Science & Technology

The solar system’s quickest planet.

Welcome to the fastest moving planet in our solar system: Mercury. In fact, speed isn’t the only record it holds. It’s the smallest, the closest to the sun, and the one with the shortest year as well. Mercury is a world of extremes, and it’s no surprise the ancients took notice.

A Wandering Star, an Elusive Planet

Mercury orbits just 36 million miles from the sun – less than half the distance from our star to Earth – and the effects of gravity are a bit more extreme. In fact, that’s the defining theme of the tiny planet: It’s a world of extremes. Because the sun pulls on it more than any other in the system, Mercury has the fastest orbital speed and the shortest orbit. Combine those facts and you get a year – that is, one full trip around the sun – of only 88 Earth days.

As fast as the tiny planet zips through its orbit, though, it actually rotates quite slowly. Mercury spins on its axis, completing one full day and night cycle, in 176 Earth days – just over two Mercury years. This spin-orbit resonance, as it’s called, is the result of tidal forces from the sun, which slowed the planet’s rotation over billions of years. But, due to the planet’s size, its own gravity is much lower. If you weigh 100 pounds on Earth, for example, you’d weigh only 38 pounds there!

Mercury’s closeness to the sun had other effects as well. Thanks to the gravity and the heat, any softer materials would never have lasted on the planet’s surface, and there’s no real atmosphere to speak of. The planet – which is just slightly bigger than our moon – is pretty much a big ball of iron covered in a thin layer of rock. Daytime temperatures can reach as high as 800 degrees Fahrenheit, with nighttime temperatures falling to 290 degrees below zero.

Mercury is visible to the naked eye, but thanks to how close it is to the sun, it doesn’t hang around long. That, combined with its short orbit and high speed, made the ancients who discovered it realize it was different from the other bright lights in the sky, leading some to call it a “wandering star.”

Mercury, the Winged Messenger

It’s small, it’s fast, it’s there, then it isn’t. Is it any wonder this “wandering star” was named by the Romans and the Greeks (who called it Hermes) for their messenger gods? Mercury in Roman mythology wore winged sandals and delivered news at lightning speed. Hermes shared similar powers and duties among the gods in Greek belief. Both represented communication, travel, and quick thinking – appropriate attributes for the swift planet.

British composer Gustav Holst also featured Mercury in his musical suite titled The Planets. “Mercury, the Winged Messenger” captures the quick and lively spirit associated with the small planet. The rhythms are fast and the melodies playful.

As fun as Mercury may sound, it’s unlikely we’ll ever land on its surface, much less set up colonies there. The extreme conditions would be far too difficult to overcome for the potential benefit of parking people on the planet. But that doesn’t mean we aren’t trying to study it. In the 1970s, NASA’s Mariner 10 flew by Mercury three times, sending back to Earth the first close-up pictures. In 2011, the MESSENGER spacecraft orbited Mercury, mapped its surface, and shockingly discovered ice hidden in shadowy polar craters.

And there’s more to come. The European Space Agency and Japan are working on a mission called BepiColombo. The idea is to orbit Mercury and learn more about the planet, such as what it’s made of, how it formed, and, importantly, how it has survived so close to the sun.

Mercury may be small, but there’s still plenty to learn about it. What other surprises might the wandering star hold?

1. Mercury is the closest planet to the sun. It’s also the smallest, fastest moving, and has the shortest orbit in the whole solar system.
2. One full solar day on Mercury lasts just over two of Mercury’s years.
3. The Romans and Greeks imagined Mercury (or Hermes, as the Greeks called it) as the messenger of the gods.