A giant planet is a diverse type of planet much larger than Earth. It is sometimes referred to as a jovian planet, with Jove being another name for the Roman god Jupiter. Giant planets are usually primarily composed of low-boiling point materials (volatiles), rather than rock or other solid matter, but mega-Earths do also exist. There are four such giant planets in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar giant planets have been identified.

Giant planets are sometimes known as gas giants, but many astronomers now apply the term only to Jupiter and Saturn, classifying Uranus and Neptune, which have different compositions, as ice giants. Both names are potentially misleading; the Solar System's giant planets all consist primarily of fluids above their critical points, where distinct gas and liquid phases do not exist. Jupiter and Saturn are principally made of hydrogen and helium, whilst Uranus and Neptune consist of water, ammonia, and methane.

The defining differences between a very low-mass brown dwarf and a massive gas giant () are debated. One school of thought is based on planetary formation; the other, on the physics of the interior of planets. Part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history. This core may be partially or completely dissolved and dispersed throughout the hydrogen/helium envelope. Gas giants are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable molten core with a rocky composition.

Jupiter and Saturn's outermost portion of the hydrogen atmosphere has many layers of visible clouds that are mostly composed of water and ammonia. The layer of metallic hydrogen makes up the bulk of each planet, and is referred to as "metallic" because the very high pressure turns hydrogen into an electrical conductor. The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood. though it remained an informal category until a quantitative definition was proposed for it in 2026. However, per definitions, they would be instead considered as very low-mass white dwarfs, rather than high-density diamond planets.

The possibility of massive solid planets up to forming around massive stars (B and O-type stars; ) has also been suggested based on mass-radius relationships for rocky planets, proposing that the protoplanetary disk around such stars would contain enough heavy elements, and that high UV radiation and strong winds could photoevaporate the gas in the disk, leaving just the heavy elements.

Per a model, one hypothesis suggested so-called blanets, fundamentally similar to other planets, orbiting around a rotating supermassive black hole at least a million solar masses () may harbor masses comparable to that of massive solid planets. Although the runaway accretion of the gas onto blanets to become gas giants is possible, it is likely difficult. Nevertheless, this would also depend on how fast are the orbits of blanets filled with gas.

Super-puffs

A super-puff is a type of exoplanet with a mass only a few times larger than Earth's but a radius larger than Neptune, giving it a very low mean density. They are cooler and less massive than the inflated low-density hot-Jupiters. The most extreme examples known are the three planets around Kepler-51 which are all Jupiter-sized but with densities below 0.1 g/cm<sup>3</sup>.

</references> <!-- end of reflist -->

Bibliography

  • SPACE.com: Q&A: The IAU's Proposed Planet Definition, 16 August 2006, 2:00 AM ET
  • BBC News: Q&A New planets proposal Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
  • SPACE.com: Q&A: The IAU's Proposed Planet Definition 16 August 2006 2:00 am ET
  • BBC News: Q&A New planets proposal Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
  • Gas Giants in Science Fiction: [http://www.sf-encyclopedia.com/entry/gas_giant]
  • Episode "Giants" on The Science Channel TV show Planets