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Planets (and Exoplanets)

We all live on Earth – our beautiful, blue-green planet. While Earth is the only one in our solar system with life, it’s definitely not the only planet! But no human has been on any one except Earth. Attempts at a Mars landing are still in the works. So without having landed humans on another planet, what do we know about the planets in our universe? 

An illustration of the Earth in space and the moon.

The Earth is only one place out of the many we have yet to explore fully!

What Exactly Is a Planet?

Have you ever heard about the Pluto debacle? Well, once upon a time, our solar system had 9 planets, and Pluto was the very last one. Now, however, Pluto is no longer considered to be one by scientists. To understand why this happened, we need to go to the official definition! 

An illustration of Pluto.

Pluto has been demoted to only a dwarf planet!

The full definition of a planet has 3 parts. It has to: 
  1. Be in orbit around a star. In our Solar System, this means that it has to move around the Sun. Orbits are always elliptical, or oval in shape. Planets’ orbits, in particular, tend to be quite circular!
  1. Have enough mass to have a nearly round shape. This is because smaller celestial objects don’t have enough self-gravity. Because of this, they can’t pull themselves into a sphere!
  1. Have cleared their orbits of other objects and floating debris. This means that they have to be big and have strong gravitational fields. If its gravity is strong enough, it will either pull other floating objects in, or push them out of its orbit! 

Dwarfs, Satellites, and Exoplanets

Be careful not to mistake planets with satellites or dwarf planets! Satellite is a broader term. Satellite refers to any celestial object that orbits another celestial object. So the Moon is a satellite of Earth, and Earth is a satellite of the Sun! Satellites can also be natural or man-made! 

An image of a satellite over the Earth.

A man-made satellite, watching over Earth from outer space.

A dwarf planet is almost exactly like a normal one, except for the last criteria. A dwarf planet, like Pluto, isn’t big enough to have a strong gravitational field. As a result, they can’t clear their orbits of other objects! 

An illustration of a series of dwarf planets.

Here are some examples of dwarf planets that we know of!

There is another category that you should be aware of: exoplanets! These are just planets that orbit other stars than our own Sun. Put otherwise, exoplanets are ones that are outside of our solar system. The solar system itself only has 8. But scientists estimate that there are about 10 trillion exoplanets in just the Milky Way galaxy! Imagine how many exoplanets there will be in our entire Universe!

How Do Planets Form? 

Planets and the Sun formed from the same basic ingredient – a solar nebula. A solar nebula is just a massive cloud of spinning particles in outer space. The solar nebula contains a lot of different elements, like hydrogen, helium, and various others! Once the Sun began to form, the rest of the solar nebula became a spinning disk.
As the particles rotated, gravity made them clump up and start combining together. These first formed planetesimals, the beginning stages of proper planets. The planetesimals continued to stick together, and finally accumulated into planets! 

The Different Types of Planets

Within the solar system, there are 3 main types of planets! 

Terrestrial (Rocky)

This is kind of an umbrella term for any planet with a solid, rocky surface. Terrestrial planets formed from silicate rocks and a range of other metals. The first four planets in the Solar System – Mercury, Venus, Earth, and Mars – are all terrestrial planets! 

An image of the texture of the surface of a rocky planet.

The surface of Mars, a terrestrial planet, is rocky and solid.

Gas Giants

These are planets that are mostly gases like hydrogen and helium. Astronomy experts believe gas giants have molten cores inside of their gas layers! Gas giants tend to be a lot bigger than terrestrial planets. We have 2 gas giants in our solar system: Saturn and Jupiter! 

An illustration of the planets in the solar system.

Compare the size of Saturn and Jupiter to the 4 terrestrial planets. The gas giants in the solar system are by far the biggest planets!

Ice Giants

Ice giants, like Neptune and Uranus, used to be classified under gas giants. Scientists found out recently that these two planets were different! Neptune and Uranus contain heavier substances than gas giants! Ice giants, then, are composed of heavier elements like oxygen, sulfur, carbon, and nitrogen

An image of the planet Neptune

Neptune is the second last planet, and the smallest ice giant, in the solar system!

There are more types of planets, but these don’t exist in our solar system. If you want to, click here to learn more about them! 

Other Great Resources


NASA Spaceplace on Exoplanets: https://spaceplace.nasa.gov/all-about-exoplanets/en/

What are Gas and Ice Giants: https://www.scienceabc.com/nature/universe/what-are-gas-giants-and-ice-giants-and-why-are-they-called-so.html

Terrestrial Planets: https://www.space.com/17028-terrestrial-planets.html

How do Planets Form: https://lco.global/spacebook/planets-and-how-they-formed/

Facts about the Planets for Kids: http://www.planetsforkids.org

Dwarf Planets: http://www.differencebetween.info/difference-between-dwarf-planet-and-planet



Planets of the Solar System: https://www.youtube.com/watch?v=RJ2bQWH6GCM

Exploring our Solar System: https://www.youtube.com/watch?v=Qd6nLM2QlWw

NatGeo Solar System 101: https://www.youtube.com/watch?v=libKVRa01L8


Written by: Minh Nguyen