The term “planet” is derived from the Greek word of the same spelling, and means “wanderer.” For a body to be classified as a planet, it must meet the criteria laid down by the International Astronomical Union (IAU), which is it must orbit the Sun, must be large enough to self-gravitate into a spherical shape, and must dynamically dominate or “clear” its own orbit path.

Originally, we had 9 planets in our Solar System but this was reduced to 8 when Pluto was
reclassified by the IAU in 2006 as a “dwarf planet.” Pluto was declared as “not dynamically dominant and sharing its own orbit.” It is now the largest object in the Kuiper Belt (the Kuiper Belt is a disc in the Solar System extending outwards beyond the orbit of Neptune).
Mercury, Venus, Mars, Jupiter, and Saturn have been known since Roman times as they could be easily seen. Latin was the language of the Roman Empire, and astronomers in Europe wrote in Latin.

So these 5 planets ended up being named after Roman gods. In fact all the planets in the Solar System except Earth and Uranus are named after Roman gods.

Mercury – named after the messenger of the gods.
Venus – named after the goddess of love and beauty.
Mars – named after the god of war.
Jupiter – King of the Roman gods (also known by astronomers as the King of the planets).
Saturn – named after the god of agriculture and wealth.

So, what about the other planets in the Solar System, including our own planet, Earth?
Nobody knows for sure how Earth got its name. It derived from the old English words “eor(th)e” and “ertha” and simply means “ground.”

The name is at least a 1,000 years old. Uranus, Neptune, and Pluto were simply too far away to be seen with the naked eye, and it wasn’t until telescopes came along that they were discovered. Uranus is the only planet named after a Greek god.

Uranus – was named after the Greek god of the sky and heavens. It was discovered by William Herschel in 1781 and originally named “George’s Star” after George III. However the name was changed to Uranus after much argument when it was discovered that it was not a star at all.

Neptune – was named after the god of the sea. This planet was mathematically predicted by Urbain Le Verrier and confirmed by Johann Gottfried Galle in 1846 by telescopic observation at the Berlin Observatory.

Pluto – was named after the god of the underworld. Discovered by Clyde Tombaugh in 1930, and subsequently named in the same year by an 11-year-old Oxford girl called Venetia Galle.
Here are a few facts about our planets:

Mercury – smallest planet and closest to the Sun but not the hottest.

Venus – has retrograde (opposite) spin about its axis, so the Sun rises in the West. Hottest planet in the Solar System due to thick clouds of carbon dioxide trapping heat from the Sun (the “greenhouse effect”). Surface temperature can be as much as 400 degrees Centigrade, hot enough to melt lead or cook a pizza. Pressure on it surface is about 90 times that on Earth’s surface, or the same as being at a depth of 3,000 feet under the sea. Its day is longer than its year. One day is 243 Earth days long, whereas one year is 225. Also known as “Earth’s Evil Twin” as Venus is about the same size as our own planet.

Mars – once thought to support life. Early observers saw what they thought were canals on the planet’s surface, but in fact, they were lines caused by early telescope lenses and dusty conditions on the planets surface. Even as late as the 1960’s it was thought that there could be life on Mars.

This was somewhat disproved in 1964 when Mariner 4 conducted a fly by and sent back
photographs of what looked like a dead and hostile planet.

Jupiter – the biggest and brightest planet in the Solar System. Jupiter has the shortest day of just 10 hours.

Saturn – the ringed planet. If Saturn could fit into your bath, it would float.

Uranus – takes 84 years to orbit the Sun. Its discoverer, William Herschel was 84 years old when he died (what a strange coincidence). It has a 2-ring system, and like Venus it has a retrograde spin.

Uranus and Venus are the only 2 planets to spin on their axis in a clockwise direction. All the rest spin anticlockwise. Uranus also rolls around its orbit of the Sun at nearly 90 degrees meaning that each pole receives 42 years of daylight, and 42 years of darkness. Had a flyby by Voyager 2 in 1986, the only spacecraft ever to do so.

Neptune – has very dynamic storms with winds up to 1500 miles per hour. It is the windiest planet in the Solar System. Has a ring system of 6 rings. Had a flyby by Voyager 2 in 1989. Like Uranus, this flyby was the only ever spacecraft to do so.

Is there a “Planet 9?” There is talk of a hypothetical object somewhere in the outer reaches of the Solar System having an effect on objects around Neptune (Extreme Trans Neptunian Objects) but nobody knows for sure.

So, watch this space!

Contributed by Michael Lowe, a member of Sherwood Observatory.