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Our solar system contains three types of planets. Between the four terrestrial planetsâMercury, Venus, Earth, and Marsâand the distant ice giants of Neptune and Uranus, sit two gas giants: Saturn and Jupiter.
These planets are mostly composed of hydrogen and helium gas. Researchers now appreciate that gas planets are more complex than first thought. New findings have implications for our understanding of how these planets formed and will help design future missions to potentially visit them.
How do gas giants form?
Gas giants originate from one of two processes. The first method is called core accretion, explains Ravit Helled, a professor of theoretical astrophysics at the University of ZĂźrich. This starts with the birth of a new star, when molecular clouds collapse under gravitational pressure. Whorls of gasâcalled protoplanetary disksâstart to spin around these new stars. Within these gas disks will be heavier particlesâdust, rock, or any elements heavier than helium. These particles can clump together and then suck in gas from the surrounding disk, forming a giant planet mainly composed of gas.
A second method that may form gas giants called disk instabilityâthis is a newer theory that still causes some controversy among planetary theorists. According to this idea, when massive protoplanetary disks cool down, they become unstable and can produce clumps of rock and gas that evolve into gas giants. Importantly, this proposed formation process happens much more quickly than core accretion. Helled says that Saturn and Jupiter likely formed via core accretion, but that disk instability may âexplain very massive planets at large orbits or giant planets around small mass stars.â
Landing on a gas giant
Regardless of how they form, the structure of gas giants is nothing like that of terrestrial planets like Earth. Jupiter and Saturn donât have a surface in the same way Earth does. Instead, their atmosphere simply gets thinner until there isnât enough density left to call the surrounding air part of the planet anymore. âThere is no location where you can say, okay, this is where the planet stops,â says Helled.
A spaceship attempting to âlandâ on Jupiterâs âsurfaceâ would have to overcome some significant obstacles. Once you enter the cloud of gas that roughly marks the beginning of a giant like Jupiter, temperature and pressure steadily increase as your head toward the planetâs core, and gaseous hydrogen and helium morph into liquid form. While our solar systemâs gas giants are far from the sun, the core of a gas giant is likely to be incredibly hotâJupiterâs is estimated at around 43,000 degrees Fahrenheit. Youâd also have to pass through the thick clouds of ammonia found in Jupiterâs upper atmosphere.
If you make your ship from tough stuffâtougher than any known substance on Earthâthat could survive these conditions, it might make it to a gas giantâs core. What it would find there in the alien murk is still unclear.
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