We can understand the formation of planetary systems by studying our own in detail.

Key point: Robotic spacecraft explore the planets

We can use the tools of the astronomer - remote imaging, spectroscopy - to explore planets too. However, the planets are so close (relatively speaking) that we can also send out spacecraft to visit them and send back information at a level of detail that would be impossible to obtain remotely.

For the moon, we went in person to observe and gather rocks, and we have sent probes to observe all the planets except Pluto (the Galileo spacecraft is shown at Jupiter to the right). A planetary probe can gather much more detailed data than we can get from the distance of the earth, and the exquisite images of the planets we will show soon are a result.

(from NASA and the Air and Space Museum,  http://www.nasm.si.edu/apollo/AS11/a11.htm) and from Galileo Project, http://www.jpl.nasa.gov/galileo/images/artwork.html )

In addition, by sending spacecraft to the planets, we can drop instruments into their atmospheres and learn about the gases, temperatures, clouds, and winds below the levels we can see from outside. To the left is the probe carried to Jupiter on the Galileo spacecraft, and to the right is an artist's concept of it parachuting down into the atmosphere of the planet. (see http://galileo.jpl.nasa.gov/gallery/jupiter.cfm)

And in some cases, we have even touched down and explored the surface of a planet directly. Here is the Sojourner rover from Mars Pathfinder, testing its wheels on the surface. Notice the tracks from previous runs!  http://mars.jpl.nasa.gov/MPF/ops/rvrmovie.html Here is a chance to see an entire mission to Mars, the Mars Exploration Rover, as simulated by Cornell  student Dan Maas(reload to restart lecture animations)