Mercury-The Smallest Planet: 5 Unknown Facts

Mercury is the planet, closest to the Sun. It is also the smallest planet in the Solar System. Due to its proximity to the Sun, it takes the shortest orbiting time in the Solar system of just, 87.97 Earth days. Sunlight takes approximately 3.2 minutes to reach the planet. 

The planet gets its name after the Greek god Hermes (translated into Latin as Mercurius). Mercury is regarded as the god of commerce, moderator between gods and mortals, and the messenger of the gods.

The orbit of Mercury lies within the Earth’s orbit as an inferior planet, and its estimated distance from the Sun, as examined from Earth, is never more than 28°. As the planet lies closest to the Sun, it can only be found near the western horizon after sunset or near the eastern horizon before sunrise, generally during twilight. In the sky, it may seem like a small, bright star-like object, but is comparatively far more challenging to detect than Venus. 

Quick Facts

Rotation time-period59 Earth Days
Revolution time-period87.97 Earth Days
Distance from the Sun57.91 million km
MoonsMercury has no moon.
Gravity3.7 m/s²
Radius2,439.7 km
Mass3.285 × 10^23 kg (0.055 M⊕)


In terms of density, Mercury comes second after Earth. It has a metallic core expanding over a radius of around 2,074 kilometers, making approximately 85% of the planet’s radius. According to some evidence gathered by astronomers, the inner core is in partly molten, or liquid state. The outer shell of the planet with a thickness of about 400 kilometers is comparable to Earth’s outer shell (i.e., the mantle and crust).


The formation of the planet Mercury took place about 4.5 billion years ago. The planet’s gravity brought swirling gas and dust together to form this small planet near the Sun. Like the other planets within the asteroid belt, Mercury has a central metallic core, a gravelly mantle, and a solid crust.


The surface of Mercury resembles that of Earth’s moon. It is marked by several impact craters and depressions that resulted from collisions with asteroids, meteors, and comets. Craters, large rock-structures, and other features on the planet are named after famous deceased musicians, artists, or authors like notable children’s author Dr. Seuss and dance legend Alvin Ailey.

The planet was hit by large asteroids early in the solar system’s history that created enormous impact basins on the surface, including Rachmaninoff (306 kilometers or 190 miles in diameter) and Caloris (1,550 kilometers or 960 miles in diameter). The major part of the planet’s surface comprises large areas of smooth terrain. While some parts have cliffs expanding over some hundreds of kilometers and rising to a mile high. These mounts were formed as a result of the cooling of the planet’s interior cooled that contracted over the billions of years since the planet formed.

The major part of Mercury’s surface appears to be greyish-brown in color. The bright streaks that we can notice on the planet’s surface are called “crater rays.” These streaks are produced when a large asteroid, meteor, or comet strikes the surface. Such strikes release a considerable amount of energy, which produces such an impact that digs a massive hole in the ground surface and breaks a large amount of rock present under the point of impact.

A part of this crushed rock material is thrown far away from the crater in a pattern that falls on the surface, creating the rays. The fine rock particles tend to be more reflective than large pieces. These fine particles are more in number, so the rays formed, look brighter. However, the extreme space environment that involves dust impacts, gaseous reactions, and solar-wind particles results in the rays/streaks to darken with time.

Due to its proximity to the Sun, the surface temperatures of Mercury are extreme, both cold and hot. During the daytime, the planet’s surface temperatures can reach as high as 430 degrees Celsius or 800 degrees Fahrenheit. Mercury does not have any atmosphere to retain the heat, so during night-time temperatures on the planet’s surface can drop as low as minus 180 degrees Celsius or minus 290 degrees Fahrenheit.

It is considered that there may be water or ice present in the North and South poles of the planet, compressed inside deep craters in areas of permanent shadow only. The internal regions may have conditions favorable enough to preserve water or ice in spite of the extremely high temperatures on the parts of the planet lit by the Sun.

Mercury Atmosphere

Mercury does not have any well-defined atmosphere. Instead, the planet possesses what we can term as a thin exosphere that is comprised of atoms blasted off the surface by the striking meteoroids, asteroids, and solar winds. The main constituent gases of the exosphere include hydrogen, oxygen, sodium, helium, and potassium.


The magnetic field of Mercury is offset compared to the planet’s equator. Even though the strength of Mercury’s surface magnetic field is just one percent when compared to that of the Earth, it interacts significantly with the solar wind’s magnetic field causing massive magnetic tornadoes of hot, fast solar wind plasma on the planet’s surface. When the ions brought by these winds strike the surface, they tend to blow off the neutrally charged atoms and direct them to a hoop high into the sky/ the exosphere.

Potential for Life on Mercury

Mercury’s environment is not favorable for housing living organisms. The extremely high temperatures, uneven terrain, frequent meteoroid strikes, direct exposure to solar radiations, etc. make this planet unfavourable for organisms.

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Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers. Let's connect through

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