Planets: A brief introduction

Updated: Jan 20, 2021

“Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives - Carl Sagan

We live and thrive in this beautiful blue planet called Earth. It feeds us, grows us and nurtures us. Everything we have, we have taken from mother Earth. What do we know about the place that bears us all? Primarily we all know for a fact that Earth is a planet, and it revolves around a star but what are planets? How do planets form? How did our solar system form? Let us step ahead and discuss!

What are planets?

We cannot just go around any star system and name things floating around as planets, there are guaranteed criteria for any spherical mass to be called one.

According to the International Astronomical units (IAU) to be called a planet, the astronomical object must fulfil the following conditions:

  • It should orbit a star.

  • It must hold a spherical shape (hydrostatic equilibrium)

  • It must be large enough so that its gravity clears the path it orbits around the star.

According to the above definition, bodies fulfilling all these criteria are Planets. Of course, there is always room for exceptions. Pluto-like planets only satisfy the first two conditions hence placed among "dwarf planets. There is much debate happening around these criteria to define them, as we keep on discovering more and more, our definitions for 'what a planet is?' will also evolve. Let's leave it to the experts for now and move on to how planets form.

How do planets form?

Though there are millions of planets throughout the Milky Way, we need not look beyond our solar system to understand how planets form.

Unsurprisingly, how stars form has a lot to do with how planets form. There are many theories about the formation of our solar system. The nebular hypothesis is the most notable and widely accepted among them.

Our Universe is a cornucopia of regions with uneven distribution of gas and dust. These regions are responsible for the formation of structures called Nebula, in our case, it's called the solar-nebula.

Solar Nebula is nothing but a giant cloud of spinning at a high velocity.

Here's what happens in that cloud of gas and dust. As the cold gas cloud spins at higher velocities, the centre of the gas cloud condenses. Density in the centre increases the spinning speed and results in a flattened out pancake-looking disc called an accretion disc.

This animation shows how material around a young star is shaped into planets over billions of years. (Credits: NASA visualization explorer)


Eventually, the temperature and pressure at the centre of the disk continue to increase till the central region collapses to form a Protostar.

If the accretion disk around the protostar still has matter enough to form an entire star system, it rains its material into the star till it emerges as a Main-sequence star. The material left in the accretion disc continues to spin around the protostar. These particles continue to bombard with one another, forming clumps of material. These clumps stick together till their gravity gathers up even more material, it creates thousands of moon-like protoplanets.

Protoplanets continue to grow, gathering up more mass till they form actual large-enough planets that revolve around the star at the centre.

Why are planets round?

Now we know all planets came from shapeless clouds of gas and dust, but all we see through our telescopes are spherical balls moving about, why is that? Why didn't we get to have conical moons or a giant joystick-shaped Jupiter? That would have been fun right? But nature has its reasons.

Candidly "The heavenly bodies are round because Gravity allows them only to be round". Energy and gravity conspire together to make celestial objects round. As Newton famously said, anything that has mass experiences gravity. When gravity pulls equally in all directions, the only shape a body could assume in space is a sphere and nothing else.

Types of planets:

In our solar system itself, there are mainly three types of planets- they are rocky planets, gas giants and dwarf planets. The inner planets of our solar system are rocky (Mercury, Venus, Earth and Mars) and rest are gaseous (Jupiter, Saturn, Uranus and Neptune).

The nebular hypothesis can provide an answer. The heat and solar winds caused by the sun swept the lighter gases farther out into the developing solar system. Hence he rocky terrestrial planets are closer to the Sun, while the gas giants Jupiter Saturn Uranus and Neptune formed in the shivery outer region of the solar system. Pluto falls under dwarf planet category along with Eris, Ceres, Makemake and Haumea. We classify these planets as dwarfs because they haven't cleared the path they revolve around the star.

There are other types of planets, such as rogue planets, exoplanets, Trojan planets, Goldilocks planets and more. Stay tuned for more fascinating stories from our Universe. Subscribe to get stories direct to your inbox!

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