Planets are fascinating celestial bodies. From rich environments (like the one on Earth) to desolate wastelands, you can find all sorts of worlds with intriguing terrain. Some even contain traces of water, indicating their unique formation journeys, bringing us to the central question of this article – how do planets form?
Several theories explain the mystery. This entry will explore those theories and take a closer look at the formation of specific planets in our solar system.
How Do Planets Form? Mystery Unveiled
Planets are formed in discs comprised of dust and gas. These so-called protoplanetary discs swirl around a central star, where the particles collide, stick together, and form planetary cores. Experts believe that our sun had a protoplanetary disc more than 4.5 billion years ago that triggered the formation of planets.
There are three schools of thought regarding the development of planets. The first and most accepted is core accretion. As previously mentioned, it involves dust colliding, grouping due to gravity, and growing into planetary cores.
Solar winds swept away helium, hydrogen, and other lighter elements from nearby regions, leaving only rocky materials that created terrestrial worlds. However, the winds weren’t so impactful in areas farther away from the sun. Instead, they allowed lighter elements to mingle and merge into celestial bodies. In turn, planets, moons, comets, and asteroids were formed.
While this theory is credible, it’s not perfect. One of the biggest challenges is time. It presupposes that planets sometimes take tens of millions of years to form, which doesn’t explain how some bodies developed in just 1 million years or quicker.
Pebble accretion is the theory that resolves this challenge. Computer simulations demonstrate that tiny pebbles within the sun’s dust disk grouped in growing protoplanets. They merged so quickly that protoplanets developed rapidly into full-fledged planets. Think of it as children packing on weight to become adults.
In these simulations, experts found that larger objects were bullies, taking pebbles from smaller masses to develop faster. As a result, they ate most of the rocks themselves, which allowed them to grow into giant planets.
Another theory that challenges core accretion is gravitational instability. In this model, clumps of gas and dust bound together shortly after our solar system was formed. Eventually, the clusters slowly grouped into giant planets.
In some cases, it took planets 1,000 years to form this way. These bodies quickly trapped lighter gasses and reached an orbit-stabilizing mass fast that kept them from colliding with the sun.
Planet Migration
Initially, astronomers believed that planets were formed in a fixed position. They couldn’t migrate from their place of “birth.” This assumption was disproved when scientists discovered that planets could march through their neighborhoods.
In the earliest years of our solar system, giant planets had near-circular orbits that were much smaller than today’s trajectories.
They were surrounded by discs of ice and rocks, stretching out just beyond the present orbit of Neptune. As these planets mingled with smaller objects, they scattered them into the sun. This process caused colossal worlds to exchange energy with smaller ones, sending Uranus, Neptune, and Saturn farther away into our solar system. Over time, these small bodies reached Jupiter, launching them near the system’s edge or expelling them from it.
The movements between Saturn and Jupiter drove Neptune and Uranus into eccentric orbits, sending the two smaller planets through the discs of ice and rocks. Some materials were flung inward and crashed into the planets. The rest of the mass traveled outward and created the Kuiper Belt. This region is chock-full of icy bodies and holds the remains of the primordial solar system.
As Uranus and Neptune traveled outward, they often traded places. Their interactions with debris caused them to settle into a circular path, eventually assuming their current position. The two planets were lucky to remain in the solar system. However, others had a tragic fate, as they were banished from the system.
How Do Planets Form Individually? Earth, Mars, Jupiter, and Saturn Journeys Explained
We’ve provided a general answer to the how do planets form question. While the process is the same for most masses, some have had unique developments. Let’s take a look at some of the most exciting examples.
Earth
Earth is an inner terrestrial planet. Like the other three planets of this type (Mars, Venus, and Mercury), it’s relatively rocky and small.
Initially, Earth didn’t look anything like it does today. It was scorching hot and mainly consisted of molten magma. Nothing could thrive in such an extreme environment, but things changed 100 million years after the formation.
The planet started cooling and developing oceans. Then, heavy elements sank past the bodies of water, penetrating deep toward the planet’s center. During this process, Earth became grouped into layers, leaving molten materials at the core and lighter materials at the surface.
Other and possibly even more important events took place in this period.
During the early formation of the planet, comets continuously bombarded the ground. They did wreak havoc on the surface, but they may have also contributed to the creation of life. Namely, astronomers theorize that the comets carried water from distant parts of the solar system to help develop the oceans.
Hundreds of millions of years after the bombarding, photosynthesizing bacteria emerged. They began to release oxygen into the air through photosynthesis. Gradually, these organisms transformed the atmosphere, making it suitable for more complex organisms.
Mars
Like Earth, Mars was formed from heavier, rocky materials. As it had a vast amount of rocky matter, it could withstand the sun’s heat more easily.
It went through a similar formation as Earth. First, dust and many other particles collided, mixed, and separated billions of years ago. Then, by way of accretion, the tiny particles helped create bigger and bigger bodies. Finally, the bodies became so large that their gravitational pull attracted colossal chunks of materials to speed up the formation. Consequently, massive bodies swept up more material and eventually grew into Mars and other planets.
As Mars formed, its surface was sizzling due to the pummeling from asteroids and other bodies during the accretion and decay of radioactive elements.
Shortly after the formation, the interior melted wholly or partially. The materials that made up the planet reorganized, where denser elements (iron sulfide and iron) formed the core. Lighter, silicate-rich materials surrounded the center and created a mantle. The substances that had the lowest number of silicates built the crust.
For about 200-300 million years, Mars may have had a strong magnetic field powered by fluids flowing in the core. But as the planet cooled, the field died.
Another substance that died relatively shortly after the creation is water. A long time ago, water flowed on the surface, as discovered by robotic rovers. These devices uncovered evidence that rivers and lakes were scattered across the planet. This means that the atmosphere had proper density and could have retained enough heat to keep the liquid running.
The situation is dramatically different today. Even though there’s ice in polar caps under the planet’s surface, you can’t find any liquid water on the surface.
Jupiter
Jupiter is the largest planet in the solar system. It could have even become a star if it had been around 80 times larger during its development.
The current models of the origin of this planet suggest that it had a solid center that contained about 10 Earth masses. It formed through the accretion of ice bodies and had an atmosphere created by gasses released during the accretion. As the core mass increased, it attracted gasses from a nearby nebula, resulting in the hydrogen-helium envelope that makes up Jupiter’s atmosphere.
Saturn
Saturn is also approximately 4.5 billion years old. It was created from a disturbance in our solar nebula. Scientists assume it collided with another nebula or suffered an enormous shock from a nearby supernova.
Either way, there were no mysteries surrounding the results. A protostar was developed at the core, surrounded by a disk of scorching material. The inner parts contained heavier elements, forming four terrestrial planets, while the remaining area was cool enough to retain ice.
Saturn is smaller than Jupiter, so the temperature dropped more quickly. Astronomers think the outer atmosphere sometimes reached around 15K (-432° Fahrenheit).
Nevertheless, the planet contained droplets of helium that raced toward the core. The friction produced by the droplets heated the planet so intensely that it generated two times as much energy as it obtained from the sun. This made the surface relatively hotter than some other planets.
Saturn’s rings deserve special attention.
One theory that explains their formation is that they’re remnants of old moons that once orbited the planet. This presupposition suggests that the moons were somehow ripped apart. They had icy layers and coats that were demolished, crashing in Saturn and forming the rings. The effect might have been triggered by the planet’s gravity and gravities from other moons.
Another theory is that rings were created as comets, asteroids, dust, and debris hurled near Saturn. Saturn’s gravity pulled the materials toward the planet, trapping them inside an orbit. Then, the trapped objects might have been torn apart by gravity from Saturn or other debris.
Moreover, the masses may have run into each other and broken into smaller pieces. Finally, the fragments flattened and turned into rings.
Can You Directly Observe the Formation of Planets?
Scientists have only recently started observing the formation of planets.
A few years ago, they detected the creation of a planet near the star called HL Tauri. The scientists used an advanced technique that measures the planet’s gravitational pull on the host star. All the metrics suggested that a new world was being born. Furthermore, astronomers detected dark rings that indicate the formation of new planets, pushing dust as they develop.
Images of these discs are essential to observing the development of planets. They show various features, like rings, spirals, and gaps, that indicate how the discs evolve and allow planets to form. The planets themselves can even create some, but astronomers are still investigating why this happens.
Once the discs are picked up, professionals rely on extensive theoretical knowledge and cutting-edge computer models to determine if they can harbor planets. In some cases, they try to pinpoint the properties of the planets, including their orbital distance and mass.
If scientists are lucky, they can take pictures of newborn planets. The first image of a world forming inside a disc was produced in 2018. Scientists captured a newly-formed planet around PDS 70, a star in the constellation of Centaurus.
Experts spotted another planet formation in 2021. This newborn was captured more than 500 light-years from our planet.
The area piqued astronomers‘ interest because it was home to one of the most mesmerizing stars in the universe – AB Aurigae. The most well-known feature of this star is the gorgeous, intricate disk surrounding it. The waves and clumps in the disk indicate a gravitational collapse, which generally points to the birth of a planet. And scientists hit the jackpot in this disk.
They discovered a planet embedded in a halo of gas and dust about 1 billion miles away from its star. Due to such an enormous distance, core accretion couldn’t be used to explain the formation. So instead, the experts provided more proof for the gravitational instability theory.
Stoked by flaming energy from the rapid creation process, the planet is blazing hot (approximately 3,600° Fahrenheit. It’s so hot that it glows, making detection much easier. Furthermore, the central star illuminated the swirling dust and gas around the newborn. Scientists discovered the development with the cutting-edge Subaru Telescope and the iconic Hubble Space Telescope.
With technological breakthroughs, experts hope they’ll be able to detect planet formations more often.
New Planets at Every Corner
Scientists have tried to answer the question of how planets form. Only a few viable theories have been put forward, but they do a great job explaining this complex process.
However, there’s no telling what we’ll uncover hundreds of years from now. We may discover bewildering formations in unknown parts of the cosmos. Who knows – one of them could be our new home.