Final answer:
A gas giant planet, also known as a Jovian planet, is a large planet predominantly composed of gases like hydrogen and helium, without a solid surface, and with a dense core. These planets are immensely larger than Earth and are defined by their significant mass and low density. Examples in our solar system include Jupiter, Saturn, Uranus, and Neptune.
Step-by-step explanation:
A gas giant planet, also known as a Jovian planet, is a large planet that is composed mainly of lighter materials such as ices, liquids, and gases, specifically hydrogen and helium. The best-known examples of gas giants in our solar system are Jupiter, Saturn, Uranus, and Neptune. These planets are named after 'Jove,' the Roman name for Zeus, which is also synonymous with Jupiter, the largest of these planets. Unlike the terrestrial planets, such as Earth, that have solid rocky surfaces, gas giant planets have no well-defined solid surface and are characterized by their vast size and low density. They do possess smaller, denser cores beneath their thick atmospheres, which consist primarily of hydrogen and helium. Jupiter, the largest gas giant, is so massive that about 1,300 Earths could fit inside it.
The composition and structure of gas giants suggest that they formed under different conditions than the rocky terrestrial planets. Observations and explorations have shown that while Jupiter and Saturn are mostly made up of hydrogen and helium, like the Sun, their immense pressure compresses these gases into states such as metallic hydrogen, which isn't found on the terrestrial planets. These planets also give off more heat than they receive from the Sun due to processes such as gravitational contraction.
When astronomers explore gas giant planets in other solar systems, understanding their mass and composition can help determine whether they are similar to the gas giants in our own system. For instance, by measuring the density of a planet, such as in the case of the planet HD 209458, it can be concluded if a planet is a gas giant or not based on its radius and the resulting density.