The Planets

Pttm, Going Postal
The planets of our solar system

Part One – Mercury

In this series of nine articles we shall look at each planet in our solar system, starting with Mercury and working out to Neptune, but I shall also include the dwarf planet Pluto as we have some wonderful pictures and more information about it.

To give an idea of the size, you can see here that Mercury is a bit bigger than our Moon:

Pttm, Going Postal

The Prologue:

First we need to define what a planet is.  It all came to a head in January 2005 with the discovery of the trans-Neptunium object Eris, a body more massive than the smallest then-accepted planet, Pluto.

The International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its decision on the matter. This definition, which applies only to the Solar System, states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has “cleared its neighbourhood” of smaller objects around its orbit. Under this new definition, Pluto and the other trans-Neptunium objects do not qualify as planets. The IAU’s decision has not resolved all controversies, and while many scientists have accepted the definition, some in the astronomical community have rejected it outright.  So there you go, still controversy!

Throughout this series, I would like to give a huge H/T to NASA.  I have used other sources as well, such as and  which is a great site for showing you where to look.


Sun-scorched Mercury, the smallest planet in our solar system, is only slightly larger than Earth’s moon.  Like the moon, Mercury has very little atmosphere to stop impacts and it is covered with craters. Mercury’s dayside is super-heated by the sun, but at night temperatures drop hundreds of degrees below freezing.  Ice may even exist in craters.

Distance from the sun: About 36 million miles, slightly more than 1/3 as far as the Earth.

Year: About 88 Earth days.  Day: About 59 Earth days.  Diameter: 4,879 Km (About 3,032 miles), less than half of Earth’s.

Temperatures: Minus 173c on the side away from the sun; 427c on the side facing the sun.

Mercury rotates in a way that is unique in the Solar System.  As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.  As seen from the Sun, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.

Mercury’s axis has the smallest tilt of any of the Solar System’s planets (about ​130 degree), and its orbital eccentricity (one for OT this) is the largest of all known planets in the Solar System. At, aphelion (furthest away from the sun) Mercury is about 1.5 times as far from the Sun as it is at perihelion (nearest to the sun).

Mercury got its name from the Romans who believed the gods were in charge of everything on Earth.  Mercury is their messenger god who had wings on his helmet and shoes and could travel very quickly.

The spacecraft Mariner 10 was the first to visit Mercury.  It flew by in 1974 and 1975. Not even half of Mercury was seen then. Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months.  After that, nothing was sent to Mercury for more than 30 years.  NASA’s MESSENGER spacecraft launched 3rd August 2004 flew by Mercury in 2008 and 2009.  In March 2011, it began to orbit Mercury.  The MESSENGER mission was in my view spectacular and revealed much about the planet.  It ended on the 30th April 2015 when it was deliberately crash landed on the surface as it  had run out of fuel.  Among its many accomplishments, the MESSENGER mission determined Mercury’s surface composition, revealed its geological history, discovered its internal magnetic field is offset from the planet’s centre, and verified its polar deposits are dominantly water ice.  We have even discovered the planet is actually shrinking.

Where to find it:

Mercury in the morning sky

Obviously as we and all the planets rotate around the sun, you can see, or not see Mercury over time.  It is always low down, but bright enough to see with the naked eye.  Check the above link regularly to see where planets and stars can currently be seen.

© Phil the test manager 2017