1. Notes 2-1 The Formation of the Solar System 2/6/09

2. The Sun and planets began as a cloud of gas, just like the one shown in this picture. The cloud was probably made of hydrogen and helium gas (the same gas which goes into a helium balloon. Such a cloud is called a Nebula. The gas in a nebula is used to make new stars.

3. Scientists believe that the solar system was formed when the cloud of gas and dust was disturbed, maybe by the explosion of a nearby star. Such an explosion is called a supernova. A supernova makes waves in space which travel outward from the center of the explosion.

4. When the wave from the supernova reaches our nebula, it squeezes the cloud of gas and dust. Squeezing makes the cloud start to collapse. Gravity pulls the gas and dust together into a tighter ball. The wave continues on its way through space, leaving behind a forming solar system.

5. Because the cloud was spinning, it flattened into a frisbee shape, just like a ball of pizza dough becomes flat when a chef spins it in the air. The frisbee shaped cloud grew hotter and denser in the center, with a disk of gas and dust surrounding it that was cool.

6. As the cloud flattened, the gaseous material inside was forced to begin changing into solid form. These little particles of solid material were warm, soft and sticky, and whenever they touched each other, further clumped together to form larger balls of solid material somewhat the way pieces of "silly putty" would stick together if they came into contact. The hot and dense part in the center we call a proto- sun, meaning that it was a forming sun. It was not a star yet, but eventually would become our Sun. The large balls of clumpy stuff, which could be boulder sized to asteriod sized, we call "planetismals", meaning that they were miniature planets. Eventually only a few large clumps of this material remained in the forming solar system, and they became the core of "proto-planets".

7. As shown in this picture, while they were forming in the flattened disc of the solar nebula, the nucleii of the planets-to-be (called proto-planets) drew material to themselves from the cloud of gas and dust around them. The bigger protoplanets were able to attract even more gaseous material unto themselves.

8. The position of the planets in the solar nebula greatly affected their size and composition. This is because of the effect of how cold it was in the nebula.

9. The nebula was a lot warmer close to the proto-sun. The blue line shown in the picture shows the point at which the temperature became cold enough for gases to become ice. At this point and further out, beginning with the forming Jupiter, the materials that forming planets began to extract from the cloud were ice, as well as rocky material and gas molecules. Retention of ice resulted in these proto-planets becoming giant, massive planets. Planets which formed closer to the proto-sun were smaller, and more rocky

10. This story explains the forming planets, but what of their moons? There are three theories about how moons came to be in place: 1.) moons came out of their parent planet 2.) moons were debris which were captured by their parent planet 3.) that a planet and its moon(s) formed together out of the primordial nebula. This image shows seven of the very small satellites of Saturn photographed August 25, 1981 by Voyager 2.

11. Many of the moons surrounding other planets are really captured asteroids. One sign that a moon is really a captured asteroid is that it is not round. That is, a moon may be shaped like a rock and not rounded like a planet. Moons such as these are the two moons of Mars; Phobos and Deimos. Another sign that a moon may be captured is if it orbits in a direction opposite to that of the mother planet. An example of a moon of this kind is Neptune's moon Neried. This image shows Phobos (left) and Deimos (right), the two moons of Mars.

12. The co-formation theory explains the origins of moons as objects which formed out of the initial cloud of gas and dust at the same time and roughly the same place as the parent planet. As shown in this picture, while they were forming in the solar nebula, the beginning of the moons-to-be (called proto-moons) drew material to themselves from the cloud of gas and dust around them. This theory seems to explain the many icy moons around the giant planets in the outer solar system

13. The cooler temperatures which played a role in the formation of the planets seems also to have played a role in the formation of these moons. Just as the planets became more and more icy the further from the sun, moons seem to be more and more icy the further from the planet they orbit. For example, Jupiter's moon Io (close to Jupiter) is very rocky, but Europa (farther from Jupiter) has more ice, Ganymede and Callisto (further still from Jupiter) have lots more ice.

14. Currently, the theory that best explains all the evidence about the formation of the Earth's moon is that the Earth was struck by a large object, possibly as large as Mars. This theory says that enough debris was scattered from the Earth to form the Moon. Scientists can make movies of how this crash might have worked. The movies show that energy from such a crash produces a stream of completely vaporized rock from the impact. It is thought that the Moon formed from this cooled material.

15. Besides planets and moons, the solar system contains comets and asteroids. The place from which the comets come is thought to be a giant sphere surrounding the solar system. This sphere is called the Oort cloud. It is named after a scientist called Jan Oort who first thought of the idea in 1950. Thus comets are said to come from the Oort cloud.

16. Beyond the orbit of Pluto is a region which contains objects which look like planetismals. The objects are very small, and not very bright. It takes hundreds of years for these objects to go around the sun. Because they move so slowly,finding them is difficult. Where these objects came from and how they got there is still a mystery, but they probaby condensed with the planets as part of the original solar nebula. The region is called the Kuiper Belt, after Gerard Kuiper who first thought of it. The first object in the Kuiper Belt was discovered in 1992.

17. More than 100,000 asteroids lie in a belt between Mars and Jupiter. These asteroids lie in a location in the solar system where there seems to be a jump in the spacing between the planets. Scientists think that this debris may be among those which potentially may have formed a planet, but were prevented from doing so by the immense gravity of Jupiter. Asteroids are small rocky objects with round or irregular shapes which can be up to several hundred km across.

18. At this point in planetary history, of every planet and moon with a solid surface was bombarded with the remains of the material which formed them. Even though the solar system was finished forming, there were still probably a lot of smaller planetesimals and debris around, too. The gravity of the large planets would attract nearby planetesimals, which would hit the planets and leave a crater on the planet's surface. This was the very last stage of the process whereby planetismals swept up material in their paths.

19. With all the bombardment by leftover debris, the proto-planets and proto-moons warmed up. Elements which comprise the planets and moons will part into separate regions of the body, if the body is warm enough, thus forming layers in the interior of a planet as shown in this picture. This is the akin to the process whereby an oil & vinegar salad dressing will part into regions made solely of oil and solely of vinegar. The term for this process is differentiation. Planetary elements which separate include the heavy iron and the lighter silicate rock. Iron falls to the center of a planet and forms a core, silicate material might forms a mantle on top of the core.