During Week 4 you learned how stars form from collapsing fragments of relatively high density gas and dust clouds. Not all the collapsing material ends up in the star - some of the material ends up in the form of planets and asteroids orbiting the star, and the star’s radiation and particle (electron, proton) pressure forces the rest of the material (gas and dust) out of the planetary part of the system. All this evolution happens AFTER FORMATION OF A DISK of gas and dust, according to theoretical models. As some of the disk material falls onto the star, apparently the star can also expel a bit of that material in bi-polar jets of material that we see associated with forming, but still unstable, young proto-stars (T-Tauri stars with H-H objects).

The focus this unit is on the process of planet formation during this late stellar formation period. Gathering corroborative evidence to support the model of planetary system formation is an on-going research effort in astronomy. The Hubble space telescope spends some of its time imaging near-by young stars in star-forming regions in hopes of seeing dusty disks around young stars. Another research effort that has recently had some success is observational evidence that fully-formed planets orbit relatively near-by stars. Research in these two areas has been difficult, in part because stars in this early, relatively short, evolutionary stage are far from numerous. Findings thus far suggest that
*  dusty disks are commonplace for very young stars, and
*  that very massive (gas?) planets orbit about 5-10% of near-by sun-like stars in surprisingly small orbits. 
As of mid-2006, astronomers have discovered near 200 extra-solar planets in close orbits around Sun-like stars.  This discovery helped stimulate theoretical research on migration of planets (and small stuff like comets) through gravitational interactions early in the history of a planetary system.

Active exploration of our planetary system began about four decades ago and remains a priority of the U.S. (and other countries also) Astronomy program. Surprises continue to surface about apparently dead worlds such as our own Moon, and even our home planet has its share of mysteries. I suggest you keep some focus-type questions in mind as you read the text material this unit:

1. Why is the overall shape of our planetary system disk-like?

2. What is the general rule for orbital and rotational motions of objects in the solar system, what are the exceptions, and how does the general rule square with the theory of formation of the system?

3. Why do the inner planets have rocky composition, while the composition of the outer planets (excluding Pluto) is mostly hydrogen and helium?

4. Why do the moons of the outer planets NOT have thick atmospheres with hydrogen and helium, unlike their parent planets?