Formation of the solar system
The earth, our home, is the third of 8 planets orbiting a star, our Sun. Together they make up the solar system. To understand how our planet formed, we need to know how solar systems are created and how they develop.
Around 4.5 billion years ago there was a cloud of dust, ice and gas floating in space called a molecular cloud. A nearby old star exploded – a supernova explosion – and the shockwave disturbed the cloud making denser areas. Slowly, gravity pulled material towards these dense parts, further increasing its mass and so pulling in yet more material, a process called gravitational collapse. The physics of conservation of angular momentum caused the material in the cloud it to spin faster and faster, to heat up and to flatten into a disc shape rotating about centre.
Within the centre of the cloud hot hydrogen gas was squashed together until eventually the atomic nuclei started joining together and a process called nuclear fusion started. This produced heat and light and made a stable body hot bright body called a star. The Sun is the star at the centre of our solar system. Today astronomers can see stars being born in the same way within molecular clouds far away.
Formation of the Earth and other planets
The Sun is 99.86% of the mass of the solar system, but it’s worth talking about the remaining 0.14% as that’s what we are made of. As the Sun started to shine it was surrounded by a rotating disc, made up of dust, ice and gas called a solar nebula.
Within this disc a process called accretion started, where dust grains started clumping together in time forming larger bodies called planetisimals which are about 10km across. This is very like the way raindrops in clouds on earth, where tiny droplets of water join together until they are big enough to fall as rain. Scientists recently studied a planetisimal called 2014 MU19. It is very irregular and has a very low density: the dust and ice is only lightly joined together by gravity.
Planetisimals are rare today. Most collided together to form bodies around 100km in diameter called protoplanets. Some of these survive in the belt between Mars and Jupiter and are called asteroids. It’s thought that in total about a few hundred of these protoplanets were formed within the Solar System. Over time these protoplanets collided and when they did they tended to form larger and larger bodies. Eventually most material ended up in the limited number of planets were are familiar with today.
Not all planets are the same. The sun heated up the centre of the disc, forming a region called the Inner Solar System where it was too hot for water or methane in the solar nebula to form solid ice. Therefore planets that formed here (Mercury, Venus, Earth and Mars) were mostly formed from the dust and are now called the rocky planets. Further out where it is colder, the gas giants (Jupiter, Saturn, Neptune and Uranus) formed from cores of rock and ice together. Being large they also captured gas within the nebula and are now much bigger, containing huge volumes of ice and gas.
Planetary differentiation
As planetisimals and protoplanets collided and joined together, the force of impact made the material extremely hot. Also bigger protoplanets are able to stay hotter inside for longer. This heat meant that the dust of the nebula is compressed and melted to form dense rocky planets.
The original dust material was very rich in Iron, which together with smaller amounts of Nickel and Sulphur form dense metal alloys. Within hot and maybe molten planets, this denser material sank down into the centre of the planet formed a metal core, a process called planetary differentiation. The material left behind, surrounding the metal core was mostly made of Silicon, Oxygen, Iron, Magnesium, Calcium and Aluminium. These elements together form minerals based on regular lattices of Silicon and Oxygen. These silicate minerals form most of our planet – they are within most of the rocks we see.
The exact mechanism by this the iron and rock separated out is not known. Perhaps following collisions the entire planet was molten and liquid iron separated out from liquid rock (magma). Even if the rock was solid, liquid metal may have slowly sunk between cracks and gaps to flow toward the centre of the planet. Whatever the details, we do know it happened, both from studies of our own earth and from meteorites.
Meteorites and Comets
Meteorites – small fragments from elsewhere in the solar system that fall to earth – are very varied. Some contain ancient dust grains from the original cloud. Others are pieces of small planets that were broken up by later impacts. Iron meteorites are from the cores and stony meteorites the remainder of these broken up planets. Pallasites, the most beautiful of meteorites, are a mixture of the two, with crystals of a silicate mineral called olivine set within metal. Without evidence from meteorites we would not be able to tell the story of how our planet was formed.
Not all things that hit the earth from outside are meteorites. Comets are bodies that come from the cold outer reaches of the solar system. They are mixtures of dust and ice and are common in the Kuiper Belt and Oort cloud, areas of the solar system that sit beyond the orbits of the planets. Pluto, which until 2006 was classified as a planet, is now classified as a dwarf planet, along with other bodies that sit within the Kuiper Belt.
Formation of the Moon
Planets commonly have moons orbiting them, just like planets orbit the Sun. They may be meteorites captured by the planet’s gravity, or form from material orbiting the planet. But the earth’s moon is far too large to form that way.
The most popular theory for how our moon formed is that is that there was an enormous collision between the earth and a protoplanet called Theia. Things move fast in space. We measure the speed of cars in units of kilometres per hour. The speed of objects in space is measured in kilometres per second. So when two planets collided, the impact was enormous. The planets were smashed into pieces and partly merged together. Huge volumes of the earth’s rocky mantle were pushed out into orbit around the planet. This then formed into our moon. The impact would have caused an enormous amount of heat to be released. Some studies suggest the entire earth could have become molten, with a global ocean of magma formed right up to the surface.
It’s very hard to know for certain to know if this theory is correct, as it happened so long ago. But it does explain the fact the Moon is very large and has a small core. Also we know from rock samples brought from the Moon that it’s geology and isotope chemistry is very similar to that of the Earth’s. Scientists are working hard to better understand how the Moon formed, by studying Moon rock chemistry with specialist equipment or by creating detailed computer models of the impact.
Late Heavy Bombardment
Over time the chaotic molecular cloud formed colliding planetisimals and then hot differentiated planets and then eventually the calm regular solar system we know today, with a small number of planets in stable orbits. This was a gradual process and collisions with large bodies were important for the earth’s early history even after the formation of the Moon.
The Late Heavy Bombardment is a period of time between 4.1 and 3.8 billion years ago when the rate of collisions was particularly high. The cause is not known, but one theory is that the large outer planets moved their orbits, disrupting the asteroid or Kuiper belts and so pushing many asteroids or comets out of stable orbits into paths that caused them to hit the earth, moon and other planets.
Impacts on Earth
Finding direct evidence of ancient impacts on earth is difficult as so few rocks from that time still exist. The earth is unusual in having plate tectonics and active erosion, processes that act to destroy old rocks at the surface. It’s only since the 1980s that earth scientists have worked out how to find traces of ancient impacts.
When a large object hits the earth at huge speed the impact creates a huge hole in the ground, a crater. The material from the crater is thrown out over a large area forming layers of distinctive rocks. Some is molten rock that quick cools to form glass. Some, called tektites, were formed from relatively recent impacts (less than a million years ago) and can even be found at the surface today.
Impacts create special minerals that cannot be formed by normal processes on earth and the meteorite may be rich in elements that are rare on the earth’s surface, such as Iridium. The Chicxulub impact, that hit when the dinosaurs became extinct, spread out Iridium to form a distinctive layer across the entire world. Within ancient sedimentary rocks we are finding more and more of these layers.
When large meteorites or comets hit earth they make a big mess, but they don’t hit very often. Smaller pieces arrive every day. If you’ve ever seen a ‘shooting star’ you’ve seen a small fragment entering the atmosphere and heating up due to friction. Really tiny pieces are falling to earth all the time: one might be falling onto your roof right now! A recent study of material trapped in gutters on buildings found microscopic fragments that had fallen from space.
Water and Life
We’ve talked about rocks and metal, but our earth is covered in water and teeming with life.
If the impact of Theia did create a global magma ocean, then all of the water on earth would have boiled away, leaving the planet as dry and lifeless as the moon. All life we know of is dependent on liquid water, but we don’t really know where earth’s water came from. It’s likely that much of it came from impacts of smaller meteoroids and especially comets, rich in water ice. Some of the water in your glass may have come originally from a frozen comet that hit the earth long ago.
As well as water, comets contain organic compounds. These weren’t formed by life, just that they are certain types of molecules that contain carbon. We don’t know where life formed or how, but we do know that it needs water and organic molecules, both of which were brought to earth by comets.
When humans send probes to other planets, they try really hard to make them sterile to avoid contaminating other planets with earth organisms. That’s because travelling through space doesn’t necessarily kill such tiny and tough forms of life. Impacts can send rocks out into space and eventually onto other planets. We know of pieces of Mars that arrived here this way. In the early history of the solar system, tiny organisms may have travelled between planets hitching a lift on these fragments. Maybe life actually first started on Mars (when it was young and wet) and then came to earth via a meteorite. We’ve no direct evidence of this (yet) but it’s a great reminder that our planet is part of the solar system and still interacts with it in surprising and important ways.
First publication by Xiaoduo Media in Front Vision. Front Vision is a Chinese online science magazine for children. My original English text produced with permission.