Scientists estimate there are ~1 septillion (1 followed by 24 zeros) planets in the observable universe. This figure is based on current research, which constantly expands with new discoveries. These celestial bodies are found across billions of galaxies, with most orbiting their own star systems, while some are also classified as rogue planets – interstellar objects of planetary mass which are not gravitationally bound to any star.
The Solar System is dominated by two planets – Jupiter and Saturn – whose imposing appearance has captivated stargazers ever since they could be viewed from Earth. If even dwarf planet Pluto was counted, Jupiter would literally find itself in the centre of all the planets put together, and accordingly, it holds much significance in the workings of the Solar System. A singular important similarity between Jupiter and Saturn is their planetary categorisation – Gas giants. Gas giants are a distinct class of planets in the Solar System and beyond, characterised by their massive size, thick atmospheres, and the lack of a solid surface. These planets, primarily composed of hydrogen, helium, and other gases, stand in stark contrast to rocky planets like Mercury, Venus, Earth and Mars, and are some of the most fascinating objects in the universe, offering scientists a wealth of knowledge about planetary formation, atmospheric science, and the possibility of life in extreme conditions.
Sometimes referred to as “giant planets”, these large planets primarily consist of hydrogen and helium, with small amounts of other elements and compounds. They have a massive outer atmosphere, but lack a well-defined solid surface. While the term “gas giant” originally applied only to Jupiter and Saturn, it is now often used to refer to any large planet that is predominantly composed of gas.
In definition, both Jupiter and Saturn are enormous in size, with the former being the largest planet in the Solar System. While not technically classified as gas giants, Uranus and Neptune are also sometimes referred to as “ice giants” due to their composition, which contains a higher proportion of heavier elements like water, ammonia, and methane. Despite these differences, they share many characteristics with gas giants, especially in their massive atmospheres.
Composition and Structure
Gas giants can be significantly larger than other types of planets, including rocky ones like Earth, due to their ability to retain large amounts of gas.
As mentioned above, gas giants are predominantly composed of hydrogen and helium, the two lightest elements in the Periodic Table, with traces of other gases such as methane, ammonia, and water vapour. These elements form the bulk of their atmospheres, contributing to their low densities, compared to rocky planets. The lower densities of gas giants are due to the fact that the materials they are made of are much lighter than rock or metal.
Their atmospheric pressure increases dramatically with depth, and the lower layers are often composed of a mixture of gases and liquids, creating complex and turbulent weather patterns. Beneath the thick atmosphere, gas giants may have a small, dense core made up of heavier elements like iron, nickel, and rock. This core is thought to be surrounded by layers of liquid hydrogen and helium, with deeper layers of metallic hydrogen, a form of hydrogen that behaves like a metal due to the intense pressure found inside these planets. While the core is small compared to the overall size of the planet, it plays an essential role in generating the planet’s magnetic field.
Gas giants are often surrounded by a complex system of rings and moons. Saturn is famous for its extensive and visually striking ring system, which is primarily made of ice and rock particles. Gas giants’ numerous moons may be quite large and geologically active. Jupiter’s moon Europa has a subsurface ocean, while Saturn’s moon Titan has a thick nitrogen atmosphere and lakes of liquid methane.
How were gas giants formed?
The formation of gas giants is a complex process that is not entirely understood, but is generally believed to follow a two-stage process:
A. Core Accretion
The first stage involves the formation of a solid core. This process begins with the accumulation of ice, rock, and other materials in the protoplanetary disk around a young star. Over time, these particles collide and stick together, gradually building up a large solid core. Once the core reaches a certain size, it has enough gravitational pull to begin attracting gas from the surrounding disk
B. Gas Accretion
After the solid core is formed, the planet begins to capture large amounts of hydrogen and helium from the surrounding gas. This gas accumulation can happen relatively quickly, and the planet can grow to a massive size in a short period. The gas giant’s core plays an essential role in this process, as it provides the gravitational pull needed to capture the surrounding gas
Gas giants play an important role in shaping the dynamics of the Solar System. They are often considered “vacuum cleaners”, because their massive gravitational fields can attract and capture comets, asteroids, and other debris, preventing many of these objects from colliding with inner planets like Earth. Their gravity also helps to influence the orbits of other planets and objects in the Solar System.
The gravity of gas giants, particularly Jupiter, has a significant impact on the trajectories of other bodies in the Solar System. Jupiter’s strong gravitational field is believed to have deflected potential cometary impacts that could have otherwise posed a threat to Earth. This has led some scientists to refer to Jupiter as the “Protector of Earth”. The gas giants’ gravitational forces also help stabilise the orbits of the other planets, especially in the outer regions of the Solar System.
Gas giants are also believed to have played a crucial role in the formation of the Solar System. They may have formed in the early Solar System from the remnants of the solar nebula, a cloud of gas and dust that surrounded the young Sun. Their enormous size allowed them to attract vast amounts of gas, leading to the formation of their thick atmospheres. The gas giants’ presence may have also affected the distribution of material in the early Solar System, influencing the formation of the inner rocky planets and the asteroid belt.
Exploration beyond realms
Gas giants have been the subject of intense scientific study, especially since the advent of space exploration. The exploration of these planets has provided valuable insights into the nature of planetary atmospheres, weather patterns, magnetic fields, and planetary formation. The two gas giants of our solar system have been extensively studied through space missions like NASA’s Galileo spacecraft (which orbited Jupiter from 1995 to 2003) and the Cassini spacecraft (which orbited Saturn from 2004 to 2017). These missions revealed important information about the planets’ atmospheres, ring systems, and moons. The discovery of active volcanoes on Jupiter’s moon Io and the subsurface oceans on Europa, as well as the detailed study of Saturn’s moon Titan, has significantly expanded our understanding of the potential for life in extreme environments.
Meanwhile, the study of gas giants is not limited to our Solar System. Advances in telescopy have allowed astronomers to detect exoplanets – planets orbiting stars beyond Pluto. Many of the exoplanets discovered so far are gas giants, and the study of these distant worlds is providing new insights into planetary formation and the diversity of planetary systems in the Milky Way. Some exoplanets, known as “Hot Jupiters”, are gas giants that orbit very close to their parent stars, leading to extreme temperatures and interesting atmospheric phenomena.