[[caption-width-right:250:King of the Planets]]

->''Jupiter instead cooled down below the threshold for fusion, but it maintained enough heat and mass and pressure to cram atoms very close together, to the point they stop behaving like the atoms we recognize on earth. Inside Jupiter, they enter a limbo of possibility between chemical and nuclear reactions, where planet-sized diamonds and oily hydrogen metal seem plausible.''
-->-- Sam Kean

Also known as Earth's Big Brother.

The biggest gas giant in UsefulNotes/TheSolarSystem, massing over 300 times more than Earth. Jupiter's mass is so great as to single-handedly pull the center of gravity for the entire solar system into the empty space beyond the Sun's surface. It's mostly hydrogen and helium, although various other compounds provide its colorful bands and storms, and scientists believe a rocky core sits at its center. Its most prominent surface feature is called [[ExactlyWhatItSaysOnTheTin the Great Red Spot]]. It's a cyclone, a whirling storm 3 times as big across as the Earth. Much of the energy for Jupiter's weather appears to come from the very slow contraction of the planet.



Its gravity probably stunted Mars by starving it of material when Jupiter migrated towards the Sun, when it should have gotten as big as Earth and Venus, and when it migrated back out thanks to Saturn, kept another planet from forming where the main asteroid belt is now. This migration also pushed Uranus and Neptune into their distant orbits, along with making them switch places (Uranus was originally the outermost planet). There's a strong possibility that this also ejected a planet from the Solar System, there's evidence[[note]]a lithium spot on the Sun[[/note]] another gas giant plunged into the Sun[[note]]The recent [[https://en.wikipedia.org/wiki/Grand_Tack_Hypothesis Grand Tack theory]] proposes Jupiter could have migrated inward to Mars' distance to the Sun (1.5 A. U.), messing with both the asteroid belt and any planets that could have formed there, with the current terrestrial planets forming of what was left after it migrated outward.[[/note]], and a growing body of evidence[[note]]the bizarre orbital configurations of certain trans-Neptunian objects, some of which stumped astronomers for years[[/note]] that yet another ice giant, or at least the solid object that would have otherwise become the core of an ice giant, was launched into the outer solar system beyond the Kuiper Belt. This is also why there's no "super-Earths" in the Solar System, despite them being far more common in alien solar systems than Earth-sized bodies. That's not all. The Grand Tack Theory also shows that Jupiter pushed icy material from the outer solar system into the inner, giving Earth a massive amount of water, a lot of it on the surface, but most of it located 300 miles under the crust. Venus and Mars also received that gift, but they were unable to keep their water for various reasons.

Jupiter is also well placed to deflect those nasty comets and asteroids away from us in the inner solar system. It definitely took one for the team when Comet Shoemaker-Levy 9 came calling in 1994. However, its influence on comets is a two-edged sword -- some comets which otherwise would have made a single pass through the inner solar system and headed back out into deep space never to be seen again, were instead deflected by Jupiter into short-period Solar orbits that pass through the inner solar system over and over. (Comet Halley probably had this happen to it ''twice.'') Regular dark impact spots have indicated that Jupiter probably gets hit by a large comet or asteroid as often as once every few years, having had recorded asteroid strikes in 2009, 2010, and 2012. (As if on cue, another asteroid [[http://www.slate.com/blogs/bad_astronomy/2016/03/29/jupiter_hit_by_asteroid_or_comet_in_march_2016.html hit Jupiter]] on March 17, 2016.)


Scientists now believe that gas giants like our Jupiter are actually rare, and that most gas giants eventually become "hot Jupiters". Our own Jupiter would have been one as well had Saturn not existed - and had Jupiter migrated closer and closer to the Sun, it would have wiped out the inner planets with its gravity (and as explained in the third note, this ''could'' have happened.)
