Which is the Smallest Planet in the Solar System?

Mercury, the nearest planet, is the smallest by diameter, mass and volume of the eight official planets. It revolves around the Sun at an average distance of 36 million miles (58 million km), taking it only 88 days of the earth to complete a lap, named after the fast Roman deity. 

To the eye of the surface of Mercury, the Sun is 3 times larger and 7 times brighter than it is on the surface of earth and the daytime temperatures on the surface reach 800°F (430°C) plunging to -290°F (-180°C), with no large atmosphere present.

What is a Planet?

In 2006, the International astronomical union (IAU) defined that current definition of the planet: an object in space should orbit the Sun, be almost spherical due to the hydrostatic equilibrium, and should clear its orbit of other debris. 

This rules out the dwarf planets such as Pluto, Ceres and Eris, which are dominant but not clearing. Mercury meets all the requirements of a rocky terrestrial world.

What is Mercury's Actual Size?

The radius of mercury is 1,516 miles (2,440 km), which is roughly thirty-eight percent of the size of earth, so it is a blueberry in comparison to a nickel-size earth.

It has an equatorial diameter of 3,032 miles (4,879 km), and a mass of 3.301 x 1023 kg, or about 5.5 percent of that of Earth. 

It has a massive iron core (comprising 85 per cent of its radius, about 1,289 miles or 2,074 km across) making it, with Earth, the second-densest planet, which is probably somewhat molten.

Comparison with Pluto

Pluto is only 1,473 miles (2,370 km) in diameter, which is less than that of Mercury (3,032 miles (4,879 km)) and it is only 0.0022 Earth masses in mass compared to Mercury (0.055). 

However, Pluto can only be classified as a dwarf planet according to the IAU guidelines because it is found in the Kuiper Belt with other objects that it has not cleared yet. Mercury, on the other hand, is a planet of its hot inner circle.

Why is Mercury the smallest?

Of known planets in the IAU, none is smaller; such objects as Pluto are not classified. Mercury is a rock that was formed 4.5 billion years ago out of the nebula dust and gases of the Sun, which may have vaporized the lighter material to leave a dense metallic core. 

The MESSENGER mission by NASA (2008-2015) surveyed its surface and confirmed its exact size and eliminated smaller planetary competitors.

Interesting Facts About Mercury 

• Mercury completes one rotation in 59 earth days yet has a solar day of 176 earth days because it is eccentrically in an orbit and the axial tilt is 2deg, resulting in odd sunrises when the Sun rises, sets and then rises once again. 

• It has neither moons nor rings, has a cratered, Moon-like surface with huge basins such as Caloris (960 miles wide) and contractional cliffs of the planet. 

• There is a thin exosphere of oxygen, sodium, hydrogen, helium and potassium, blown off by the wind of the sun; polar craters can contain ice or water permanently in shadow. 

• This offset magnetic field (1 percent of Earth) creates magnetic tornadoes which channel the solar wind to the surface.
  
  
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