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How Planets are Formed: Everything You Need to Know

The Universe, as we know it today, is littered with stars, planets, and innumerable celestial objects that have been floating through space for billions of years. And somehow, we all ended up on a planet that supports the miracle of life as we know it, the Earth. But how are planets formed? Here’s everything that you need to know

The most widely accepted theory on how planets are formed, the protoplanet hypothesis, posits that solar systems around the universe originate from rotating discs of space dust, covered in frozen gasses, which have collided and stuck together over millennia and slowly transformed into planets.

The nature of the universe is something that scientists have been debating for decades, and figuring out exactly how the universe came to be is an endeavor as old as humankind itself.

And, while the Big Bang Theory on how our universe is widely known and, to a lesser extent, understood among us everyday people, most won’t have heard of the protoplanet hypothesis (also known as the nebular hypothesis).

In this article we’ll learn exactly how planets are formed, along with all the main theories surrounding their formation…

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The Protoplanet Hypothesis

It simply isn’t possible for us to truly understand the conception of our planet, the Sun, and the Universe, because the Earth is at least four billion years old, and the Sun is several million older, and they are only a fraction of the Universe’s age.

But throughout hundreds of years of hypotheses, dating back to ​​Galileo Galilei’s controversial theory, proposed more than 400 years ago, that the Earth revolves around the Sun and not the other way around.

Thanks to the incredible developments in science and technology, we have been able to observe our skies with far greater accuracy than Galileo, and we can see far further into the depths of the Universe. And, with countless Astronomers spending their careers patiently observing the heavens, we have managed to piece together as many parts of the puzzle as possible.

And, without getting into a time machine and going back to when the Earth and other planets originally formed, we have been able to use the instruments like super telescopes at our disposal to stitch together a hypothesis that provides a fairly compelling explanation about the origins of planets.

Evidence suggests that stars and their planets condensed from vast clouds of cosmic gasses. And the protoplanets hypothesis is the leading theory that outlines the birth of stars, planetary systems, and moons from the gaseous nebula.

The theory suggests that the cloudy nebula condensed after pressure changed in the nebula. It is unknown what may have caused this; it could have been an exploding supernova or passing star.

The nebula’s cloud then collapsed to form a disc or halo of material, which would rotate around the gravitational center. So strong was the pressure in the gravitational center that loose hydrogen atoms came into contact with other floating molecules to form helium – which led to the birth of a star, in the case of the solar system, The Sun.

Want to learn more about stars? Check out my other articles: “How are Stars Formed?“, “Are Stars Considered Planets?“, and “What Are Stars? Everything You Need to Know

As the Sun matured and got larger, it devoured almost everything in space around it, except for about 1% of what remained of the matter in the halo/disc. That remaining matter would kickstart the formation of our planets.

In this tumultuous period, the solar system’s infancy, large bodies of gas, dust, and debris were violently floating around and colliding with one another, eventually sticking together and combining to form larger pieces of dust and gas, becoming what resembles a meteor.

As these new, bigger rocks grew, so did their gravitational pull, which attracted smaller rocks that collided and stuck to the rapidly expanding celestial bodies that formed our rocky planets. The sun pushed gasses to the outer orbits (which is why gas giants like Jupiter/Saturn/Neptune are further out, and more rocky planets like Earth, Mars, and Mercury are closer to the sun).

The smaller, undeveloped, young “protoplanets” collided to form bigger bodies of rocks or “planets” as we know them today. There is evidence of this theory to be found all over our solar system, with debris scattered all over the show, albeit to a far lesser extent than the solar system’s early days.

For example, an asteroid belt between Mars and Jupiter exists, which would have led to the formation of more planets, if not for the fact that Jupiter is so large and its gravity kept these “leftovers” from the infancy of our solar systems under lock and key.

How The Earth Was Formed

Even if we can explain how planets came into existence, the simple fact of the matter is that we live on a giant chunk of rock that is so incredibly unique – it’s the only planet that supports life as we know it. So what was it about the formation of our life-giving utopia that has given it the unique ability to be a haven for living organisms? To understand this, we need to take a closer look at what happened here on Earth.

Planet Earth has a “squashed” spherical shape (an oblate spheroid), with a heavy metal core and a lighter surface crust, with a thin outer atmosphere that provides breathable air. These attributes make it truly unique, and the theory behind how the Earth acquired its unique features is well documented. This is how our planet was able to form vast oceans, arable land, forests, mountains, freshwater rivers, and so on:

In the Earth’s infancy, it probably would have more closely resembled Saturn and Jupiter and was a mere gigantic volume of gas and dust. As the density of the planetary body grew through particle collisions, it would take on a solid state, leading to the creation of the Earth’s crust and inner layers.

The Earth’s core is made up of iron and molten metals, and these molten metals would eventually lead to the formation of massive volcanoes all over the planet. All of this volcanic activity was relentless, and all of the emissions that resulted from these global volcanic eruptions formed what we know today as Earth’s atmosphere.

Furthermore, these volcanoes played a key role in forming the Earth’s crust and creating planetary anomalies like islands.

The volcano emissions that formed the earliest iterations of the Earth’s atmosphere, made up of hydrogen and helium, were accelerated by a meteor shower that hit the Earth, which led to emissions of carbon dioxide and water vapor.

However, the presence of sulphuric gasses from the volcanic eruptions would have made the Earth uninhabitable at the time. Once all of the gasses within our atmosphere condensed, it rained for the first time.

Gradually, the Earth cooled down, and the surface formed a thin crust, but under the crust, hot rock continued to react, which moved the crust below, breaking it apart (plate tectonics) – a process that continues to this day. These subterranean plates shift around, meet each other, collide, crumble and create mountains or deep underwater trenches.

Over time, thanks to the presence of water on our planet (which could take liquid form due to the composition of our atmosphere), photosynthetic bacteria would release oxygen into the Earth’s atmosphere.

Once oxygen entered our atmosphere, it found its way into our oceans, giving rise to marine life forms, which would eventually evolve into all the species of animals that we know today.

The final change to the Earth’s atmosphere’s composition resulted in what we know today, comprised of roughly 21% oxygen and 78% nitrogen.

There is one final piece to the puzzle that makes it possible for life to thrive on Earth – Our magnetic field. This is an invisible phenomenon, diverting high-energy particles from the sun, such as solar winds (a stream of plasma and other particles), among other potentially damaging forces that make other parts of the universe uninhabitable, away from the Earth.

This means there’s relatively little radiation that makes its way from the sun through to the Earth. Scientists are still learning more about the purpose and origins of Earth’s magnetic field, but the standing belief is that it’s the iron and heavy metals that make up the Earth’s core that has led to this phenomenon.

How Do We Verify The Protoplanet Hypothesis?

We are talking about a moment in history where there were no humans around to observe what was happening, so the difficult question for everyone is, “how do we prove the protoplanet hypothesis?” There are two methods that Astronomers are using to test the hypothesis: Observation and modeling.


By using powerful super telescopes, such as the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the South African Large Telescope (SALT) in South Africa, or the ​​Multi-Mirror Telescope (MMT) in Arizona, astronomers can observe dusty disks/halos from other parts of the universe, where young planets are being formed everywhere.

These planets outside the Solar System (exoplanets) can be seen from billions of lightyears away by the Hubble Space Telescope (which operates from space).

We can observe primitive solar systems and planets forming around the vast expanses of the known universe, how things have changed over time, and whether the way things play out is consistent with the theory.

The Hubble Space Telescope can see beyond the solar system

But the problem with this, of course, is that the events in the protoplanet hypothesis occurred over a period of billions of years, meaning we’d be waiting for a very, very long time to verify this theory. This is why astronomers have turned to modeling to arrive at an answer faster.


To test their hypotheses, astronomers use computers to calculate whether their theories are viable. By accounting for various conditions, they can run a simulation, adding variables to the equation to see if it is consistent with their theory.

The simulation can recreate the conditions of the young Earth, speed things up over time, and add variables such as a passing star and see what kind of changes that would make to the composition. If the model remains robust after several runs, accounting for additional variables, the theory carries some weight.

However, modeling isn’t necessarily accurate. There are countless complications, variables, and stumbling blocks that we simply don’t know about, which could easily throw off some of the most widely accepted theories. Accounting for every moon, every asteroid, rock, particle, and an infinite range of external factors is simply impossible. There’s a lot about our Universe that is and may always be shrouded in mystery.

How Were The Gas Giants Formed?

We may know how rocky planets like ours were formed, but the process was very different for the gas planets like Jupiter, Saturn, Uranus, or Neptune.

Check out my article “7 Planets that are Made Out of Gas

In the Solar System’s infancy, in the early stages of the accretion disk (the flat, condensed structure that surrounded the young Sun), the Sun wasn’t large enough to be considered a star, but eventually, as it gained mass, would become a protostar which gave off solar flares that would push gasses further from its orbits than the rocky protoplanets.

The Sun pushed particles, especially the lighter particles like hydrogen, helium, etc., to the solar system’s outer limits, along with “ices” like water, methane, and so forth. This meant that the heavier, rockier elements, like iron, stayed in the middle of this young Solar System. These would collide and grow and turn into dwarf planets. From there, many theories have emerged about how the gas giants were formed.

As these terrestrial objects began to enter orbits, collapse into one another, and create bigger and bigger planets, a few of them (four) possibly moved to the outskirts or could potentially have formed a lot further away from the sun. We aren’t certain about exactly where the earliest iterations of the gas giants originated, but we do know what came after.

As these terrestrial bodies blew through the gasses that were pushed to the outskirts of the solar system, they would start to gain mass, accumulating a massive body of various gasses at a far greater rate than the rockier planets closer to the sun (which is why they became “giants”). Think about the gas giants’ accumulation of mass as something akin to a snowball effect.

One such example was a rocky protoplanet that was roughly one and a half times the size of earth, which accumulated all of these gases, primarily helium, turned into Jupiter. A little further out, similar elements came together to form Saturn, while, further out from there, where very light elements such as ammonia, methane, and water were present is where Uranus and Neptune were formed.

However, a lot of this theory is grounded in speculation because we don’t know what the surfaces of the gas giants actually look like, and we don’t know that they even necessarily have cores. Getting close enough to inspect what’s underneath all of the tumultuous storms that we can observe on Jupiter, Saturn, Uranus, and Neptune is not possible right now

What About The Moons?

Starting with our own moon, it is believed that the brightest object in our sky was born out of a catastrophic collision between the Earth and a Mars-sized protoplanet in the earliest stages of our planet’s formation.

The remaining debris orbited the Earth, much like the accretion disk that formed around the Sun, with smaller fragments of rock and debris from the impact colliding and clumping together to form what is now the moon that appears in our skies every night.

While the definition of what constitutes a moon can be somewhat arbitrary and inconsistent, let’s consider them to be the same as a natural satellite. A satellite is any body of mass that follows a distinct orbit around a planet. Natural satellites are always much smaller than the planets they’re orbiting, but our moon (the Moon) is distinctly large. It is 0.273 times the diameter of the earth.

One of Neptune’s moons, Triton, for example, is 0.055 times its diameter, while Saturn’s Titan is 0.044 times its diameter, Jupiter’s Ganymede 0.038 its diameter, and Uranus’ Titania is 0.031 times its diameter. 

The only planet with a Moon that’s larger in comparison to ours is Pluto, with Charon being 0.52 of the tiny planet’s diameter. Charon is believed to have also have been born out of a celestial impact involving Pluto and another protoplanet.

Where Do Saturn’s Rings Come From?

It is commonly believed that Saturn’s rings are a consequence of the gas giant’s massive gravitational pull but resulted in a completely different phenomenon to what was observed with the accretion disk. Saturn’s rings were likely formed when asteroids, comets, or even small moons were broken up while orbiting around the planet.

After continued collisions, they broke up into smaller pieces rather than being clumped together. When the pieces gradually spread around, they formed the rings that we observe today.

However, did you know that Jupiter, Uranus, and Neptune also have rings that are just a lot smaller and harder to see? We only learned about Jupiter’s rings in 1979 when the Voyager 1 Spacecraft flew past our solar system’s largest planet.

Are There Any Earth-Like Planets Out There?

So now that you know how a planet is formed let’s take a look at some of the other planets that we’ve discovered outside of our Solar System that have the potential to support life as we know it on Earth.

Planet Gliese 581 d

One such planet is Gliese 581 d, a planetary body that’s six times the size of the Earth that falls within the “Goldilocks zone” (a position close enough to the planet’s sun not to be too cold, but far enough not to be too hot) and located 20.2 light-years away from us.

But we may not be so keen to go because, even though it could support life, theoretically, because water can exist there and it has at least one ocean, it has an average surface temperature of -18ºC (-0.4ºF). Another is ​​HD 85512 b, a planet 3.6 times bigger than the Earth, which is covered in clouds.

However, the problem with this planet is that it’s tidally locked, meaning one side of the planet is perpetually facing the sun while the other remains plunged in a state of perpetual darkness. 

Planet Kepler-69c

Kepler-69c is perhaps the most famous planet that falls into the habitable zone of its solar system. However, as Earth-like as it may seem, the planet appears to have properties far more similar to Venus’, which makes it a far from favorable environment for life to thrive in.

Planet Tau Ceti e

Tau Ceti e, a planet that has temperatures in a similar range to ours (70ºC/158ºF), which is 1.8 times Earth’s size and supports a strong atmosphere, has a rocky surface similar to our and is relatively nearby (relatively being the operative word) at just 11.9 million lightyears away from us.

Planet Gliese 667 Cf

Gliese 667 Cf, a planet with three suns (although it only orbits one of them), also falls within the “Goldilocks zone” due to its unique features, but it only receives about 60% of the amount of sunlight that the Earth does.

Planet Kepler-62f

Kepler-62f – 1.4 times the size of the Earth – is very similar to our planet but has a much smaller, cooler sun, and there’s a very high probability that water can exist there.

However, at 1200 lightyears away, it’s hard to think that getting there will be an easy job, regardless of how much our technology could improve in the future.

Planet Gliese 667 Cc

Gliese 667 Cc is a planet that’s roughly 3.8 times the size of earth, which probably looks more like Mars.

However, it is 85% similar to the Earth, with its rocky surface, but ventures very close to the limits of the “Goldilocks zone”, resulting in -18.85ºC (-1.93ºF) temperatures. Nonetheless, it receives roughly 90% as much light as we do on Earth, making it a promising candidate for a habitable planet.

Planet Keppler-2e

Keppler-62e, which is also a bit far out at 1200 lightyears away, has a high probability of water, is about 1.6 times larger than Earth, and has similar characteristics, such as a cloudy sky, meaning that it will likely have similar warmth and humidity to what we experience here.

Planet Gliese 581 g

Gliese 581 g is one of the closest examples we have to an Earth-like planet, however, because it has distinct oceans and continents that look almost just like our own.

It’s also “only” 20.3 lightyears away. It is rocky and supports a strong atmosphere similar to ours, but the temperatures are rather off-putting, at -37 to -12ºC (-34.6ºF to 10.4ºF).

There is an incredibly high probability of life on this planet, but it is still up for debate, which is why it will always live in the shadow of our “Future Home”, the planet Kepler-22b

Planet Kepler-22b

The Kepler-22b is 2.4 times the size of Earth, which lives in the sweet spot of the “Goldilocks zone”, with temperatures estimated to be between -11ºC (12.2ºF) and 22ºC (71.6ºF), and it doesn’t even have its own atmosphere!

However, the planet does appear to be covered with oceans and lacks the rocky surface that we’d ideally like. It is located approximately 620 light-years from Earth. The jury is still out on Kepler-22b, however, and some suggest that it may even be a gas planet, and we have to do a lot more research to find out about its true nature.

How The Origins Of Our Universe Can Shape Our Future

Studying the origins of the Universe and how planets are formed can often seem like a very academic exercise. However, this isn’t just about formulating abstract theories that have no relevance to our current reality – the protoplanet hypothesis tells us a lot about our environment and what makes the miracle of life such a miracle. It tells us about how the climate changes and how our atmosphere’s composition makes a massive difference to its conditions.

By using modeling to figure out how our planet came into being, we maya also be able to predict how the effects of climate change can change our environment and affect how habitable our planet is. And, if we reach a point where intergalactic space travel is possible, understanding how our planet was created and how it has changed over time allows us to search the heavens for candidates that could serve as a new miraculous, live-giving host to humankind.

It is only with the help of astronomers that we can unlock the secrets to the recipe that allows life to exist on earth. 

More About Stars and Planets…

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