Most school children learn that the giant planet Jupiter is a "gas giant". This means that the largest planet in our solar system is composed mostly of gases such as hydrogen and helium. Well, just as our definitions of what a planet is-as in the case of Pluto-are being redefined, we may have to refine our definition of what a gas giant planet is.
A discovery made just last month has revealed that Jupiter has a rocky core more than twice as large as previously measured. This makes big Jupiter appear less like a gas giant and more like a quasi-terrestrial planet, at least deep below its ultra-dense atmosphere.
A research team led by Burkhard Militzer at the University of California ran a series of detailed computer simulations that showed how individual hydrogen and helium, at the atomic scale, behave deep inside Jupiter. Hydrogen-helium atoms behave strangely in the extreme pressure and temperature environment of the big outer planets.
Militzer's colorful computer model shows that the inner of Jupiter core is an immense rock approximately 16 times Earth's mass. This rocky core, perhaps made of iron, nickel and silicate materials, amounts to five percent of Jupiter's total mass. The new model challenges the old, standby theories: Most astronomers thought Jupiter's core was either seven Earth masses or they argued that the big planet lacked any kind of core except for maybe a mushy interior.
According to an interview with Space.com, Militzer said his team's high-tech simulations show a big rocky object in Jupiter's central region surrounded by an ice layer. Little ice exists on the planet beyond the core region; this is contrary to a theory apparently overturned by last week's discovery.
"This is a very different result for the interior structure of Jupiter than other recent models, which predict a relatively small or hardly any core and a mixture of ices throughout the atmosphere," Militzer told Space.com.
The new research results now show a Jovian planetary core that is composed of extremely dense layers of metals and rocks surrounded by methane, ammonia and water ices. Above these outer core layers is an atmosphere of mostly hydrogen and helium. At the planet's very heart, a metallic ball of iron and nickel makes up the deep, inner core.
With last month's finding, it now looks like Jupiter's interior more closely resembles what recent data has revealed about the interiors of Neptune and Uranus; these smaller "gas giants" have rocky cores, too, surrounded by icy hydrogen and helium. However, Neptune and Uranus lack the ultra-dense and forbidding atmospheres of Jupiter and Saturn.
As Jupiter accreted from the solar nebula 4.5 billion years ago, its heavy rocky core captured-and held on to-hydrogen, helium, and other gases.
What's in the Sky: On Dec. 7, look for the Dog Star or Sirius (the name that inspired the popular satellite-radio company). This bright star, of special meaning to the ancient Egyptians, rises in the southeast during mid evening. Since Sirius is the brightest star in the night sky, it's very easy to locate. Sirius is close to our Sun-it's a mere 2.6 parsecs (8.6 light-years) away. The Little Dog Star or Procyon, is Sirius' nearest neighbor (to its left from Earth). The Little Dog Star is located 3.5 parsecs (11.41 light years) away. Why the term "dog star"? Perhaps the stars were observed during the dog days of summer? We really don't know. Both dog stars are visible until morning.
Lou Varricchio, M.Sc., is a former NASA senior science writer. He is involved with the NASA/JPL Solar System Ambassador program in Vermont and is a senior member of the U.S. Civil Air Patrol.