Gas giant

Artist's impression of a gas giant planet forming in the disc around the young star HD 100546

Artist impression of ultra fluffy gas giant planet orbiting a red dwarf star

Gas giants are a class of planets that are primarily composed of hydrogen and helium. Unlike terrestrial planets, which have solid surfaces, gas giants do not have a well-defined surface; their outer layers transition smoothly from the gaseous atmosphere into deeper liquid or metallic phases due to the high pressures within. The most well-known gas giants in our Solar System are Jupiter and Saturn, with Uranus and Neptune often being classified as ice giants due to their higher composition of volatiles compared to hydrogen and helium.

Characteristics

Gas giants are characterized by their large masses and low densities. These planets have thick atmospheres composed mainly of hydrogen and helium, with traces of other volatiles like water, methane, and ammonia. The immense pressure in their deep atmospheres may lead to the formation of exotic materials, such as metallic hydrogen in the case of Jupiter and Saturn. Gas giants also typically have extensive ring systems and numerous moons.

Atmosphere and Weather

The atmospheres of gas giants are sites of complex weather phenomena. Due to their rapid rotation, these planets exhibit strong banding patterns, with alternating wind zones and storms. The Great Red Spot on Jupiter is a notable example of the massive storms that can occur on gas giants. These storms are driven by the planets' internal heat, which also contributes to their banded appearance.

Internal Structure

Despite their name, gas giants are believed to possess solid cores beneath their thick atmospheres. These cores are likely composed of rock and metal and are surrounded by layers of metallic hydrogen and helium. The transition between these layers is gradual, with no distinct boundary.

Formation and Evolution

Gas giants are thought to form in the outer regions of star systems, where temperatures are low enough for volatile compounds to condense. The leading theory of their formation involves the accretion of a solid core, which then accumulates a massive atmosphere from the surrounding protoplanetary disk. Over time, gas giants may migrate closer to or further from their host star, affecting their composition and the architecture of their planetary system.

Exploration

Humanity has sent several spacecraft to explore the gas giants of the Solar System. Notable missions include the Pioneer and Voyager flybys, the Galileo orbiter around Jupiter, and the Cassini mission to Saturn. These missions have provided invaluable data on the atmospheres, rings, and moons of these planets.

Gas Giants Beyond the Solar System

With the advent of exoplanet discovery techniques, many gas giants have been detected orbiting other stars. These exoplanets range in size from being comparable to Neptune to several times the mass of Jupiter. Their discovery has challenged existing theories of planetary formation and migration, highlighting the diversity of planetary systems in the universe.