, surface temperatures at night can drop to as low as -290 degrees Fahrenheit / -180 degrees Celsius. So, temperature changes on Mercury are the most extreme in the Solar System.

Temperatures on Mercury
Temperatures on Mercury

The extreme change in temperature on Mercury is due to its thin atmosphere. Mercury is surrounded by a thin atmosphere, or a layer of gases, that does not keep in heat well.

Orbit and Rotation

Mercury has two types of motion: orbit around the Sun and spin around its centre. It orbits the Sun very quickly. It orbits at an average rate of about 48 kilometers per second. It completes one orbit around the Sun every 88 Earth days, so a year on Mercury lasts 88 Earth days. On the contrary, Mercury spins very slowly. It takes nearly 59 Earth days to rotate once about its center.

Spinning slowly while orbiting the Sun quickly results in an unusual situation. It takes about 176 Earth days for the Sun to rise, set, and then rise again on Mercury. This means that a day on Mercury lasts about 176 Earth days. So, the unusual situation is that, on Mercury, a day is longer than a year.

History and Exploration

Mercury has been known since ancient times because it can be seen without advanced telescopes. Recently, it has been visited by two spacecraft: Mariner 10 and Messenger.

NASA’s Mariner 10 was the first spacecraft to visit planet Mercury. It was an unmanned American craft that flew by Mercury in the 1970s. It mapped about 45% of the planet’s surface. In 2004, the United States launched a second unmanned craft, called Messenger, to Mercury. This craft completed mapping the planet’s surface.

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  • The name of planet Mercury is taken from the Roman god of commerce and travel. In Greek mythology, Mercury was known as Hermes, who was the messenger of the gods. It is probably named so because it moves very quickly across the sky.
  • In ancient times, Greek astronomers thought that Mercury was two different planets in the sky: The Mourning Star and The Evening Star. They called the one they saw at sunrise Apollo and the one they saw at sunset Hermes.
  • Mercury’s diameter is 4,878 km. This makes it about two-fifths the size of Earth. It would take 18 Mercury planets to match Earth’s size.
  • Mercury is slightly bigger than Earth’s Moon, and it also looks like our Moon.
  • Jupiter’s moon Ganymede and Saturn’s moon Titan are both bigger in size than Mercury.
  • Mercury is nearly two times bigger in size than Pluto.
  • Mercury does not have any moons orbiting it, and it does not have any ring systems.
  • Mercury and Venus orbit the Sun within Earth’s orbit. This makes them inferior planets.
  • Even though Mercury is the closest planet to the Sun, it is not the hottest, but Venus is. This is because Mercury has no greenhouse effect, which causes the heat that comes from the Sun to quickly escape into space.
  • Sunlight takes about 3.2 minutes to travel from the Sun and reach Mercury.
  • Mercury only has one sunrise every 180 Earth days.
  • The Sun on Mercury appears three times bigger than it does on Earth, and the sunlight there is seven times more powerful.
  • Even though it is very hot on Mercury, there is frozen water on its surface. Some of the deep craters at the poles are never exposed to direct sunlight, and temperatures there are very low.
  • Mercury is the second-densest planet in the Solar System after Earth even though it is very small. This means it is very compact. This is because it has a large iron core.
  • Mercury is bright, and we can see it from Earth. However, it cannot be seen easily because it is inside the Earth’s orbit. This means that to look at Mercury, you have to look towards the Sun’s direction. The bright sunlight makes it difficult to see Mercury. So, the best time to see Mercury from Earth is in the morning or evening twilight, or when there is a solar eclipse.
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