This post is going to cover the middle gassy planets of our solar system, and what makes them different to other planets.
If you want, jump Straight to:
What is a Planet?
To this day, the definition of planet is up in the air. A planet is widely accepted as something that orbits a star, has a large gravitational force (but is still less than a star’s), and has a clear orbit path. Our knowledge of planets is still growing, and so our understanding of what a planet is will change as well.
What Kind of Planets Are There?
Just like stars, planets come in many types, our solar system being very rich in planet diversity. Planets are classified into four groups: terrestrial planets, gas giants, ice giants and dwarf planets. From that they can be classified even further. Think about which class you would place each planet in our solar system.
In the middle of our solar system are the gas giants. These are planets much larger than Earth, mostly made up of hydrogen and helium, which is why they’re called gas giants.
Yet, gas planets are unique and have varying characteristics, so there needs to be a classification system that works for all. Thus, I present to you Sudarsky classification!
Class I: Ammonia Clouds
These planets have an ammonia-filled outer layer. Planets in this class are fairly far away from their star and beyond the frost line (the point where ammonia and methane are in liquid/ ice form).
Jupiter and Saturn fit right into this Category.
Class II: Water Clouds
Class II planets are close to the star they orbit and their temperature is high enough for lots of water to exist as a gas (water vapour). The water vapour makes large clouds that dominate the atmosphere.
We don’t have any of these in our solar system.
Class III: Cloudless
Cloudless giants are hotter than water cloud giants. The chemical compounds in this planet can’t condense and form clouds since the temperature is too high, and there usually isn’t a sufficient amount of the right chemicals anyway.
There aren’t any planets like this in our solar system.
Class IV: Alkali Metals
Even hotter than class III are the alkali metal giants. Methane played a large part in the atmosphere of the planets before, but in class IV carbon monoxide dominates. As you may have guessed from the title, there’s a large amount of alkali metals in their atmosphere, such as potassium and sodium.
Class V: Silicate Clouds
Silicate cloud giants are the hottest of all classes, with atmospheres filled with silicate clouds and iron vapour. These may glow red as they release thermal energy.
Of course, this system is not complete and needs more tweaking as we discover more exoplanets. But, it’s fairly good for our current level of knowledge.
Gas giants in class IV and V are also called Hot Jupiters (since they’re similar to Jupiter, but hotter).
If there are hot Jupiters, are there cold Jupiters? Can we label planets as Saturns or Neptunes too?
Astronomers like using Earth, Jupiter and Neptune as descriptors. Hot Neptunes (rare) are given to planets close to their star with a similar size to Neptune, and hot Earths (even rarer still) are Earth sized.
I couldn’t find a case of a cold Jupiter or Earth. But Neptune is already a cold planet, so a cold Neptune would probably just be … a Neptune?
But what about size?
There are many cases of Super Jupiters, Earths and Neptunes, where these are over twice as large. In the case of Jupiter, however, the Super Jupiter is 2-13 x as large, since anything beyond that has enough gravitational force to become a brown dwarf star.
On the other side, there are lots of Mini Neptunes (which could also be called Super Earths), Sub Jupiters and Sub Earths!
Gas Giants in Our Solar System
- Average radius: 69’200 km
- Semi-major axis: 779 million km
- Eccentricity: 0.048
- Orbit: 11.8 Earth years
- Sidereal day: 9 hours 50 minutes
- Surface temperature: -145°C
- Gravitational constant: 24.8 m/s²
- Tilt: 3.13°
- Moons: 79 (and still counting)
The largest of all planets in our neighbourhood is Jupiter. Jupiter’s radius is 71,500 km at the equator, but is slightly squished at the poles where the radius is 66,900 km. By volume, Jupiter is roughly 1,300 x bigger than Earth. That’s massive!
It is 779 million kilometres away from the Sun, so takes much longer to make one full orbit than the terrestrial planets (11.8 Earth years). On the contrary to such a long Jovian year, Jupiter rotates very quickly, completing a rotation (one day) in only 9 hours and 50-56 minutes.
Unfortunately, we don’t know as much about gas giants as terrestrial planets, so some of this is theory, not proven.
We expect a dense metallic hydrogen core and helium and other rocky materials, although there is a fairly high chance it’s contents have been carried away from the centre, meaning there is no more core.
Most of Jupiter is metallic hydrogen with small amounts of neon and helium in the lower regions. In the upper layers it is hypothesised that there is an abundance of carbon, specifically diamond crystals.
Above this is the atmosphere, made up of hydrogen. The pressure and temperature in the atmosphere are high enough that there are no distinct liquid and gas phase changes.
The Great Red Spot
The Great Red Spot has been a massive interest for centuries. The Great Red Spot is an anticyclonic storm rotating anti-clockwise. It’s 16,350 km wide, making it 1.3 x larger than Earth’s diameter!
The GRS has been observed since 1830, but the storm has most likely existed before this, quite possibly from 1665. To this day, it’s very much unknown how this storm was created, but thanks to recent observations we are building up more knowledge of the GRS.
Looking at Jupiter Through a Telescope
Jupiter’s atmosphere is incredibly large, and full of interesting sights!
It’s atmosphere is made up of zones and belts. These clouds are made up of ammonia compounds which are neatly arranged into its well-known stripes (normally called bands). The lighter stripes are called zones, and the darker are named as belts. There are many storms within them and on their boundaries, but the most famous is the Great Red Spot.
Definitely spend a night observing Jupiter. With even just a regular telescope, you’ll be able to see the zones, bands, and even the Great Red Spot! With a large telescope, you may also see some smaller storms in the atmosphere!
- Radius: 57,350 km
- Semi-major axis: 1,434 million km
- Eccentricity: 0.054
- Orbit: 29.5 Earth years
- Sidereal day: 10 hours 30 minutes
- Surface temperature: -178°C
- Gravitational constant: 10.44 m/s²
- Tilt: 26.73°
- Moons: 62
Our second gas giant is Saturn. It is the second largest planet with a radius of 60,300km at the equator and 54,400 km at the poles, making it 764 x larger than Earth, not quite Jupiter sized but enormous still.
Saturn’s distance to the sun is 1,434 million km, meaning one Cronian year is 29.5 Earth years! Similarly to Jupiter, Saturn rotates quicker than the terrestrial planets, taking only 10 hours and a half to complete a day.
Just like Jupiter, Saturn is mostly made up of hydrogen and helium, but not much is known about all of its contents.
It is suggested that Saturn has a rocky and icy core surrounded by metallic hydrogen and helium. The pressure and temperature within the core is very high. These make up a fairly large part of Saturn’s volume.
The pressure and temperature decreases with distance, yet is still high enough for Saturn’s gases are able to exist as liquids. Above the core layers lies a large layer of liquid hydrogen, and “helium rain”. Although the evidence is small, there may be a chance of liquid helium within this layer. The helium exists as droplets within the hydrogen liquid, not mixing similarly to oil droplets in water.
After the liquid layer lies an atmosphere of hydrogen and helium gas and clouds. These create thin stripes visible with a powerful telescope. Like Jupiter, within the atmosphere are storms and extremely fast winds creating jet streams reaching 5 x the speed of winds in Earth.
Saturn’s Hexagonal Jet Stream
After being observed by Voyager I and the Cassini Orbiter spacecraft, astronomers have discovered a remarkable “hexagon” jet stream at the North Pole of Saturn.
This six-sided jet stream is 30’000km wide, an enormous 2.4 x the Earth’s diameter! The speed of the stream reaches about 300 km/h. This Cronian storm is unlike any other in the solar system. Take a look at the magnificent images by Cassini.
Arguably the most well-known feature of Saturn is it’s wondrous Rings. The entire satellite is split into small rings, going from D to A then G to E.
The majority of these rings are made up of ice chunks, ranging from a few micrometers to a couple of kilometres in diameter! Although the origins of the rings are not well known, it’s likely that the ice came from comets orbiting the sun. A very small fraction of the rings are also composed of rocky material, most likely from debris.
Among these rings are large gaps, such as the Colombo gap in the C ring, and the Encke gap in the A ring. Some of these are caused by moons. In the image on the right there is a thin gap between the large grey area and the small but very bright outer strip. This is the Keeler gap, and snugly within this gap lies the moon Daphnis.
Although this moon is small, it still has a gravitational force, and also an effect on its surroundings. As Daphnis journeys through the Keeler gap it tugs on the particles, creating small ripples like those on a pond. Daphnis is too small to actually pull the ice and rock along with it.
Looking at Saturn Through a Telescope
As the planets get further away from Earth, details on them become difficult to see with a regular telescope. Looking at Saturn, the rings are visible and, of course, Saturn itself.
The last time I looked at Saturn, it was with a f/11 scope with high magnification. With that telescope, I could see the large gaps within Saturn’s rings, and even some details of thin stripes on Saturn too. Overall, the planet looked a very light yellow-brown colour. I expect with a regular small telescope, Saturn will appear as a small dot with parts of the rings visible, but not very much detail.
What Makes Gas Giants Unique?
Initially, planets were just clumps of debris. By gravitational force, the debris clumped together into larger chunks. This repeated more and more as the planets grew bigger, because as the total mass increased, so did its gravitational pull.
This is how we expect a planet’s core is formed.
Unlike the terrestrials, these two gas giants had the advantage of having an abundance of hydrogen and helium to pull in. As they did, the mass and gravity of the planets increased rapidly, allowing them to pull in more and more gas. Eventually the hydrogen became the dominant substance within the planet, followed by helium, giving Jupiter and Saturn the title gas giant.
As the planets finished forming, what we end up with are giant planets with high amounts of hydrogen and helium gas.
This also explains why Jupiter and Saturn rotate faster than the other planets
Jupiter and Saturn’s days are small since their rotational velocities are larger than the terrestrial planets and ice giants, but why?
The explanation can be found within conservation of angular momentum. You may know linear momentum and the equation p = mv, or the fact that momentum is always conserved. Angular momentum also always obeys this rule.
Angular momentum can be defined by this equation (don’t worry, no need to know it in detail):
L = I ω
Momentum = moment of inertia x angular velocity
Let’s apply this equation to a forming planet. The moment of inertia (I) would be the distance some gas that’s being pulled in is from the planets center. The angular velocity (ω) is how fast the planet’s spinning.
As the gas gets closer, the moment of inertia decreases. But, if the angular momentum (L) needs to stay the same, then the velocity must increase to balance everything out (i.e the planet needs to spin faster).
The same concept can be applied ballet dancers pulling their arms and leg in when spinning to rotate faster, or playground spinning wheels.
That will be it for gas giants. Next up, Laufey, Loki, and the Jötunn!
Oh wait, those are frost giants, not ice giants.
Ice giants will be next.