The Rocky Planets


Zofia Patel

How did we get here? Standing on this rocky planet that is speckled with green and covered in large swaths of blue and white, the only planet we know to have life, one might often wonder, how did we get here in the first place? Why are we here, and more importantly, why now? To give you the answer to these questions, we will be going back in time about 4.5 billion years.

Our solar system came into existence about 4.5 billion years ago from a dense cloud of dust and gas that collapsed, most likely due to a shock wave generated by a nearby exploding star. When the cloud of dust and gas collapsed, it formed what we call a solar nebula. In the center of it gravity starts to pull things in. Eventually the pressure of these things becomes too great for the hydrogen atoms and they start bonding together, creating helium in a process called nuclear fusion. This eventually creates our small Sun, which amasses more than 95% of the matter in its reach. The Sun also created something called solar winds (charged particles streaming out of the Sun) that swept away lighter particles and elements which prevented any gas giants from

 forming in the inner circle of planets, but in the outer circles where the solar winds don’t often reach the gas and small particles are not swept away which allows the outer ring to be a perfect domain for the gas giants.


We now come to Mercury the smallest planet and the closest to the sun only 0.39 au 1 from the Sun. There are multiple theories on how the planets were created but the most widely believed and accepted of these theories for the terrestrial planets is the core acceleration theory. Mercury first forms its core then draws in lighter materials to form the outer layers of the planet (the crust). The planet’s gravity also drew in nebulous pieces that would make up the planet’s atmosphere. The planet’s proximity to the sun however would cause the planet’s atmosphere to be ripped away. 

Earth’s Twin, Venus


The planet named after the Roman goddess of beauty is quite a sight to behold with its many colors. But, like the goddess Venus, its beauty is also deadly. The planet Venus 

was formed the same way that Mars was, pulled together by gravity, larger more dense pieces fell to the center and lighter pieces became the crust. Just like Mercury, Venus also once had an atmosphere, but that was destroyed, filled with dangerous greenhouse gasses. Today, thanks to the excess amount of carbon dioxide and traces of nitrogen in the atmosphere, the planet’s surface is now hot enough to melt lead (the melting temperature of lead is 622 f), making it the hottest planet in our solar system even though Mercury is closer to the Sun. The air pressure on Venus is also quite great, more than 90 times that of Earth’s. 

When first discovering Venus astronomers thought it was covered in water and even prepared for a water landing on the planet, but as you can tell, the planet was not covered in water. Venus was also the first planet to be explored by spacecraft and rotates in the opposite direction meaning Venus’s sun rises on the west and sets on the east. 


We are finally here, in our own little corner of the galaxy, planet earth. Like all planets, ours had a similar beginning. Rocks bumping into each other creating larger rocks, lighter debris forming the crust nebulous pieces forming the atmosphere Earth’s magnetic field also was probably developed during this time. Now that the planet is formed, you might be wondering how did the water get there? Well, the answer is quite simple. During the planet’s early life there were a lot more meteor showers and the meteors that were hitting Earth were made of rocks and ice which provided the planet with water and since our planet is in the Goldilocks zone the water didn’t fully evaporate or fully freeze. The also suffered an incredibly large hit in its early life probably resulting in the creation of both our moon and the Gulf of Mexico.


The Red Planet

 The infamous red planet, the one planet most people know about and arguably the most interesting of all the planets, and despite Venus being Earth’s twin, Mars is by far the most Earth-like planet. This planet was created in the same old way but with a twist that fully changed the entire planet’s formation. We all know that Mars is a really small planet but why is it so small? Well there are many theories pertaining to Mars’s small size all based on the lack of material in that certain area of space all centered around our very own gas giants, Saturn and Jupiter.   

The first theory goes that early in the creation of our solar system Jupiter started migrating toward the Sun, sucking up a lot of the debris in the area that would have formed Mars and that the planet would have continued to move forward if not for the creation of Saturn which pulled the gas giant back to its current place in the solar system with its own large gravity. The second theory is similar to the first but in this one both Saturn and Jupiter start migrating towards Mars before tacking back like a sailboat to their current places in our solar system. “Provided that Jupiter changed direction close to 1.5 AU, the growth of Mars would be successfully stunted while leaving enough material closer to the Sun to form Earth and Venus,” John Chambers of the Carnegie Institution for Science wrote and published in the journal Nature. There are other theories but those theories are less clear and defined. 

Going back to the planet’s physical building. Like all planets, Mars became incredibly hot because of the force of the collisions making up the planet. Soon, because of all that heat, the denser materials like iron snake to the center of the planet creating a core. Mars also at one point had a magnetic field but that dissipated when the planet finally cooled off. The planet also had an atmosphere at some point because of the active volcanoes which constantly spewed lava into the air as well as carbon dioxide and water vapor which gave Mars a strong atmosphere. The atmosphere also would have protected Mars and the high atmospheric pressure would have allowed for water on the planet. Ravines and cracks on the planet’s surface indicate that the planet could have at one point been covered in liquid water, but if that is true, where did all the water go? Well, previous theories suggest that it all evaporated when the atmosphere was mostly destroyed. On the contrary though, new theories now suggest that it is still there, just trapped in the crust of the planet. 

One half of our solar system, just a few rocky surfaces inside the first asteroid belt doesn’t sound that exciting, does it? But as you have seen, these planets have a fascinating and dense history filled with explosions and collisions. Keep in mind though, that there will always be more to learn and more to be discovered.  


1-Atomic units

Overview | Venus – NASA Solar System Exploration