The formation of solar system and earth

Aayushi Tiwari
15 min readNov 22, 2020


Kon’nichiwa mates. Welcome to the last post of the Chronology of the Universe series. In the previous posts, we have talked about many topics like star formation, inflation, fundamental forces and particles, black hole. I tried to cover everything that can explain the evolution of the universe from the big bang to now. After the star formation, the biggest event for us was the formation of the earth and solar system. We will see how the solar system formed and what makes the earth different for life to appear.

It is already 4.5 billion years when our solar system formed out of a gas cloud. I couldn’t emphasize the fact that how important it was for us to have conditions so optimal that we thrived on earth as a life form. In the past few years, astronomers were able to find many stars surrounded by exoplanets with many being in the goldilocks zone. As every now and then scientists are able to find many Earth-like exoplanets. We might find other forms of life on them too.

When you look at the stars and the galaxy, you feel that you are not just from any particular piece of land, but from the solar system

Kalpana Chawla

The solar system

Our solar system is located at the outer arm of the milky way galaxy with the sun being at its center. It has 8 planets and 5 dwarf planets like Pluto, dozens of moon millions or even billions of comets, meteoroids, asteroids. If we have to regionalize the solar system it can go like this, inner solar system, outer solar system, the asteroid belt, Oort cloud, and Kuiper belt. Kuiper belt and Oort cloud are both made up of icy bodies and both lie beyond Neptune. The Kuiper belt is home to dwarf planets. Although Oort cloud had not detected directly yet though its presence has been noticed through mathematical models and the effect it has on us.

Sun’s solar winds create an electromagnetic bubble around the solar system called the heliosphere. The Oort cloud extends beyond the heliosphere. Naturally, solar winds don’t retain their effect beyond a certain limit where it is halted by interstellar pressure, we call it Termination shock.

Two NASA spacecraft had crossed the interstellar shock, voyager 1 in 2004 and voyager 2 in 2007 but it will take them millions of years to cross the Oort cloud.

Formation of the solar system

Around 4.5 billion years ago a gas cloud was swirling in the Orion arm of the milky way galaxy. That was the solar system nebula. The almost evenly spread gas cloud started contracting to form the sun and planets. We think that a supernova explosion nearby had triggered this contraction. Just imagine a normal cloud puffy, and how easily you can cross through them. There are many videos on YouTube in which pilots fly through clouds while touching them. It was the same for the solar nebula.

A sunlit sky

Several light-years across nebula stared to contract why? Because at one place gravity became dominant(sun). The surrounding gas started to fall into that center. Even when everything was dispersed the nebula had an overall rotation. As the gas started contracting more and more the whole system rotated and when it rotated more and more it got flattened. The rotation of matter had converted the potential energy of particles into kinetic energy and the nebula became hotter near the center. When the increase in temperature the innermost center of the nebula had a nuclear fusion, the sun shone for the first time. The energy released due to fusion made an outward pressure push against the gravitational force from outside. A delicate balance formed between external gravity and internal energy/pressure push made the star afloat in the cosmos without bursting.

If you want to go in-depth about star formation refer to my blog post on the stelliferous era and star formation.

The mass distribution and shape

The solar nebula had 73% hydrogen, 25% helium, and 2% metals. The high temperature near the sun made only rocky planets form and gaseous planets formed far away from the sun where their hydrogen and helium were safe from the scorching sun.

The clouds contracted started spinning. This had made the contraction even faster and the infalling matter attracted faster. As more matter fell into the center (highest gravitational pull) spinning matter turned into a disc shape. The center of the disc that formed our sun had 90% of the mass of gas cloud/nebula. The leftover mass formed our planets. The newborn protostar was surrounded by an accretion disc that had debris dust and rocks. As the whole system spun rock and dust start colliding and combining. They made a giant ball kind of structure and the diameter also increased up to 1 mile. Now, these protoplanets were big enough to gravitationally attract other materials.

Formation of planets

The most basic way of explaining the formation of planets is that as the remaining disc moved away from the center the temperature dropped and tiny dust particles condensed to form bigger objects. Big enough to attract other big objects. Metals (iron, nickel), silicon-based compound, hydrogen-based compound (methane, ammonia, water) aggregated at T 1600 K, T 500- 1300 K, and T- 150 K respectively. Due to the effect of solar winds only denser matter was able to condense near the sun that transformed into rock planets and far away regions became dominated by the gas giants.

The first particles of planets were not bigger than dust and some rocks. When the sun was formed every remaining material revolve around it due to gravity. The smaller gas particles clumped together on their way to circling. The more aggregation the more increment of mass and higher gravitational pull that made it easier to attract more mass. This method called accretion.

When matter attracted each other and collected sufficient mass they become one stage before the planet, called planetesimals. After most of the matter collected into planetesimals, they became dominant features of the orbital plane. After that, it was a very violent process. Planetesimals collided with each other bursting into pieces. The smaller planetesimal fell apart only the biggest one survived this collision course. They combined and grew into protoplanets then eventually planets.

The core of rocky planets got heated due to pressure and radioactive decay. Leaving a small crust the whole planet burns and melt which give ample time for elements to get arranged according to density. The inner planets have solid bodies and heavy metals inside them. On the other hand, gaseous planets have an icy core(water or carbon) and thick layers of hydrogen and helium.

With the help of mathematical models and observation, scientists have forged some models of planet formation. let’s dive right into them.

Core accretion model

One of the earliest models is the core accretion model. When the solar nebula collapsed it gave rise to the sun at its center. Sun had 99.9% mass of the nebula and the remaining material encircled it. As time passed the remaining matter started to clump together under the influence of gravity to form planets. Because of solar winds only heavier elements forged together in the inner solar system. Outer regions had less effect of solar winds to gases piled up to make gas giants. In 2005 scientists found an exoplanet with a massive core orbiting star HD 149026 which has strengthened the core accretion model.

Disc instability model

The core accretion model was all fine for rocky planets but if we focus on the formation of gas giants it doesn’t sit right. It took millions of years for planets to form if we go with Disc accretion but due to power full solar winds this much time was not there to form gaseous planets. This new model proposes that gas was clumped from a very early period probably just after the birth of the sun. As time passed these gases clumped together to form gas giants. According to this approach, gas planets formed very fast in very little time and they stabilized also.

Nice model

The nice model simply states the possibility of planet migration. First scientists thought that all planets must have formed at their star systems (in our case solar system). But after the discovery of exoplanets and 12 million-year-old PSO J318.5–22 (rouge planet) some are thinking that maybe planets can migrate.

One theory was presented to explain it. When gaseous planets interacted with small bodies (comet, asteroid, meteor) they exchanged energy with them. As a result, the smaller bodies are directed towards the sun and planets’ orbit’s increases. The orbits of Neptune, Uranus, and Saturn have stretched. When the body enters Jupiter’s gravitational field it can either throw the object out of the solar system or at the edge of it making the Kuiper belt. The gravitational interactions between Saturn and Jupiter had sent Neptune and Uranus even more out of the solar system. Other smaller bodies have collided with each other making materials of the Kuiper belt. It is thought that some solid rocks that had the potential of becoming planets were destroyed maybe some giant planets were thrown out of the solar system.

Formation of moons

The moons formed in a very similar manner as planets. Out of four rocky planets, only earth has a proper moon. Our moon formed when a Mars-size planet collided with earth. This massive impact threw rock and debris out in space which then aggregates together to create our moon. Mars has 2 moons both are some rouge asteroid captured by the planet. The Jovian planet’s moon has formed in a similar way to planet formation. When gaseous planets compacted and formed the remaining gases started revolving around them. They aggregated, spun, and flattened in the same manner before materializing into moons. It is also possible that some of the moons formed along with planets and then captured by planets like mars.

The constituent of the solar system


Our sun is the largest object in the solar system that comprises almost 99% of solar nebula mass. It can easily fit 1000 earth inside it but still a small pebble in comparison to other giant stars. The distance between the sun and earth is 93 million miles on the other hand distance between the sun and the galactic center is 26,000 light-years.

The solar wind(basically an electric current) creates a magnetic field around the whole system. The sun, its gravitational and magnetic field drives seasons, aurorae, climate, weather, and ocean current here on earth. There are some noteworthy points about the sun. Firstly it has a spin of 25 days on the equator and a span of 35 days on poles. Secondly, the sun has 92.1% hydrogen and 7.8 % helium. It is ringless with a 27 million-degree Fahrenheit temperature. In many of NASA’s videos and photos’ of the sun, there are visible dark spots on its surface.

Sun has influenced many religions, cultures, myths, and folklore. In almost every culture our yellow ball is someway linked to some divine energy, take Hindu, Aztec, Egyptian, Roman, Greek, tribes of the south and north America, Chinese. In pop culture, many characters and stories are woven around sun-like superman and the movie sunshine.

Inner solar system


The closest planet to the sun is mercury which is just a bit large than our moon naturally handling the tile of the smallest planet. Mercury is not the hottest planet and receives 7 times more light than Earth. It is 58 million km (0.38 AU) away from the sun. It takes mercury only 88 days to make a complete orbit around the sun. Sodium (Na), hydrogen (H), helium (He), oxygen (O2), and potassium are major gases in its environment. It has no ring and no moon along with no evidence of life. NASA’s Mariner 10 and NASA’s messenger had first explored mercury.


Venus has a very similar size and density to earth and that’s why it is called earth’s twin. Though there are many differences between both of them. Venus’s atmosphere consists of mainly carbon dioxide. It has a yellow hue due to the high concentration of sulfuric acid. Both of these gases create a high greenhouse effect which makes Venus the hottest planet in the solar system. On its surface pressure is 90 times more than earth. NASA’s Mariner 2 had flown by it in 1962 making it the first planet to measured.

To this date, many space agencies had studied Venus. The soviet union’s spacecraft Venera 13 is had only landed on its surface dying shortly after. Venus is 67 million miles away from the sun. It is extensive mountain craters and valley system resulted due to comet bombarding. Currently, the spacecraft Akatsuki is mapping this moonless planet. From very ancient times Venus had religious values like ancient Greeks and Romans. Personalize for love and beauty, is associated with women.


Earth stands at 3rd position from the sun and holds the title of the only planet with life. It is the only planet with liquid water and the biggest rocky planet in the solar system. All the other planets are named after god and goddesses but “Earth” is derived from old Germanic. It is 93 million miles away from the sun. We have a liveable atmosphere with oxygen and nitrogen dominant. Most of the planet is covered in water and it has one moon.


Associated with the god of war in both Roman and Greek mythology because of its red color, planet mars can be a possible place for human colonies. It is almost 1.5 AU away from the sun and it takes about 687 to revolve around the sun. Mars is a dust cold place with a very thin atmosphere. It is the most explored body in the whole solar system and many scientists believe that in the past Mars had liquid water and a stable atmosphere. There are mountain systems, plains, valleys, and extinct volcanoes on the red planet which indicates a geologically active past. It has two moons phoebes and Deimos. mars is symbolized as masculinity and has associated with men in pop culture.

Outer solar system

The gas giants

Jupiter is the biggest planet in the solar system standing at 5th number. It has historical importance as Galileo had found moons of Jupiter disproving the geocentric model. It takes 12 earth years to complete one orbit. Jupiter has a brilliant red spot 2 times the size of the earth that is actually a storm surge. Saturn is the second-largest planet with mostly hydrogen and helium. It takes 29 earth years to complete one orbit. Saturn has 53 conformed moon with dozens awaited to be conformed.

Uranus is named after the Greek god of the sky. It is 19.8 AU away from the sun and completes one orbit in 84 years. It is an ice giant with 27 moons. The last and eighth planet of the solar system is Neptune, 30.1 Au away from the sun. It has 14 moons and first visited by voyager 1. it is the most windily active icy world. All gaseous planets have a ring system with Saturn having the most majestic.

Asteroid belt

The first line of comets and meteors lies just after mars we call it the asteroid belt. It lies between Jupiter and Mars. Scientists believe that it was created at the time of the solar system but the matter it in wasn’t able to form a planet due to Jupiter. The comets from it sometimes interact with the earth’s gravity and fall into our atmosphere. For the scientific purpose, they are very important as they tell us about the formation and composition of the solar system.

Kuiper belt

After Neptune comes very exciting outer space about which we are still very unclear. We have two large structures Kuiper belt and Oort could. Don’t confuse them together as the Kuiper belt comes before Oort cloud and it is Minatare in comparison to Oort cloud (it is as large as 5000 to 10000 light-years across). The Kuiper belt is full of icy meteorites and asteroids with a donut-shaped body. It also had formed from the remains of the early solar system and shaped by giant planets like an asteroid belt. Its famous resident id Pluto and other dwarf planets also reside in it. All the objects of it collectively called Kuiper belt objects (KBOs) and are smaller than our moon. It is spread across 33 to 50 AU (1 AU is the distance between sun and earth).

Mercury and Venus are both moonless but surprisingly many objects of the Kuiper belt have moons revolving around them. Haumea is a dwarf planet and resides inside the Kuiper belt it also has a ring. Many of the KBOs wobble and make random motions in their orbits which points towards a mysterious planet( planet X) which have a very strong gravitational influence. The main kipper belt runs from 30 AU to 50 AU which has a maximum number of objects.

Oort cloud

Oort cloud is a giant sphere of space debris encircling the solar system. It is a leftover of the solar nebula and made up of mostly icy bodies equivalent to mountain size. Although It is yet to directly measure the Oort cloud however researchers think that many long-distance comets like C/2013 A1 Siding Spring which will not return to the inner solar system for 7,40,000 years is from the Oort cloud.

It is an enormously large structure with its inner edge is between 2000 to 5000 AU from the sun. The outer edge can be as far as 10,000 AU from the sun. Now you can imagine how thick it is. When a comet from Oort cloud approaches the sun their icy surface vaporizes and produces a coma with two tails, one gas, one dust. The tails can reach up to hundreds of miles and it is visible from far away. After crossing the sun tails disappear.

Dwarf planets

Pluto, smaller than our own moon, has five moons in its orbit, including the Charon, a moon so large it makes Pluto wobble. Even tiny asteroids can have moons. We have 5 dwarf planets found in the Kuiper belt Make make, Ceres, Haumea, Eris, and Pluto.


  1. one of the best post on the solar systems is by NASA Our solar system. Do read other posts too they are very informational.
  2. post, How did the solar system form? is quite informative on formation models.
  3. The Arizona education post on the formation of the solar system is concise and detailed.
  4. If you want some video explanation then refer to The origin of the solar system and The Formation of the solar system.

The life on earth

Life on earth as we see now has gone through many trials and errors to reach this stage. Countless evolutionary dead ends, unique experiments, and immense diversity have brought us this far. If you can somehow make and time machine and can go back to when life just began or even at the time of the Cambrian explosion yow will see totally alien life form. So different from what we have today. We will see in future posts all about how this all happened on earth.

This post is the last on the chronology of the universe series. It was difficult to successfully write several posts on it along with procrastination and festival (Diwali). The next blog series will be based on the evolution of earth as a planet and the stages of life on it. It will cover different geographical ages, extinction events, and life forms as they developed during the course of 3 billion years. So, stay tuned for the new blog series and do Revelation.

Our solar system is a fantastically bizarre. There are worlds with features we never imagined. Storms larger than planets, moons with under-surface oceans, lakes of methane, world lets that swap places…. and that’s just at Saturn.

Phil Plait

visit my blog The Scientific Revelation for more reads.



Aayushi Tiwari

Hey everyone. I am a bibliophile and love writing. I am trying to sharpen my hobby of writing regularly. I am always up for new things to learn.