An Introduction to the Solar System (Part 1)

Carl introduces you to the planets, asteroids and dwarf planets that make up our solar system and that have surprised us with their unexpected properties and beauty since we sent eyes to visit them…

Depending on when you went to school, or indeed how conservative your school is on updating their textbooks, your picture of our solar system may be a little out of date or even incomplete. Our solar system is full of wonders and surprises that you probably didn’t even know were there.

If you want to skip to a particular part of our backyard, click on any of the links below and you will be taken directly to the information on that body.

• The Sun
• Mercury
• Venus
• Earth & The Moon
• Mars
• Ceres & The Asteroid Belt
• Jupiter & The Jovian Moons
• Saturn & The Saturnian Moons
• Uranus
• Neptune
• The Trans-Neptunian Dwarf Planets & Beyond

So, if you are strapped in and ready for a tour of our solar system, we will begin with an object that accounts for over 99.8% of the mass in our solar system and is our nearest star…

The Sun

The brightest object in the sky is the Sun. Our nearest star bathes our planet in light and warmth and is its primary source of energy. The Sun is a nearly perfectly spherical ball of hydrogen, helium and some heavier elements, with around 75% of it being hydrogen and around 23% being helium.

The Sun shines because deep inside the star, nuclear fusion takes place. This is where hydrogen is converted to helium through a number of different processes. One of those processes, and the one which produces most of the helium inside the Sun, is called the proton-proton (p-p) chain reaction. As the name suggests, there is a number of steps in the process of converting hydrogen into helium. It all starts with two hydrogen nuclei (protons) that collide with each other to form deuterium () as well as a positron () and an electron neutrino ().

The positron immediately collides and annihilates with an electron, releasing two gamma-ray photons of light. After this, the deuterium created in the first step goes on to combine with another hydrogen nuclei (proton) to form a light helium isotope, helium-3 ().

Beyond this step, there are a number of paths that all ultimately culminate in the production of the helium we all know and love, helium-4 or , using the light isotope created earlier. These are fusion processes and combine smaller elements to produce larger ones.

Credit: SOHO (ESA & NASA)

These processes release a lot of light and a lot of energy which heat the interior of the Sun. Due to the high densities involved inside our Sun, the gamma-ray photons released in these processes are absorbed within just a few millimetres and re-emitted in a random direction and with a lower energy. This means that it takes a photon emitted in the core around 30 million years to reach the surface of the Sun!

Light that has taken so long to get to the surface has warmed planet Earth and allowed life to flourish on this small ball of rock and water. Humans have appreciated it as a source of energy as well as life for an extremely long time – we have worshipped it and sacrificed for it, all in an attempt to appease and influence it. Even now we keep a twenty-four hour watch on our star (in 3D no less) through missions such as SOHO and STEREO in an attempt to discover its secrets and better understand the complex processes that power and shape that ball of hydrogen that sits at the center of our solar system.

Mercury

The closest planet to the Sun, Mercury is the smallest planet in the solar system. It has no atmosphere to speak of and although known to exist since around the 14th century BC, we know surprisingly little about this dense ball of rock. This is due in part to the complexities involved in sending our spacecraft to visit the tiny planet.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Only two spacecraft have ever visited Mercury, with one of those two currently in orbit around the planet right now (NASA’s MESSENGER). The other spacecraft was NASA’s Mariner 10 which flew by the planet on a number of occasions in the mid-1970s, providing our first intimate view of the inner most planet. These two missions have shown that the tiny planet has a magnetic very similar to Earth’s, which scientists thought not to be the case. Both these past and future missions, such as the joint Japanese-European BepiColombo, have and will attempt to answer questions such as why Mercury has such a significant magnetic field, what the composition of Mercury’s core is and discover more about the planet’s geological history. With the fastest route to this planet taking six years, we may not truly begin to understand our smallest neighbour until the next decade!

Venus

Named after the Roman goddess of love and beauty, the name hides the menacing past and truth about Earth’s sister planet. While Venus’ size, mass, gravity and composition are similar to Earth’s, its atmosphere differs in significant ways that make it more like Earth’s evil twin. The planet remained a mystery until the twentieth century when its secrets began to be gradually revealed by human kind’s bombardment of robotic spacecraft, sent there to investigate our twin.

It is widely thought that the early Venus was once similar to Earth, with oceans and a similar atmospheric composition. Due to its proximity to the Sun and its inability to absorb carbon into rocks on its surface, green house gasses built up in Venus’ atmosphere to the point we see today. Its atmosphere is almost exclusively carbon dioxide () with around 4% Nitrogen as well as other trace molecules and elements. These create the dense atmosphere and extreme temperatures we observe today – on the surface, temperatures can reach over 450 degrees Celsius (840 degrees Fahrenheit)!

This composition allows for such weather as clouds that rain down sulphuric acid, typical wind speeds of over 200 miles per hour, surface pressures close to 100 times greater than here on Earth as well as more terrestrial phenomena like lightening. Venus is one inhospitable and violent place with observation revealing its recent past being no less violent. Once we managed to peer through the planet’s thick atmosphere at its surface, it was revealed the surface was shaped by widespread volcanic activity with evidence still mounting that volcanic activity continues to this very day.

Earth & The Moon

Our journey through the solar system brings us to the planet that we were all born on, have lived our lives on and will eventually die on. The planet with blue skies and red sunsets, blue oceans and green lands. The planet that, as far as we know, is unique in that it harbours the only known forms of life in the observable universe. From our blue ball of rock we have looked out into the solar system and beyond in an attempt to discover what exactly is out there, and the processes behind their creation and behaviour.

Planet Earth is the densest of the eight planets in the solar system and the largest of the four inner rocky planets. Over time we have come to know planet Earth as many things – from a blue marble to a pale blue dot – but ultimately, planet Earth is and always will be known as our home. Slightly larger than Venus, Earth holds on to a comparatively thin atmosphere made up mostly of molecular nitrogen (~78%) and oxygen (~21%). Unlike Venus, Earth has the ability to ‘scrub’ the carbon dioxide from the atmosphere through absorption by rocks and organic life such as plants which is why carbon dioxide only accounts for 0.04% of the atmosphere – in stark contrast to Venus.

Compared to other planets in the solar system, the Earth’s only known natural satellite – the Moon – is the largest relative to its planet’s size. With a diameter of almost a quarter of the Earth’s, the Moon has had visible and important effects on the Earth with the tides being widely known to be caused by the gravitational interaction between the Earth and Moon. Lesser known effects may be that the Moon has had a role in shaping the climate on our planet over its history.

Believed to have been formed through the collision of a Mars-sized object with the early Earth, the Moon has orbited Earth for around 4.5 billion years which is around 40 million years after the formation of the solar system. It is mostly composed of the same material found in the Earth’s crust which is evidence for the impact theory of formation presented here (and which is also the most widely accepted explanation of the Moon’s formation). If you look up at the Moon on any night of any day of the year, you will always see the same face gleaming back at you. This is because the Moon is tidally locked to the Earth – the time it takes for the Moon to rotate about its own axis is the same as the time it takes the Moon to orbit once around the Earth.

Credit: NASA/JPL/USGS

Besides the Earth, the Moon is the only other celestial body that human beings have ever visited. This may all change within the next few decades, with several planned missions to the Moon, and then Mars, in the works by multiple space agencies and privately owned companies.

Header image credit: NASA