Notes on Gas Planets in our Solar System

Notes on Gas Planets in our Solar System

When we hear the term Gas Giants, our minds race directly to several planets located within our Solar System, Jupiter, Saturn, Uranus, and Neptune. These planets share several defining characteristics that allow them to be lumped together into the same category. In particular, it is the high percentage of helium and hydrogen within their over-all composition that places them apart from other more terrestrial solar bodies. However, the term Gas Giant is a bit misleading in that it tells us very little about the actual composition of these unique worlds.

Gas giants are composed of a high percentage of solid material. Due to the extreme pressure that exists within the core of a gas giant, hydrogen may be converted into a metallic solid or liquid form. Frequently, there are other materials interspersed within this solid matrix as well. Although all gas giants contain high amounts of hydrogen and helium within their overall composition, they may also contain heavier materials such as methane and ammonia. The differences in composition can tell us a great deal about the circumstances under which each planet formed.

Within our solar system, only Saturn and Jupiter are considered true gas giants. Neptune and Uranus are now categorized as ‘ice giants’ since their composition is mainly composed of heavier elements such as carbon, oxygen, sulfur, and nitrogen with hydrogen and helium making up only about 20% of their actual mass. During formation, scientists believe that Neptune and Uranus incorporated a majority of their material as gas trapped within water ice or as ice crystals of various other elements. They are much smaller in size than Jupiter and Saturn, and current research into their overall formation is less understood than their larger cousins.

Would Saturn Float if Placed In a Bathtub?

This is a common quote which has some truth to it but fails to encapsulate the actual reality of giant, gaseous planets. To truly understand the nature of “gaseous” planets, you are going to have to use your imagination. The gases we speak of actually do not exist in the gas state of matter that we are used to in our everyday life. The components of these planets are more of a gas/liquid mix leaning more towards a fluid consistency. This state of matter is referred too as a “crucial point.” A crucial point is a state in which gases and liquid phases are not indistinguishable.

So will Saturn float? The statement is odd for several reasons. But let’s forget about the scientific oddity of this state and address why this is a statement. The fact is that by the numbers, water, as we know it on Earth, is denser than Saturn. Density is the relationship between mass and volume. Buoyancy (an object’s ability to float) is the relationship in between the density of the fluid and object and the weight of both the fluid and objects. Simply put, on Earth, if an object’s density is less than the density of water, that object will float.

As with all science, it’s tough to squeeze all pertinent factors into our realm of existence… ask Flat Earthers. The problem is that physics as we experience it changes on the exponentially large scale of planets. Technically Saturn would float if it were a small spherical ball we play within a bathtub. However, water (hydrogen and oxygen) held down on a planetary-like surface significantly deeper more expansive than Saturn would cease to be water.

Why? Buoyancy is an equal downward force of gravity. Without doing the math, we can deduce the size, mass, and gravity of a ginormous vessel of water would have to be significantly larger than Saturn. The mass of this “vessel” or planet would be so great that its gravity would most likely strip hydrogen from the water molecules and kick start fusion, becoming a star. Furthermore, before this takes place the depth of such a massive ocean would be so pressurized, water would reach its critical point, transforming into something else, maybe a planetary mantle?

And still, planets do have large, dense metallic cores. The core of Saturn presumably wouldn’t float since it would most definitely be thicker than water. So there you go. Yes, Saturn is less dense than water here on Earth, but to say it would float in a bathtub is a little disingenuous since it’s essentially comparing apples to oranges.

Classification of Giant Planets

There are a few models of classifying gas giants. One model, the Sudarsky’s Gas Giant Classification categorizes gas giants based on their temperature, albedo, and distance from their star. The more straightforward classification of gas giants is the Planetary Habitability Catalog.

Sudarsky's Gas Giant Classification

David Sudarki and his team performed a theoretical categorization work on gas giants. There are five different classifications that ‘Gas Giant’ planets can fall into. Their work was published in 2000 and is still used for the categorization of newly discovered exoplanets. Gas giants are classified as Ammonia Clouds, Water Clouds, Cloudless, Alkali Metals, and Silicate Clouds. Within our solar system, Jupiter and Saturn are both classified as type Ammonia Clouds gas giants.

Class I: Ammonia Clouds

Ammonia clouds dominate planets in this class, and we usually found in the outer regions of a star system. They exist at temperatures less than about −120 °C or −190 °F. The temperatures for a Class I planets require either a cool star or a distant orbit. 47 Ursae Majoris c, 47 Ursae Majoris d, Upsilon Andromedae e, and 55 Cancri d are possible Class I planets.

Class II: Water Clouds

Planets in Class II Water Clouds class form clouds of condensed water vapor instead of ammonia clouds. Planets with temperatures below or around -23°C or -10°F usually display these characteristics. Even though the clouds on Class II planet would be similar to those of Earth, the atmosphere would still consist mostly of hydrogen and hydrogen-rich molecules such as methane. Examples of possible Class II planets are Gliese 876 b and c, Upsilon Andromedae d, and Kepler-90 h

Class III: Cloudless

Class III Cloudless planets have temperatures between about 170 °F or 80 °C and 980 °F or 530 °C. They do not form a global cloud cover because they lack specific chemicals in the atmosphere to form clouds. These planets would appear similar to Uranus and Neptune as azure-blue globes because of rayleigh scattering and absorption by methane in their atmospheres. Class III Cloudless planets exist in the inner regions of a star system, roughly as close to their stars as Mercury. Possible Class III Cloudless planets are Upsilon Andromedae c, Kepler-89e, and HD 205739 b.

Class IV: Alkali Metals

Class IV planets temperatures are above 900 K (627 °C; 1160 °F), at which point carbon monoxide becomes the dominant carbon-carrying molecule in their atmospheres instead of methane. Class IV and V planets are referred to as “Hot Jupiters.”

Class V: Silicate Clouds

Class V Giant planets are the hottest of gas giants, with temperatures above 1400 K (1100 °C; 2100 °F). Gas giants are likely to glow red from thermal radiation and reflected light. Examples of Class V: Silicate Cloud planets might include 51 Pegasi b and Upsilon Andromedae b.

Planet Habitability Laboratory Classification of Giant Planets

Jovian Giants

Jovian planets share a lot of similarities with Jupiter. Their size is usually six times the radius of Earth and beyond or 50 times the mass of Earth. These planets are generally dominated by hydrogen and helium with trace amounts of water, ammonia, and methane. Jupiter and Saturn are Jovian Giants that inhibit the Cold Zone.

Neptunian Giants

Neptunian giants are 10 – 50 times more massive than Earth and are 2.5 – 6 times larger than Earth in diameter. Neptune and Uranus are Neptunian planets within the Cold Zone. All Giant planets in our Solar System are composed of mostly hydrogen and helium. However, Neptune and Uranus can hold condensed methane in their cold atmospheres thereby resulting in their greenish and bluish hues.