What are the Gas Planets?

Astronomers love to categorize things, especially planets in our Solar System. All the time you hear people refer to the term “gas planets”, which was coined by a science fiction writer back in 1952, but what does it mean?

Even though we hear terms like “gas giant”, “gas planet”, “giant planet” or even “Jovian planets”, they’re all referring to the same thing and one of the more common questions in Astronomy; What are the Gas Planets?

Gas planets refer to the giant planets in our Solar System which are furthest to orbit the Sun and made of… yeah, you guessed it; mostly gases!



Photo by Meng Bomin, https://flic.kr/p/7jBxC9


What are the gas planets: Jupiter, Saturn, Uranus and Neptune.

Now, before you go around and spit off how much you know about the gas planets, remember one thing. Astronomers love to categorize things, sometimes they even re-categorize things.

In the case of the gas planets, Uranus and Neptune are now categorized as “ice giants” or ice planets. But regardless, Uranus and Neptune should be considered gas planets. For the purposes of this article, we’re going to include them as they’re still giant planets and full of gas.

What are the Gas Planets Characteristics?

The gas planets are mostly made up of:

  • Hydrogen (H)
  • Helium (He)
  • Molten Rocky Core

It’s tough for Astronomers to know for sure, without being on the planets themselves, but they’re pretty certain that the gas planets are made up of an Outer Layer of Molecular Hydrogen and an Inner Area of Liquid Metallic Hydrogen and more than likely an awesome Molten Rocky Core at its center.

The outer most layers are made of Hydrogen and Helium, where the atmosphere also contains water and ammonia.

The inner layer is where some say trickery takes place. It’s here that you’ll find liquid metallic Hydrogen. Yeah, you read that right, liquid Hydrogen and not gas.



Photo by Anthony Starks, https://flic.kr/p/7HyrZu


Come to find out, no, there is no wizard in the middle of these planets, despite Hydrogen typically being a gas. Under extreme pressure hydrogen can become a liquid and behave as an electrical conductor, metallic Hydrogen! This metallic Hydrogen makes up the majority of each of the gas planet’s mass.

Element Recipe for the Gas Giants:

  • Hydrogen and Helium
    • 87% to 93%
  • Ammonia, Methane and Water
    • 3 to 13%

Element Recipe for the Ice Giants:

  • Oxygen, Carbon, Nitrogen and Sulfur
    • Roughly 80%
  • Hydrogen and Helium
    • About 20%

What’s with the Categorization of Ice Planets?

For real! Well, back in the 1990’s Astronomers realized that Uranus and Neptune were different, different enough to justify them being in a league of their own; please welcome the Ice Planets.

Ice planets are mostly made of heavier elements; Oxygen (O), Carbon (C), Nitrogen (N) and Sulfur (S). About 80% of their mass is made up of these elements.

Ice planets also contain a fair amount of the lighter elements that the gas planets contain; Hydrogen (H) and Helium (He) – only about 20% of their mass is composed of these elements.

How were the Gas Planets Formed?

It’s believed that these gas planets originally formed with solid cores, much like the terrestrial planets closest to the Sun; Mercury, Venus, Earth and Mars.

The big difference, is that the planet’s core would have been really, really big! They’d have to be big to maintain orbit that far away from our Sun. Some estimate that the core of gas planets would need to be roughly about 10 times the size of Earth’s mass to support the formation of a gas giant planet.



Photo by Duncan Hull, https://flic.kr/p/9kUnGm


So after these massive planets were just chilling in orbit for millions of years, they eventually started to attract and collect left over gases from the formation of their solar system’s star.

Stars are born in Giant Molecular Clouds, which are massive collections of dust and gas. When these giant molecular clouds (also referred to as Nebula) get to be so large, they begin to collapse on themselves. The outer matter compresses down so heavily on the inner matter, which now has nowhere to go and boom, nine months later a baby star is born. Fair enough, nine months is an exaggeration.

What are the Gas Planets Distance from the Sun?

  • Jupiter: 5.2 Astronomical Units or 511,256,940 miles from the Sun
  • Saturn: 9.54 Astronomical Units or 883,080,169 miles from the Sun
  • Uranus: 19.2 Astronomical Units or 1,784,751,499 miles from the Sun
  • Neptune: 30.1 Astronomical Units or 2,825,856,541 miles from the Sun
  • Earth: 1 Astronomical Units or 92,955,807 miles from the Sun

Check out this infographic on How Big is our Solar System?

What are the Gas Planets Masses?

  • Jupiter: 318x Earth’s Mass
  • Saturn: 95x Earth’s Mass
  • Uranus: 14x Earth’s Mass
  • Neptune: 17x Earth’s Mass
  • Earth: 1 Earth Mass



Photo by Lunar and Planetary Institute, https://flic.kr/p/7emdda


What are the Gas Planets Density?

  • Jupiter: 1.33 grams per cubic centimeter
  • Saturn: 0.71 grams per cubic centimeter
  • Uranus: 1.24 grams per cubic centimeter
  • Neptune: 1.67 grams per cubic centimeter
  • Earth: 5.514 grams per cubic centimeter

What are the Gas Planets Rotation Periods?

  • Jupiter: 9 Hours 56 Minutes per Rotation
  • Saturn: 10 Hours 39 Minutes per Rotation
  • Uranus: 17 Hours 14 Minutes per Rotation
  • Neptune: 16 Hours 6 Minutes per Rotation
  • Earth: 24 Hours per Rotation

The gas planet’s rotation is very different than the inner, terrestrial planets. The outer, gaseous and icy planets have a much faster rotation around their axes. This rotational speed actually causes the planets to become ever so slightly oblong in shape.

Gravity is hard at work keeping the planets spherical, but their rapid rotation causes material closer to the core of the planets to be flung outwards near the equatorial area of these planets.


Featured image by Kabsik Park, https://flic.kr/p/dUUvY

We strive to create amazing and unique content about all things related to Space and Astronomy.

Next Article10 Facts About Mercury