INTRODUCTION
Our home world the Earth is a terrestrial planet. The terrestrial planets are the four smallest and innermost planets of the solar system. They are small and rocky and for the most part pretty boring. Further out beyond the asteroid belt four more planets orbit the sun, Gas Giant Planets! Massive continuous chemical reactions of extreme weather which drives a range of fascinating phenomena, including the formation of majestic patterns across their upper atmospheres and rich subsystems of moons and rings. In our solar system today, there are four of these gas giant planets. Jupiter the oldest and largest of the planets, Saturn home to a magnificent ring system, Uranus a cold atypical world, and lastly Neptune the most distant of all. But before we take an up-close look at these worlds what do we actually mean by the term gas giant?
WHAT ARE GAS GIANTS?
The term gas giant was originally coined by science fiction author James Blish in 1952 in reference to all the large planets in the solar system. We didn’t really know much about our own gas giant planets let alone extrasolar planets back then and so in the years since the meaning has been refined somewhat. A gas giant is a comparably large planetary mass celestial body with a rocky or molten core, but topped with thick layers of hydrogen and helium in different states, the same elements which are present in young stars which is why some gas giants are known as failed stars. The gas part of the term refers to the hydrogen and helium within the planet, but it is actually kind of inaccurate because the vast majority of a gas giants planet matter is not in a gaseous state. Instead, the pressure below the outer atmosphere is so intense that most of the matter has reached what is known as its critical point, the point for a substance at which the distinction between a liquid and a gas disappears.
These huge pressurized planets tend to be at least two or three times bigger than the Earth and dozens of times more massive. They can also grow to be much larger but it’s worth noting that size doesn’t correlate with mass in this case. Heavier gas giants which exert a stronger gravitational influence will keep themselves pulled together more tightly and compactly making them smaller than cooler lighter gas giants. In any case they have vast upper atmospheres which drive extreme weather which marks the surface of each planet with bands spots and other fascinating features. But then why have these planets been able to grow so large and become so different if their cores are made of the same stuff as the inner planets? Well, the answer to this has a lot to do with the planet’s starting position. Gas giants begin life as large rocky planetary cores which grow larger and larger through the accumulation of mass as they move around their system, kind of similar to the way in which the terrestrial planets formed. The difference however is that these planets formed beyond the snow line of the solar system. The boundary beyond which it is so cold that water and other volatile substances can exist as a solid which is as hard as rock, and this provides loads of extra material for planetary formation. As such these cores sweep up lots of matter and become massive in every sense so much so that their gravity begins to attract hydrogen and helium gas, even more so if the star system is young where hydrogen and helium clouds are abundant. This accumulation of gas begins to form an envelope layer around the planet which becomes so heavy and pressurized, that the rocky surface of the planet is transformed. Turning the whole thing into a massive ball of liquefied gases in various forms under unimaginable pressures. Gas giants like Jupiter and Saturn are thought to consist mostly of hydrogen and helium with heavier elements making up only about 10 %. These heavier elements may be contained at the core but are under such high and intense pressures that their properties at this depth are likely to be poorly understood.
WHAT ARE ICE GIANTS?
In contrast the two other giants in the solar system Uranus and Neptune are quite different in size and composition to their older gas giant brothers and so warrant their own classification the ice giants. This far away from the parent star heavier volatile substances like methane, carbon dioxide and ammonia can exist in a solid form referred to as ices. Uranus and Neptune are composed primarily of these volatile substances with the majority of the mass being contained within a thick water-rich mantle layer of ammonia and methane. The methane at the surface is what drives the aesthetic blue coloring in both planets. Ice giants tend to be smaller than gas giants but their mass is still sufficient to create diamond rain, diamond icebergs and many other incredible phenomena below their surfaces. And whether it’s a gas or an ice giant both are massive enough to attract moons and plenty of them.
JUPITER
Hydrogen and helium gas is prevalent across space in young star systems which along with the multitudes of rocky material, makes them perfect environments for gas giants to spring up almost as soon as the system’s star begins to shine. The story was no different in our solar system four and a half billion years ago. Jupiter was the first planet to form and it remains the oldest and by far the largest planet in the solar system. After forming it spiraled inwards towards the sun accumulating most of the rocky material in its path allowing it to become far larger and heavier with less left over for the terrestrial planets. One of the reasons mars is quite small compared to the Earth and Venus. We’re still trying to understand this gargantuan planet’s structure. One hypothesis is that its core may not be rocky but a very dense liquid center surrounded by layers of hydrogen and helium. Its gaseous upper atmosphere encases a layer of liquid hydrogen below, but further down the gravity and pressure turns this into metallic hydrogen. Metallic in the sense that it starts behaving as a superconductor this critical point metallic ocean likely extends most of the way down towards the center giving Jupiter a substantial magnetic field. Intense reactions further down power violent weather in the planet’s five-hundred-kilometer thick, upper atmosphere we can see swells and bands in brilliant detail and aurora much more powerful than anything on earth. But the most iconic feature on Jupiter is its great red spot a storm three times larger than the earth, which has been raging for hundreds of years. Wind speeds within this storm can reach up to 600 kilometers per hour much more powerful than any wind current on earth. We exist at a good time to observe this spot on, Jupiter but they can occur on the other giant planets too including on the other gas giants in the solar system Saturn.
SATURN
Saturn was the next planet to form after Jupiter, and while similar in some respects is different in many others. Saturn is thought to be composed of a dense iron and nickel core surrounded by rocky material, which has been enveloped in liquid hydrogen like jupiter but smaller and much lighter. Ammonia crystals in the outer atmosphere give it its rich yellow appearance. Despite being the second largest planet in the solar system it is also the least dense planet and the only planet less dense than water. Whereas Jupiter has a colourful cascading tapestry of winds on its surface Saturn appears to be slightly more isotropic. Like Jupiter Saturn formed sufficiently far away from the sun beyond the snow line. When Saturn was young it collected rocky and icy material until it was a planetary body about 20 times more massive than the earth, and when it reached this mass, it began pulling in leftover hydrogen and helium. Trillions of tons of gas enveloped the planet transforming the surface under pressure 10 million times stronger than on earth. Today the planet is mostly hydrogen and helium in a variety of forms powering phenomena such as liquid diamonds and helium rain further down. Like Jupiter this weather has grouped into impressive systems most notably its beautiful hexagonal structure at its north pole. but the most striking feature of Saturn is not on the planet itself but, the brilliant ring system surrounding it all four of the gas giants in the solar system have a ring system but they are much thinner darker and fainter and so we didn’t know they existed for a long time. Saturn’s rings on the other hand are easy to spot visible from earth in great detail and so we noticed them as soon as we found the planet. They extend about three hundred thousand kilometers across but are only about nine meters thick on average. The fact that they are so dazzling and pristine compared to the other giants tells us a lot about how they formed and when. Saturn itself is billions of years old but in comparison the rings are only about 100 million years old. If they were any older, they would appear dustier. this probably means that about 100 million years ago one of Saturn’s icy moons got too close to the planet and was shredded up into trillions of pieces of icy material which fell into orbit along Saturn’s orbital plane. This created the awesome rings the essence of a dead icy moon reborn as one of the most bedazzling sights in the solar system.
URANUS
Further out than Saturn lie the most distant and coldest worlds the ice giants, Uranus is one such giant while not as massive as the gas giants is four times the diameter of the earth. Meaning you could fit earth inside more than 60 times over. It is thought to have a core made of magnesium silica, the remaining 80 of the planet’s is mostly contained inside a layer of water, ammonia and methane; and the methane on the surface is what gives it its blue appearance. Unlike Saturn and Jupiter Uranus’s surface is particularly blank and featureless. Its surface covering obstructs the processes below meaning the planet emits barely any heat back out into space, giving it the coldest surface temperatures in the solar system of -224°C. Uranus orbits the sun 20 times further away than the earth, and is the only planet to do so on its side. Planets usually orbit along the same plane and in the same direction, because of the direction and motion of the collapsing cloud which formed the star and its system. But unlike the other planets the Uranus system including its faint rings and dozens of satellites all seem to orbit on their side. Scientists think that this orientation is due to a collision with another planetary mass object the impact of which tipped the entire system over on its orbital plane. Thus, its 13 faint rings appear vertical over the planet these rings are kept in place by Uranus’s 28 known moons specifically its shepherd moons Cordelia and Ophelia. Small satellites which orbit either side of the ring system exerting counterbalancing gravitational influences that keep the rings mata in place.
NEPTUNE
Finally, we come to the most distant giant of them all Neptune. It is similar in size to Uranus and despite being feathered from the sun it is a much more volatile world. Neptune has the most extreme weather found anywhere in the solar system and great cyclones play out on its surface including spot-like storms similar to the ones on Jupiter, but with much more aggressive wind speeds. Neptune records the highest wind speeds of anywhere in the solar system. With no land mass or surface to slow down currents, winds can climb to over 2,000 kmph. Out here it is so cold that nitrogen freezes on the surface of its moons but Neptune’s interior is anything but cold. its outer atmosphere consists of frozen methane crystals mixed with hydrogen and helium becoming much more pressurized the deeper into the planet you go. Deep down the interior temperature climbs to thousands of degrees creating a liquid ocean two thousand times deeper than anything on earth. While we expect similar processes to be going on within the two ice giant’s interiors Neptune emits more heat back out into space than Uranus, giving it a slightly higher surface temperature despite being further from the sun and we aren’t actually sure what the source of this additional heat is. One of the many mysteries cloaking this fascinating world. So, those are our four gas giants they are huge and inhospitable each with their own characteristics and special properties but with all that considered what could lie beyond our solar system elsewhere in the galaxy.