The solar system began to form when a massive cloud of dust and gas known as the solar nebula collapsed. This happened 4.6 billion years ago.
During the collapse of this nebula, the densest material was drawn inwards, forming our sun. This was probably caused by a nearby star’s supernova, which sent shock waves that pulled in this material.
The Nebula
A nebula is a cloud of gas and dust in space. Some nebulae are the remnants of a supernova explosion, while others form when stars are forming in their own right. Nebulae are also referred to as stellar nurseries.
Some nebulae are dark, while others reflect the light from nearby stars or emit it themselves. Most nebulae are located within the Milky Way, but some are located in distant galaxies.
Often, nebulae lie undisturbed for millions or billions of years. But gravity or the shock wave from a supernova explosion can cause swirls and ripples in the cloud. These disturbances can pull matter in, eventually coalescing into clumps and increasing their size and mass.
When these clumps reach critical mass, gravity can squeeze them even tighter. This pressure and temperature increases, and eventually they become hot enough to trigger nuclear fusion. The protons in the centers of these atoms fuse, turning them into energy and becoming a star. This is what we see in our Sun.
Another process can transform a nebula into a planet. This process is called planetary accretion.
The outer part of a planet’s nebula can be much hotter than the center, which causes it to accrete faster. When this happens, it can attract other planets, too.
This process is also able to create gas giants, which are much larger than the planets that form in our solar system. These planets are made of a mix of gases, metals and other elements. They have a much stronger pull on the material that forms them than do the planets in our solar system.
But this strong gravity is also what makes them so special. They can collect the most hydrogen and helium, which are the building blocks of stars.
Other objects in the nebula can also stick together, like atoms and molecules. These can build into bigger particles, called planetesimals. The planetesimals can then grow to be large enough to attract other planets, too.
The Sun and the planets in our solar system were born from a nebula. But there are many other places in the universe that stars and planets can form, too.
The Cloud
The Solar System began to form when a cloud of gas and dust became dense enough to trigger nuclear fusion. This process transformed a tiny amount of matter into a huge source of energy that provides us with food, water and life.
At the centre of the Solar System was a disc-shaped mass of nitric acid, helium and other gases that had been heated by sunlight and were now hot and dense enough to cause nuclear fusion. Those gases, along with other materials from the nebula, were shaped into planets with metal cores, including Mercury, Venus, Earth and Mars.
Eventually the hot, dense material in the central region cooled down and became more volatile. It turned into minerals and metals, and a less volatile solid material called ice formed in the outer regions.
As the cloud cooled, evaporation of the water from the vaporised material caused it to rise into the air. As the volume of air containing the water vapor became saturated, it cooled down and changed from gas into liquid or ice (called condensation). The water vapor then condensed back to a gas again.
There are many factors that affect how the clouds form and where they appear. Some of these are weather conditions such as warm and cold fronts, variances in the jet stream and topographical features on Earth like mountain ranges.
Another factor is air movement around the Earth, known as convection. When air rises, it expands and cools because of its lower pressure higher up in the atmosphere. When it reaches its dew point, the air becomes saturated and water vapor condenses into droplets that form a cloud.
When the water vapor condenses, it combines with other gas particles in the air called aerosols. These aerosols include salt crystals from sea spray, bacteria and ash from volcanoes. They make it easier for the water vapor to condense and become water droplets.
Once the droplets are in the cloud, they start to merge together and the cloud becomes full of them. Depending on the type of cloud, these water droplets may look like puffballs or more like patches or a sheet. Meteorologists use a classification system to group cloud types into different categories. The most common ones are cumulus, cirrus and stratus.
The Core
The Solar System consists of the Sun and the planets (rocky) and their moons (gaseous). These planets have distinct characteristics that determine how they behave in the solar system. The Sun is a central point in the solar system, and its gravity pulls the planets around it.
The Core, which is a large region of the Earth’s interior, has long been thought to be a dense, molten ball of iron (see Figure 1). It is located at the center of the planet and is responsible for generating its magnetic field.
Most scientists believe that the core formed as materials that make up the planet collided and glommed together, heating each other to a molten state. The core is primarily made of iron, nickel and some oxygen. It is also enriched in elements that dissolve in iron, such as gold and platinum.
It is a very hot, dense sphere of about 1,220 kilometers in radius and has pressures that range from 136 GPa at the core-mantle boundary to 360 GPa at its center. The temperature of the core varies between 5,200 deg Celsius and 9,392 deg Fahrenheit.
In addition to its thermal properties, the core is a powerful magnet that attracts other planets. It has been proposed that the core is the source of the Earth’s magnetic field and that it has a dynamo that generates this field by convection.
However, there are still many questions about the nature and origin of the Core. Several different mechanisms have been suggested for core formation, and the exact mechanism is still unknown.
One of the most common theories is called the homogeneous accretion hypothesis. This suggests that the metals and silicates accrete together, and as the planet heats up they accrete toward the center. This mechanism also produces a relatively uniform core.
Another popular theory is that the core formed when refractory iron and nickel condensed from a cooling nebula. This process triggered the accretion of the heavy elements in a relatively unimodal fashion, producing a core that was much more uniform than it is today.
The Planets
Our solar system formed 4.6 billion years ago from a large cloud of gas and dust, called the nebula. It collapsed onto itself, forming a disk around the young Sun and surrounding it with planets and moons.
The four inner rocky planets – Mercury, Venus, Earth and Mars – were created from matter that was drawn toward the center of the disk by gravity and collided with it. The leftover pieces that could not be shaped into a planet were left in the outer part of the disk, where they formed asteroids.
These small rocks were eventually rounded into spheres by the force of gravity. As the spheres grew bigger, they started to attract other materials. In some cases, they grew so big that they were able to pull in enough material to form a planet and moon.
Scientists have long suspected that planets begin to form early in the life of a star system, but until recently it was unclear how this process works. New modeling work suggests that the earliest giant planets can trap and stabilize gases in their orbits.
This model, known as the “disk instability” theory, suggests that early in the solar system’s life, clumps of dust and gas were bound together by gravity and over time compacted into giant planets. As the planets expanded, they were able to trap and hold on to lighter gases as they began to fade away.
It’s this accretion process that led to the creation of the outer gas giants and ice giants – Jupiter, Saturn, Uranus and Neptune – which have become the Jovian planets we know today. If the earliest giant planets had formed in a different configuration, they would never have reached their current sizes.
The giant planets have also been pushed farther from the Sun by their surrounding Kuiper Belt objects, and their orbits have been thrown off course. This is what led to the discovery of many planets outside our solar system.