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while solar systems are based on hydrogen and helium, they also also likely to include all the elements of the period table

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many generations of dying stars have injected freshly synthesized matter into interstellar space

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how can this widely scattered matter be used to form dense solar systems for planets

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if the matter was evenly distributed throughout the universe, this would be a hopeless task

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fortunately, due to the force of gravity, matter is concentrated in galaxies such as our own Milky Way galaxy

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within the galaxies, most of the matter is concentrated in molecular clouds

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these are enormous clouds, even when we observe them from as far away as the Earth

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yet because they are also very faint, they also remain almost invisible to the naked eye

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the Orion constellation which lies close to the terrestrial equator

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the three bright stars are known as Orion's belt

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if we zoom in and increase the brightness, the underlying cloud becomes visible

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Horsehead Nebula

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Orion Nebula

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this is a star-forming region

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we cannot observe how our own solar system formed, but observing places like this, we can deduce how solar systems form in general

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the size of molecular clouds can be measured in units of light years

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molecular clouds look passive but they are highly dynamic

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stellar winds push the gas around at speeds up to several hundred km/s

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even at these speeds, it would take a millions years to traverse the width of these clouds

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we use computer simulations to fast-forward this process

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can run the simulation for two million years

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newborn stars light up within the cloud as the simulation runs

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but we need to compress it by many orders of magnitude to form a solar system

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sometimes gas gets compressed to a point that gravity is stronger than the gas pressure

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the result is a runaway collapse resulting in star formation

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a famous example of star formation is seen in the Hubble image of the Eagle Nebula

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intense stellar winds have created a bubble in the cloud

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the stellar winds not only push the gas further and further away from the star through a process known as photoevaporation

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it also compresses the gas to point where a gravitational collapse is triggered

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this is exactly what we need to form a new star

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once a dense core is formed, it's gravity field will start to grow as attracts more and more gas and thus further increases a gravity field, which is called accretion

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each of these tentacle-like structures contains a new star at the tip

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eventually the gas will clear around the new star, accretion will cease, and the star will begin its life as an independent object

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as the gas collapses toward the growing star, it will rotate faster and faster, much like water running out of a bathtub

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the accretion prevents all of the gas from accreting directly to the star

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instead, a rotating disc of gas and dust is formed around the young star

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it is from these discs that planets are formed

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this explains why all of the planets in our solar system orbit in the same plane around the sun

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early solar systems

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matter from the cloud is falling in toward the disc

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most of the mass is drifting inwards toward the star

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concentrated toward the mid-plane, we find solids of different sizes

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near the start, milimeter-size particles known as CAI's condense out of the gas

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Calcium/Aluminum-rich inclusions

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calcium and aluminum condense at very high temperatures

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meteorites have CAIs, e.g. white, irregular inclusions in a rock

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further away from the star we find silicate spheres known as chondrules

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the disc is composed of dust and gas

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near the center there are no ice crystals, further out there are ice crystals

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as a result, outer planets grow faster and bigger

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the gas in the disc was only present for a few million years

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the outer planets beyond the frost line grew big enough to capture gas from the disc while it was still there

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became gas giants

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inside the frost line, smaller rocky or terrestrial planets formed, e.g. Mercury, Venus, Earth and Mars

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accretion of the planets

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a protoplanetary core

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accretes dust, condrules and CAIs

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in the last stages of accretion there were only a few large planetary formations

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one of the last of these later formations which accreted to the Earth was probably Mars-sized

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resulted in a collision of cataclysmic proportions

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the presence of our Moon was likely a consequence of this event