Initial Conditions: The temperature varies with distance from the protosun.

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1 Initial Conditions: The temperature varies with distance from the protosun. In the outer disk it is cold enough for ice to condense onto dust to form large icy grains. In the inner solar system ice can t form and does not become bound with dust. But the density is higher so the dust can accumulate.

2 The Snowman Effect: As the disk rotates the grains collide, forming larger grains like a snowball rolling down hill When the grains become larger than a few meters across we call them planetesimals. Planetesimals go on merging together. When they become large enough for gravity to affect other nearby objects, we call them Protoplanets. When the Protoplanets attract enough mass that they begin clearing out a whole toroid of the disk, the have become Planets.

3 End Products: The hierarchical formation process explains the terrestrial planets. They formed from the refractory or ice-free grains.

4 The Third Way: In the case of Jupiter, Saturn, Uranus, and Neptune something else happened. They aren t made of grains and protoplanets. If both dust and gas are added together, there s a lot more material to work with. In the outer solar system the protoplanets were much larger. Their gravity became great enough to suck gas directly from the disk, just like the Sun was doing. This allowed them to become a WHOLE lot bigger.

5 The Jovian Planets: Most of the mass (other than the Sun) in the solar system is tied up in the Jovian planets. How big each one gets depends on where it forms and how long it has to accumulate material. They form within the disk in a manner similar to the Sun, complete with their own planet systems (their moons). Given enough time and mass it is even possible for these planets to become stars(?)! That didn t happen here..

6 End of Planet Formation: After about 10 8 years, the central Star has become large enough that it can no longer hold itself up with heat energy alone. It begins to have fusion reactions in its core. This is a violent process because it doesn t start right out with P-P burning (Li and D burn first). It takes about another 10 6 years to reach equilibrium (the Main Sequence). We strong solar wind blows the remaining dust and gas out of the planetary disk. This chokes off additional mass for planet formation.

7 And That s How We Arrived: This process does a good job of getting us to the start of things in our planetary system. But this still hasn t answered two important questions. Is our system typical? Are there processes we ve missed?

8 Kuiper Belt (leftovers) is a diskshaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun containing many small icy bodies. It is now considered to be the source of the short-period comets.

9 The Roads Less Traveled: Ten years ago we wouldn t have been able to answer either of those two questions. We have now discovered more than 100 planets around other stars. None of them look like ours or even like they should be ABLE to form.

10 Are We a Happy Accident? The Orbits of extra-solar planets are puzzling. Gas Giants are found CLOSE to the star. Their orbits are not very circular (while ours are). What s going on? We think we ve underestimated the last step of dynamical filtering. Not just planetesimals and protoplanets are filtered. The mature planets -even JUPITER sized- are moved around as well. Taking this into account helps our models reproduce the systems we see. They even predict some early movement (mainly from Uranus & Neptune) in our solar system. What we DON T know is which is the norm, our stable system, or the strange ones we re seeing.

11 Terrestrial Worlds of the Solar System:

12 Evolution of Terrestrial Planets: The terrestrial planets formed via hierarchical evolution (from dust to planet). As they formed they differentiated. The heavy elements fell to the center, enriching their crusts with silicates.

13 Evolution of Terrestrial Planets: Initially the terrestrial planets were too hot to retain any kind of atmosphere. Once they cooled however, what gasses they could retain were provided either from volcanic activity or impacts. While they may have had similar origins, the terrestrial planets have had very different histories since then. Once again there is more to say than we have time for. So we ll spotlight the unique features of each world.

14 Mercury: Mercury is the smallest of all the planets, less sizable (but much more massive!) than two outer solar system icy moons.

15 Mercury: The surface of Mercury is most like that of the Moon. It s very old and crater scarred. However the Moon is the object it has the least in common with on the interior

16 Mercury: That makes it nearly double the relative size of the Earth s core. What happened? Volatile Stripping in the Protosun? High Formation Temperatures for planetesimals and protoplanets? A Giant Collision?

17 Mercury is the densest object in the solar system. It s core is composed of mainly Iron and is 65% of the diameter of the planet! That makes Mercury s Core nearly double the relative size of the Earth s core.