Giant planets, that is, not actual giants. We’ll leave the latter to Hagrid’s internist. Next week I’m off to the annual meeting of the Division of Planetary Sciences of the American Astronomical Society, or “the DPS” for short. The DPS will give the Urey Prize to Tristan Guillot in recognition of his work studying the interiors of the giant planets. Today Tristan gave a seminar here in Colorado explaining how we can learn about the structure and composition of planets such as Jupiter and Saturn, and now also some of the more than 200 planets that have been discovered orbiting stars other than the Sun.
The “why” of all this is to better understand how planets, in general, form. The giant planets are the dominant objects in a planetary system, and the habitable, or terrestrial, planets, are in some ways afterthoughts or leftovers of the evolution of the protoplanetary disk. How the giant planets get to be the way they are will help us understand how our own planet got to be such a friendly place, and how many other friendly places there might be in the galaxy. And to understand how they got to be the way they are, we have to know, well, how they are. Our knowledge of the upper atmospheres of the giant planets in our system is reasonably detailed. But these planets have no solid surface like the Earth and Mars, and that makes figuring out what is going on inside tricky.
Image credit: NASA/JPL/Space Science Institute
The “how” is going to get a big boost from the upcoming Juno mission to Jupiter. This spacecraft will orbit Jupiter with a closest approach just a few thousand km above the cloudtops, not to get a better picture of the clouds, but so that Jupiter’s gravity will have a stronger effect on the spacecraft’s trajectory and so that the strong inner region of Jupiter’s magnetic field can be explored. If Jupiter were a perfect sphere then Juno’s orbit would be a perfect and constant ellipse. The uneven distribution of mass in the planet’s interior, combined the flattening of the planet due to its rotation, result in a gravity field that causes perturbations in the orbits of objects nearby. By flying very close to Jupiter, those perturbations are magnified and will allow Juno scientists to refine our models of the interior of the planet. The magnetic field is created by the motion of metallic Hydrogen deep in the interior so measurements of the magnetic field also tell us about the interior. Scientists such as Guillot face the challenge of modeling planetary interiors where the pressures are extremely high (millions of times the atmospheric pressure at the surface of the Earth). To successfully model the interior of Jupiter, for example, we need to know how materials respond to changes in pressure at those very high pressures which can only be achieved in laboratory experiments on very small samples for tiny fractions of a second.
So what is the interior of Jupiter like? Jupiter is about 318 times more massive than the Earth, and most of that is Hydrogen and Helium gas. At the center is a core of highly smushed rock and ice that is only about 10 times as massive as the Earth. Above that, the Hydrogen gas is so compressed that is in a strange state called metallic Hydrogen, and it is here that Jupiter’s strong magnetic field is generated. Juno’s launch is scheduled for no earlier than 2010. In the meantime, Guillot and others are studying the ever-growing giant planet menagerie with information as simple as the mass, size, and orbit of the planet.