The Eight Planets in Our Solar System
A quick guide to the eight planets that orbit the Sun.
British spelling.
The solar system is estimated to be over 4.5 billion years old. At its centre is an average G-type main-sequence star or yellow dwarf star, which we call the Sun.
Eight planets are in orbit around the Sun. Here is a short description of them all.
Mercury, the first of the four rocky planets
Mercury is the smallest of the eight planets that orbit the sun, being just 4,880 kilometres in diameter. It is positioned closest to the sun at an average distance of almost 58 million kilometres.
It is thought to have formed much farther away from the sun than its present position. Like most planets, it has an elliptical orbit (oval-shaped). Mercury can be as close as 47 million kilometres and as far away as 70 million kilometres from the Sun.
The nearer an object orbits a star, the faster it travels; therefore, among all the planets, Mercury orbits the Sun the fastest, speeding along at just over 170,000 kilometres per hour and taking just 88 Earth days to complete one solar orbit, also known as a Mercurial year.
A sidereal day on this planet is very long, which is due to its very slow rotation speed; it takes over 58 Earth days to complete one turn on its axis.
A solar day is different; look at it this way: it can be the time it takes between two noons when the sun is at its highest point in the sky. On Earth, the difference between a sidereal day and a solar day is only 4 minutes, but Mercury has a solar day that lasts for 176 Earth days, making it the longest solar day of any planet in the Solar System.
The gravity on Mercury is just over one-third that on Earth, and having almost no atmosphere, its surface has no protection from the sun’s rays or incoming cosmic missiles.
Mariner 10 was launched from NASA’s Kennedy Space Centre on November 3, 1973, and became the first spacecraft sent to visit Mercury, making its first flyby of three on March 29, 1974.
Nasa’s Messenger spacecraft was next to visit; it left the Earth at the start of its long flight on August 3rd, 2004 and entered orbit around Mercury on March 18, 2011, becoming the first spacecraft to orbit the planet. The mission terminated with a controlled impact on the surface of the planet on April 30, 2015.
The surface was found to be dry and harsh and covered in ancient craters, the largest being the Caloris Basin, measuring 1,550 kilometres in diameter. It is thought to have been caused by an asteroid 100 kilometres wide that slammed into Mercury around 4 billion years ago.
Surface temperatures can reach 430 degrees Celsius during the day and can drop to minus 190 degrees Celsius at night, giving the planet an average surface temperature of 178 degrees Celsius. Mercury is slightly bigger than our moon but smaller than Saturn’s moon Titan and Jupiter’s moon Ganymede.
Mercury has no moons.
Venus, the hottest planet
Venus was named after the Roman goddess of love and beauty; it is sometimes called the Earth’s sister planet and is positioned second closest to the Sun. This planet has an average distance of just over 108 million kilometres from the sun.
Venus is also the second-brightest object in the night sky, with our moon being the brightest. Venus is a little smaller than Earth, having a diameter of 12,100 kilometres and a mass of 0.8 times that of the Earth.
It takes just over 224 days to complete one orbit around the sun, which is known as a Venusian year.
Surprisingly, a day on Venus is longer than its year, taking 243 Earth days to complete one turn on its axis. Another strange thing about Venus is that it turns on its axis in the opposite direction from the rest of the planets in the Solar System; this means that the sun rises in the west and sets in the east.
The first successful mission to Venus was Mariner 2, which was launched in August 1962. Mariner 2 passed by Venus on December 14, 1962.
As of this date, September 2023, a considerable amount of information has been gathered by some of the 43 space missions from various countries that have attempted the journey; just over 20 of the missions were successful. The surface was found to be covered with craters, volcanoes, mountains, and big lava plains.
The surface of Venus has temperatures reaching 465 degrees Celsius, hot enough to melt lead. It has a thick, heavy atmosphere stretching up to 80 kilometres from the surface and is made up mostly of carbon dioxide and sulphur clouds, which trap the heat below, causing a greenhouse effect.
Atmospheric pressure is over 90 times and gravity is 0.9 of that of the Earth. Venus was found to have a very weak magnetic field, probably due to its very slow rotation speed.
It is believed that billions of years ago Venus had a climate similar to that on Earth, with large amounts of water on its surface; however, the large temperatures produced by the greenhouse effect have boiled off the water, leaving the harsh conditions that we see today.
Like Mercury, Venus has no moons.
Earth, the special one
Our beautiful world, the Earth, is the third-closest planet to the Sun, orbiting at a distance of 149.6 million kilometres. The earth’s diameter is 12,756 kilometres, with an average temperature of 16 degrees Celsius.
One of the main reasons that life, as we know it, can live on this magical planet is because it is the right size and it orbits the Sun at the perfect distance, which is called the Goldilocks zone or habitable zone. This area of space ensures that it is neither too hot nor too cold. Liquid water exists in this habitable zone, which is needed for all forms of life on our planet.
Like everything else in the cosmos, Earth has a lifespan that will end naturally sometime in the distant future, but in the meantime, I hope that mankind will treat our only home with respect for the sake of ourselves and all the other millions of species that share our world.
The Earth is roughly 4.5 billion years old, just a little younger than the Sun. Gravity has pulled most of the heavier elements, like iron and nickel, down into its core, with the lighter elements staying at the surface and forming the crust.
Due to its massive molten iron core and its rotation, a magnetic field was created around the Earth, and that magnetic field has protected our planet ever since. It deflects most of the solar wind, whose charged particles would otherwise strip away the protective ozone layer, which protects the Earth from ultraviolet radiation.
It took a long time for the molten earth to cool down from its violent birth, but as it cooled, these new conditions allowed the first rain to fall from the sky, which began forming oceans and lakes.
It is thought that incoming asteroids and comets also contributed to the water we see today.
It is thought that around 3.5 to 4 billion years ago, some elements came together in the oceans and formed the key substances that created the first primitive life on our planet.
Over that vast time, evolution has been at work, and we can see the outcome now with the millions of different species that live in the water and on land.
Fossil stromatolites have been found in western Australia dating back 3.5 billion years. Their columns were constructed by Cyanobacteria, which is a single-cell photosynthesising microbe.
That was a game-changer for the development of life because they produced the by-product oxygen, which slowly accumulated in our oceans and in the atmosphere, which eventually led to a big leap in the evolution and diversity of organisms.
However, it must be said that it is not yet fully understood how primitive life started.
Around 500 million years ago, the first land plants appeared. Fossil records show that insects were living on the planet 400 million years ago. Dinosaurs roamed the Earth 230 million years ago and unexpectedly died out 66 million years ago. 130 million years ago, flowering plants first appeared.
Our early ancestors existed in Africa six million years ago, and modern man, known as Homo sapiens, has been around for over two hundred thousand years. Civilisation as we know it has existed for six thousand years.
The evolutionary path for life on Earth has been very long, but at last, we have arrived.
Some animals have evolved to be stronger, larger, and faster than we are; some can fly; and many never leave the water, but what sets us apart is that we have developed the most advanced brain.
That amazing brain has given us the intelligence, curiosity, and ability to carry out the science that provides a better understanding of the universe and life itself.
Carl Sagan, who was my favourite astronomer and presenter, is quoted as saying, “The human brain is a very big place in a very small space,” which is so true. I can still hear his distinct voice.
Earth is such a magical place; could it be possible that it is the only planet in the entire universe to harbour intelligent life?
There are an estimated two trillion galaxies in that gargantuan area we call the universe. Some galaxies will hold millions of stars, and others will hold billions of stars and countless planets. The chances of there being no other life in the universe must be astronomical.
Surely, by sheer numbers, billions of planets will have similar conditions as Earth, with the possibility of life existing. Perhaps sometime in the future, the question about life on other worlds will be answered.
As you know, Earth has only one natural satellite; we call it the moon.
Mars, the red planet
Mars has a year that adds up to 687 Earth days, and the 25 hours in its day are almost the same as a day on Earth.
This planet can be as far away as 400 million km and as close as 56 million km from the Earth. It is the fourth-closest planet to the sun, orbiting at an average distance of just over 228 million km.
Mars is much smaller than Earth, with a diameter of just 6,794 km. The surface temperatures near the poles can drop to minus 125 degrees Celsius and rise to 20 degrees Celsius at the equator, giving an average temperature of around minus 65 degrees Celsius. The thin atmosphere consists mostly of carbon dioxide and features clouds, weather systems, and winds.
For Mars to make a complete trip around the sun, it takes 1.88 Earth years, referred to as a Martian year. An interesting feature on Mars is the giant inactive volcano Olympus Mons, which is 22,000 metres high and is the largest volcano in the entire Solar System.
Mount Everest is the highest mountain on our planet, at a mere 8,848 metres high when compared to Olympus Mons.
Mariner 9 was launched on May 30, 1971, from Cape Canaveral and became the first artificial satellite of Mars when it arrived and went into orbit on November 14, 1971.
Launched on August 20, 1975, NASA’s Viking 1 project found a place in history when it became the first mission to safely land a spacecraft on the surface of another planet, namely Mars.
A lot has happened since then. The Mars rover Opportunity landed safely on the red planet in 2004. It was sent to look for any signs of past life. Opportunity made a few discoveries, including evidence that in the distant past, some areas of the planet stayed wet for a long time and that the conditions back then could have been favourable for sustaining microbial life.
After a severe dust storm in June 2018, Opportunity fell silent. It is thought that its solar panels became covered in dust, which resulted in the loss of power. The rover covered a total distance of 45 kilometres during its working life; how impressive a feat that is after all those years on a very inhospitable planet!
I wonder how long it will be before someone from Earth stands beside it.
Reaching Mars is hard and unforgiving. As of this date, September 2023, over 50 unmanned missions have attempted the journey; many were not successful as there is little room for error, which shows how difficult these missions to faraway destinations can and will be.
As of September 2023, Curiosity is still operating on the planet, exploring an area known as the Gale Crater. It was launched from Cape Canaveral on November 26, 2011, touching down on Mars on August 6, 2012, only 2.4 kilometres from its planned landing site, which is not a bad outcome considering its very long journey. Another amazing achievement for the human race
Mars has two tiny moons.
Jupiter, the first of the gas giants
Jupiter is the largest of all the planets in the Solar System by far, with a diameter of almost 143.000 kilometres. Its average distance from the Sun is just over 778 million kilometres; to give an idea of its size, Jupiter could hold in its volume up to 1,300 Earths.
Jupiter is the fifth planet from the Sun, with 2.5 times more mass than all the remaining 7 planets combined; its mass would be the equivalent of 318 times that of the Earth.
The atmosphere on Jupiter is made up of 75% hydrogen and 24% helium; in addition, there is water, methane, and ammonia in trace amounts, and the core of the planet is thought to contain rock and metallic hydrogen.
This giant planet takes almost 12 years to complete one orbit around the sun, which is known as the Jovian year, and the duration of its day is only 10 hours. The temperature in its clouds can be as low as minus 145 degrees Celsius.
The red spot on Jupiter that can be seen with a telescope is a hurricane that has been raging for centuries. To give an idea of its size, this red spot is three times the size of our planet.
Arguably, 92 moons are in orbit around Jupiter’s centre; its largest moon, Ganymede, is also the largest in the Solar System, with a diameter of 5,268 kilometres. The water ice on Ganymede is thought to contain more water than all the oceans on Earth.
Nine robotic spacecraft have visited Jupiter over the years; the latest one, Juno, left the Earth on August 5, 2011, and went into orbit around Jupiter on July 4, 2016, circling as close as 3,500 kilometres from its surface. It was sent to study Jupiter’s auroras, atmosphere, magnetosphere, structure, and origins.
Saturn, the ringed planet
Saturn is the sixth planet from the Sun and the second-largest gas giant. It has a diameter of 120,600 km, and its average distance from the sun is 1,427 million kilometres.
The rings surrounding Saturn are mostly lumps of frozen water, thought to be remnants of a moon that came too close to this giant planet and was torn apart by its gravitational force.
Saturn’s volume could theoretically hold up to 760 Earths and is 95 times the mass of the Earth. It takes over 29 years to make one orbit around the sun, which is called the Saturnian year, and the duration of its day is almost 11 hours.
There are 145 moons with confirmed orbits that circle Saturn.
Enceladus is the sixth-largest of these moons; it has a liquid ocean of water covered by a frozen crust. The heat needed for liquid water to exist is thought to be caused by gravitational interactions with other moons orbiting Saturn.
At this moment in time, there is no evidence of any other life forms within the solar system or indeed further out into the universe. Could it be possible that microbial life could be living on Enceladus? Maybe not, but I am sure that someday a mission will be sent there to try and answer that question.
There have been four scientific missions sent to Saturn to study the planet and some of its moons, the first of which was Pioneer 11 way back in 1979.
Voyager 1 visited in 1980, Voyager 2 passed by in 1981, and the most recent mission, Cassini-Huygens, was launched on October 15, 1997, and started orbiting Saturn in July 2004, being the first artificial satellite to do so.
On December 25, 2005, Huygens separated from the orbiter and landed on Saturn’s moon, Titan. The mission ended on September 15, 2017, when Cassini depleted its fuel supply and entered Saturn’s upper atmosphere, where it was intentionally destroyed, much to the dismay of all the people who had worked on the project for over twenty years.
Uranus. A strange name for a planet!
William Herschel discovered this planet in 1781. Its axis was found to be tilted 98 degrees, which means that it lies almost on its side as it orbits the sun. It is thought that the reason for this strange position could have been a collision with another planet in the distant past.
Uranus has a diameter of almost 52,000 km, and its average distance from the Sun is 2,87 billion km. It has a mass of 14.5 times that of the Earth. This large planet’s atmosphere is composed primarily of hydrogen, helium, and methane; a temperature of minus 216 degrees Celsius has been detected at the top of its clouds.
27 moons are known to orbit its centre. Uranus has an orbit so large that it takes 84 Earth years and 36 days to make one circuit around the Sun, which is known as the Uranian year. It has a day that lasts for 17 hours and 14 minutes, which is shorter than our day on Earth.
Voyager 2 is the only spacecraft that has visited Uranus; it made its closest approach to the planet on January 24, 1986. As of this time, September 2023, no further exploratory missions to Uranus have been approved.
Neptune, the most distant planet
Neptune is the most distant planet from the Sun. It has a diameter of 49,500 kilometres and can be thirty times further away than the distance between the Earth and the Sun.
Neptune’s average distance from the Sun is over 4.5 billion kilometres, so it is no surprise that it takes 164 Earth years and 264 days to complete one circuit around the Sun, which is known as the Neptunian year.
A day on Neptune adds up to 16 Earth hours; the mass of this planet is 17.2 times that of the Earth, and it has fourteen known moons orbiting its centre. The temperature in its clouds can be as low as minus 235 degrees Celsius, with an average temperature of minus 200 degrees Celsius. The winds on Neptune can be so strong and fast that they can break the sound barrier.
Neptune is another planet that has been visited only once. During its grand tour of the solar system, the space probe Voyager 2 paid it a visit on August 25, 1989, passing just 3,000 kilometres from its surface. As of 2023, NASA has no further missions planned for Neptune.
I hope you have found my short article about the solar system’s planets interesting and knowledgeable.
End of article
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