Is Uranus Considered a Gas Giant or an Ice Giant?

The planets in our solar system are categorized as: Terrestrial, Gas Giants, or Ice Giants. Uranus, the seventh planet from the sun, is an Ice Giant.

Terrestrial vs Gas Giants vs Ice Giants

The four planets closest to the sun are terrestrial. The four planets furthest from the sun are either gas or ice giants. 

Planet sizes
Image Credit Wikimedia

The four terrestrial planets (Mercury, Venus, Earth, and Mars) are the smallest planets in the image above. The Gas Giants (Jupiter and Saturn) are the largest planets.   The Ice Giants are the blue plants in the image above. 

Planet NameOrder from SunPlanet typeComposition
Jupiter5OuterGas Giant
Saturn6OuterGas Giant
Uranus7OuterIce Giant
Neptune8OuterIce Giant

What’s the difference between the Gas Giants, Terrestrial, and Ice Giants types of planets?

Terrestrial planets

Mercury, Venus, Earth, and Mars, the four planets closest to the sun, are terrestrial planets.  

  • A terrestrial planet has a solid, rocky, compacted surface like Earth.  
  • Doesn’t have rings
  • Few moons
  • A dense core that’s filled with heavy molten metals.
  • Valleys
  • Volcanos
  • Craters

Gas Giant planets

Jupiter and Saturn are Gas Giants. Uranus and Neptune might also be classified as Gas Giants depending on the reference material you’re reading.

  • They are primarily composed of helium and hydrogen.
  • A dense hydrogen core
  • Helium gases that twist and twirl above a solid hydrogen core
  • The core is smaller than the gasses that swirl around it.

Ice Giant planets

Uranus and Neptune are classified as Ice Giants

  • Primarily composed of helium and hydrogen, similar to a Gas Giant, but also contain:
    • Oxygen, nitrogen, sulfur, and carbon. 
  • The additional elements combine into compounds:
    • Methane-This makes the planet appear blue.
    • Water
    • Ice
  • The planet has a solid mantle that’s composed of ammonia and compressed partially frozen water. (Think about partially melted snow when the weather starts warming up. Not quite frozen, not entirely melted.)  
  • The core is proportionally more significant than the gas that surrounds it.  

Uranus discovery

William Herschel discovered Uranus in 1781. Herschel used a telescope that he designed and manufactured himself. Herschel thought that he’d found a star. Two years later, Johann Elbert Bode’s observations confirmed that the discovery was a planet, not a star. 

Herschel wanted to name the star Georgium Sidus after King George III. Bode’s recommended the name Uranus. In Greek Mythology, Uranus is the God of the Skies. 

Ice Giant?

Until the 1990s, Uranus was classified as a Gas Giant. Data collected and analyzed indicated that Uranus and Neptune had different periodic elements than have been detected on Saturn or Jupiter.  Their classification was changed from Gas Giant’s to Ice Giants. 

Fun facts about Uranus

Uranus is unlike any other planet in our solar system.   

  1. Rotates on its side
Image Credit: Lawrence Sromovsky/UW
  1. Dark and narrow inner rings
  2. Brightly colored outer rings
  3. Rotates in the opposite direction of Earth and most other planets
  4. It takes 84 years for Uranus to orbit the sun one time
  5. Thirteen rings
  6. Twenty-seven moons

How do gas giants form?

Data collected from the Spitzer Space Telescope in 2007 showed that gas giants form rapidly. Scientists estimate that a gas giant may develop within the first ten million years of the formation of a nearby star. 

Young stars may have a gas-rich debris field that surrounds them. A debris field is created from comets and asteroids colliding in that gas-rich environment. When the debris field is large enough, the debris coalesces. The debris field has sufficient gravity to attract gas and form a planetary core. 

Core accretion Versus disk instability

We need to learn a great deal of information about how planets form. The two primary planet formation methods are Core Accretion and Disk Instability. 

Scientists are not in agreement about how Uranus formed. Some scientists lean towards the Core Accretion theory. Conversely, other scientists believe that the Disk Instability theory is the most accurate planetary development model. 

One theoretical model only fits some planets. While some planets’ formation aligns accurately with the Core Accretion theory, others fall into the Disk Instability bucket. Is more than one formation model possible inside a single solar system? Scientists don’t know.

Core Accretion

Small centimeter pebbles of material collide and stick together. The clumps become larger and larger as more pebbles stick to them. These objects are called Planetesimals.  

If the Planetesimals are “growing” close to the star, heavier, metallic elements are abundant. The heavier metallic elements and the sun’s heat allow terrestrial planets to form. 

Further away from the sun, past the snow line, the planets form at cooler temperatures than planets form nearer a star. The cooler temperatures allow the hydrogen and helium to condense. The condensed elements begin to create a larger planet. As the planet’s size increases, its gravitation pull attracts nearby lighter elements, like hydrogen and helium.  

What’s a snow line? The snow line is the location in a solar system where temperatures are low enough that elements like hydrogen, helium, ammonia, methane, and carbon freeze and stick together. The further away an object is from the sun, the lower the temperature.  

Disk Instability

A gigantic cloud (called a disk) composed of gas and dust becomes so large that its gravity breaks a portion of the disk “off.” The broken-off part of the disk becomes a planet. 

The portions of the massive disks that were broken off become planet-sized clumps of material.  

Why don’t we know more about Uranus?

Uranus generates a different kind of scientific enthusiasm than other planets closer to Earth generate. There has yet to be a single dedicated exploration mission of Uranus by any space agency.  

The lack of exploration may be due to several factors: 

  • Time.
    • Voyager 1 took approximately nine and a half years to reach Uranus.  
    • Longer mission times require greater dedicated career times from NASA/ESA/JPL engineers. In an ideal situation, an engineer could work on the project from inception to conclusion, a thirty-year commitment. An ENTIRE career commitment. 
  • Surface conditions: Uranus is frozen. Literally. Temperatures drop down to -373℉.  
  • Cost: It’s easier to “sell” a costly space exploration to a very skeptical public when finding current or previous life forms is possible. 
  • Other space exploration: The scientific community votes on high-priority space exploration projects. Exploration with more votes receives higher priority. 

Learning more about the ice-giant

NASA plans a flagship-class mission to Uranus with a lift-off in 2031 or 2032. The approximate flight time is thirteen years. The mission will focus on the evolution of the ice giant, its habitability, and if ice giants like it are unusual in the universe. 

Voyager Mission

Voyager 2 flew past Uranus in 1986 on its way out of our solar system. The Voyager Mission passed approximately 51,000 miles from Uranus.  

Data collected from Voyager and the Hubble Space Telescope have allowed planetary scientists to reevaluate Uranus. Once considered a gigantic gas giant, scientists know there are weather events today on Uranus’ surface. 

Clouds travel over Uranus at over 300 miles per hour. Planetary scientists discovered a storm, larger than the size of America, traveling around the globe.  

Let’s wrap it up!

Uranus was considered a gas-giant planet until the mid-1990s. Today Uranus is classified as an ice-giant planet. A NASA flagship mission for Uranus will launch in 2031.