Named after the fleet-footed Roman god, Mercury is the smallest planet in the Solar System and the closest to the Sun, so close that it has been tricking astronomers for centuries. Hidden behind the extremely bright glare emitted by our host star, Mercury can only be observed in the either morning or evening twilight, for short period of times.

Failure to observe this elusive planet did not spare big star astronomers such as Copernicus, who blamed the miserable weather condition in northern Poland, and Kepler, who predicted the planet’s rare transit in front of the Sun in November 1631 only to die a year earlier missing out on the opportunity.

Even NASA’s Hubble Space Telescope avoided glancing at Mercury to prevent potential damage to its optics. It is no surprise then that the first detailed images only came in between 1974 and 1975, when the Mariner 10 space probe completed three close flybys of the mysterious planet.

The thousands of pictures taken by NASA’s space probe revealed a view of Mercury that seemed oddly familiar to us all: it looks a lot like The Moon!

From its greyish-brown appearance to the impact craters scattered all over the surface and the brighter streaks called “crater rays” that spread out from them. The two are even similar in size, with Mercury just slightly bigger than The Moon.

The proximity with the Sun affects the planet in several ways. At only 58 million km (36 million miles) distance, the gravitational pull exerted by our star is extreme and fuels Mercury up to a speed of 47km (29 miles) per second. At this speed it takes the planet only 88 Earth days to complete a full orbit around the Sun, a Mercurian year.

On the other hand, that same gravitational pull has a breaking effect on the planet’s spin to such an extent that Mercury completes a rotation on its own axis in 58.6 Earth days.

However, due to its elliptical, fast orbit, a full spin is not accompanied by a sunrise and sunset like it is on most other planets when Mercury is at its closest distance from the Sun. In such phase, a day measured as day/night cycle lasts 176 Earth days, or a little over 2 Mercurian years.

Without a mitigating atmosphere, the planet temperatures are heavily influenced by the Sun too, reaching extreme highs (approx. 430° C) where the surface is lit, before dropping to extreme lows (-180° C) at night. There are no seasons on Mercury, unlike most planets, as its axis is tilted just 2 degrees with respect to its orbit around the Sun.

Credit: NASA ®

Despite being a terrestrial planet with ice water potentially hidden at its north and south poles inside deep craters, in regions of permanent shadow, Mercury is not seen as a potential candidate for hosting life.

It does not have any spectacular rings or any moons in its orbit but can boast a main role in one of the most important scientific discoveries of all times!

In 1859 the French astronomer Leverrier, famous for having successfully predicted the existence of Neptune, announced a redundant anomaly in Mercury’s orbit, which he suggested it may have been the gravitational effect of yet another undiscovered planet.

The search for this hidden planet went on for decades without success until the mystery was finally solved in 1915 with the discovery that revolutionised the understanding of the universe-Einstein’s theory of relativity.

When he first announced his ingenious theory, Einstein shocked the whole world. He had shown how Newton’s laws were incomplete by demonstrating how space itself is warped by the extreme gravitational field in proximity of a large celestial body. It was a sensational discovery and incredible theory that was soon globally accepted.

The theory of relativity revealed how the Sun’s extreme gravitational field distorted space around it causing the change in Mercury’s orbit.

Einstein himself expressed great joy when his relativity-founded calculations of Mercury’s orbit were verified by the observations. He soon became the first scientist celebrity and some credit, perhaps, goes to Mercury too.