A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are really a distinct class of giant planet, being composed mainly of heavier volatile substances (which are referred to as "ices"). For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.
The defining differences between a very low-mass brown dwarf and a gas giant (estimated at about 13 Jupiter masses) are debated.  One school of thought is based on formation; the other, on the physics of the interior.  Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history.
Gas giants can, theoretically, be divided into five distinct classes according to their modeled physical atmospheric properties, and hence their appearance: ammonia clouds (I), water clouds (II), cloudless (III), alkali-metal clouds (IV), and silicate clouds (V). Jupiter and Saturn are both class I. Hot Jupiters are class IV or V.
A cold hydrogen-rich gas giant more massive than Jupiter but less than about 500 M ⊕ ( 1.6 M J ) will only be slightly larger in volume than Jupiter.  For masses above 500 M ⊕ , gravity will cause the planet to shrink (see degenerate matter ). 
Kelvin–Helmholtz heating can cause a gas giant to radiate more energy than it receives from its host star.  
Planetary rings are an interesting phenomena. The mere mention of these two words tends to conjure up images of Saturn, with its large and colorful system of rings that form an orbiting disk. But in fact, several other planets in our Solar System have rings. It’s just that, unlike Saturn, their systems are less visible, and perhaps less beautiful to behold.
Thanks to exploration efforts mounted in the past few decades, which have seen space probes dispatched to the outer Solar System, we have come to understand that all the gas giants Jupiter, Saturn, Uranus and Neptune all have their own ring systems. And that’s not all! In fact, ring systems may be more common than previously thought
In was not until 1979 that the rings of Jupiter were discovered when the Voyager 1 space probe conducted a flyby of the planet. They were also thoroughly investigated in the 1990s by the Galileo orbiter. Because it is composed mainly of dust, the ring system is faint and can only be observed by the most powerful telescopes, or up-close by orbital spacecraft. However, during the past twenty-three years, it has been observed from Earth numerous times, as well as by the Hubble Space Telescope.
The ring system has four main components: a thick inner torus of particles known as the “halo ring”; a relatively bright, but extremely thin “main ring”; and two wide, thick, and faint outer “gossamer rings”. These outer rings are composed of material from the moons Amalthea and Thebe and are named after these moons (i.e. the “Amalthea Ring” and “Thebe Ring”).
The main and halo rings consist of dust ejected from the moons Metis, Adrastea, and other unobserved parent bodies as the result of high-velocity impacts. Scientists believe that a ring could even exist around the moon of Himalia’s orbit, which could have been created when another small moon crashed into it and caused material to be ejected from the surface.
Discovery! Atmosphere Spotted on Nearly Earth Size Exoplanet in First - 4:20
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