Mercury, the Fastest Planet in the Solar System

Mercury’s Orbit & Rotational Period

When it comes to Mercury’s orbit, we find two main things that make it unique from the other planets. First is that, unlike the other planets, Mercury orbits the Sun “inferior” to Earth. In other words, it orbits much closer to the Sun than we do. First, the distance between Mercury and the Sun is only 37 million km (23 million mi). That’s less than a quarter of the distance between the Earth and the Sun. This is the reason why Mercury is so hot, It is much closer to the Sun and receives much more solar radiation than any other planet. Second, Mercury’s rotational period is very slow. It actually takes 58 Earth days for the planet to complete one full rotation around its axis.

Why is Mercury the Fastest Planet?

As we said, Mercury is the fastest planet in the solar system. How is that possible? Well, like the other planets, it is also in orbit around the Sun. But in addition to its regular orbit, Mercury also has an extremely elliptical orbit. Five planets in our solar system have elliptical orbits: Mercury, Venus, Earth, Mars, and Jupiter. Each planet’s elliptical orbit has two foci (two points of focus). While the two points of focus for Earth’s elliptical orbit are at equal distances from the Sun, the two foci for Mercury’s elliptical orbit are at different distances from the Sun. This happens because Mercury’s elliptical orbit is more circular than Earth’s elliptical orbit. The more circular a planet’s elliptical orbit is, the closer the two foci are to each other. Mercury’s elliptical orbit is so circular that its two foci are only 35 million km (22 million mi) apart. This is the reason why Mercury is the fastest planet and why it moves so quickly around the Sun. It is closer to the Sun than any other planet.

Mercury Has a Unique Orbit

Mercury has a unique orbit because it is the only planet that orbits the Sun in a direction opposite the other planets. All the other planets orbit the Sun in a counterclockwise direction, while Mercury orbits the Sun in a clockwise direction. This has led to two interesting phenomena:

- The Sun’s path through the sky: On Mercury, the Sun rises in the west and sets in the east every single day. This is because the planet’s orbit is in the opposite direction of the other planets.

- Mercury’s seasons: Since Mercury has a very eccentric orbit, the planet’s seasons last for twice as long as the seasons on Earth. In fact, Mercurial seasons last for about two Earth years.

Mercury Has no Atmosphere

Actually, all the planets in our solar system (except for Earth) have no atmosphere. The reason for this is that planets with atmospheres don’t have enough gravity to hold onto them. This is because the weight of the atmosphere is simply too much for a planet to sustain. However, the planets without atmospheres still have something called an exosphere that extends beyond their atmospheres. This is where we find the gases hydrogen and helium. Since these gases are light and are not held down by any planet’s gravity, they escape into space. Because Mercury is such a small planet, it doesn’t have enough gravity to hold onto an atmosphere.

One Final Fact: Mercury Has No Days!

We’ve talked about how Mercury has no atmosphere or seasons, and how it has a very slow rotational period. Well, Mercury also has no days. When we say there are no days on Mercury, we actually mean that there are no days as we know them. This is because Mercury rotates so slowly that a single day lasts almost twice as long as a year!