It’s no secret that the solar system is filled with mystery. It was once believed that the Sun, Moon, stars, and planets orbited the Earth. This was until Galileo famously proved that the planets orbited the Sun. So, why do planets orbit the Sun? Is there a particular reason why the planets orbit the Sun and not a planet like Earth?
Modern science has continued to prove that Galileo’s ideas were correct.
We all know that the Sun is the star around which the remaining solar system rotates around, but it may not be clear why this happens.
Here is everything you need to know about why and how planets orbit the Sun.
The Short Answer
In short, planets orbit the sun because of the Sun’s gravitational pull. The sun’s gravitational pull is similar to that of Earth but much greater. Gravity pulls things together.
In fact, the Sun’s gravitational pull is so great that it can hold all the planets (and other objects) in our solar system. This is how the planets stay close to the Sun and remain in place!
The Long Answer
Let’s look back at the long history of the solar system. Around 4.6 billion years ago, before our solar system even existed and after the Big Bang, all that was left was a huge gas cloud. This gas cloud was mostly hydrogen, with a small amount of helium and a smaller amount of lithium.
The initial hydrogen cloud formed the earliest stars. These stars consumed their hydrogen supply, resulting in a supernova. As a side note, before the supernovae, elements up to iron were formed via fusion. During the supernova, elements heavier than iron were formed. All the gold in your jewelery was formed in the death throws of an ancient star!
The supernova seeded the surrounding space with leftover hydrogen and heavier elements.
After the supernova explosion, the gas cloud (nebula) underwent gravitational collapse. This occurs due to mutual gravity.
Initially, the nebula had a slight rotation. As the cloud collapsed due to gravity the nebula began to rotate faster and faster due to conservation of momentum. In a spinning body, if mass is brought closer to the center of rotation, the system will spin faster. The often-used example of this effect is a skater bringing her arms in to spin faster.
This eventually led to a vast spinning ball of gas atoms which then collapsed, leaving it spinning faster and faster. Rotational forces forced the cloud to flatten, with the central area having a higher concentration of matter. This disk structure is called a protoplanetary disk.
At the center of the protoplanetary disk, a pro-star subsequently became the Sun.
The disk of material that remained around the Sun eventually formed planets. Rather than planets forming directly, small objects first formed. These ‘planetesimals‘ collided and coalesced to become planets.
Any gas left close to the Sun was blown to the outreaches of the solar system. This resulted in the small rocky inner planets (Mercury, Venus and Earth) and the outer gaseous planets (Jupiter, Saturn, Uranus and Neptune).
So, the reason why planets orbit the Sun is that they formed from the same protoplanetary disk. Planets are the leftovers of how our solar system was formed. The motion of the planets is a direct result of the spinning of the gas cloud that provided the material they are made of.
This hypothesis for the solar system’s formation is called the nebula hypothesis. The hypothesis also explains why the planets orbit in the same plane and rotate in the same direction (except for Venus).
Why Are The Planets In Perfect Balance?
The reason why the planets are in perfect balance is because of gravitational pull.
Two opposing forces of gravity act on the planets, so they don’t move far from their original positions. One of the forces is the Sun’s gravitational pull, which pulls the planets towards the star, and the other is the inertia of their orbit.
This inertia drives the planets outward and stops them from colliding with the Sun.
The planets act like any object with mass in that they resist changes to their speed of movement and direction. This is what is known as inertia.
These forces have to be perfectly balanced. Otherwise, the strength of gravity would spiral the planets into the Sun. Likewise, the planets would spiral into space if inertia were more dominant than gravity.
So, to put it simply, the planets are stuck between trying to fly out into space while the Sun’s gravitational pull is pulling them into a curved orbit.
Both forces are balanced, and this is why they remain in orbit.
Why Do The Planets Orbit The Sun And Not Jupiter?
We all know that Jupiter is the largest planet in the solar system, with a diameter of approximately 11 times the diameter of the Earth.
So, considering the size of Jupiter, you’d think it offers a gravitational pull strong enough to pull the other planets around it, right?
Interestingly, no. Jupiter has a fairly feeble gravitational pull, pulling you up 34 million times less than how Earth pulls you downwards.
This means that Jupiter’s gravitational pull is insufficient for the planets to orbit around.
Also, the sheer size of Jupiter means that it doesn’t technically orbit the Sun. The center of gravity of the other planets is close to the center of the Sun.
The center of mass between the Sun and Jupiter (called a barycenter) lies between the Sun’s surface and Jupiter. It is still very close to the Sun. Technically the Sun and Jupiter orbit around the barycenter.
The massive size of Jupiter means that its pull also affects how the Sun moves, even though the Sun is about 1,000 times bigger than Jupiter.
The amount of gravitational pull that Jupiter has on the Sun is one-thousandth the amount of gravitational pull that the Sun has on Jupiter.
Why Don’t Planets Get Pulled Into The Sun?
There are two reasons why planets don’t get pulled into the Sun. Firstly, as explained earlier, the planets lie in perfect balance between two forces – the Sun’s gravitational pull and the planet’s inertia.
These forces are balanced so that the planets don’t pull into the Sun or fly into space.
Secondly, the planets move too fast to plunge into the Sun. Earth, for example, travels at 67,000 miles per hour in a sideways motion. If that sideways motion were to stop, or if we were to slow down, then the Sun’s gravitational pull would pull our planet inwards.
Wait, What About General Relativity?
The astute may have noticed a simplification of how gravity works. We used Newton’s view of gravity to keep things simple. It is easier to imagine a pulling force than the bending of space-time.
Currently, our best description of gravity is that provided by Einstien. In his Theory of General Relativity gravity is seen as the bending of space-time by the presence of mass. Anything traveling through the bent space-time travels in a straight line in the curved space-time.
Even though it is an approximation Newton’s view of gravity is accurate for the orbit of all planets except for that of Mercury. Being much closer to the Sun Mercury is in a much stronger gravitational field. So strong that Newton’s theory does not perfectly describe its orbit. The planet’s orbit processes. For more information on this, see Precession of the perihelion of Mercury.
General relativity is a complex concept and is beyond the scope of this post. If you want to learn more about General Relativity, you can read Introduction to General Relativity.
Summary
So, there you have it! To put it simply, the reason why planets orbit the Sun is because of the gravitational pull of the Sun and the inertia and momentum of the planets which keeps them from plunging into the star.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
