- The cause of orderly patterns of motion
- Why there are two types of planets
- The origin of asteroids and comets
- The formation of the moon


[SLIDE 1]

How does the nebular theory explain the orderly patterns of motion in our solar system?

According to the nebular theory, the solar nebula, which was originally a large and roughly spherical cloud of cold, low-density gas, began to collapse as the result of a cataclysmic event - for example, the shock wave from the explosion of a nearby star (a supernova).

As the gas cloud shrank, the strength of gravity increased, and three important processes altered its density, temperature, and shape.

The first process was heating. As the solar nebula collapsed, the individual gas particles began to collide, turning gravitational potential energy into thermal energy. The Sun formed in the center of the cloud, where the temperatures and densities were highest.

The second process was spinning. The solar nebula rotated faster and faster as it shrunk in radius, just like an ice skater does as she pulls in her arms in the middle of a spin. This rotation kept all of the material in the nebula from collapsing into the middle. The greater the angular momentum, the more the cloud will spread out.

The third and final process was flattening. As the collapsing cloud spun faster and faster, it began to flatten into a disk. As clumps of gas inside of the disk bump into each other, the random motions get averaged out, leading to greater order and making orbits more circular.

This flat, spinning disk of gas which eventually formed into the planets explains why they all orbit the Sun in the same direction in nearly the same plane, and the collisions averaging out the random movements into more circular movements explains why the planets tend to have nearly circular orbits.

Observations of spinning disks around other stars support the idea that our solar system formed from a similar disk.

Also, computer simulations using conditions observed in interstellar clouds and applying the laws of physics have successfully reproduced most of the general characteristics of motion seen in our solar system.


[SLIDE 2]

Next, let's talk about how nebular theory explains the presence of two types of planets.

The planets began to form after the solar nebula had collapsed into a flattened disk of 200 Astronomical Units in diameter.

Planet formation began around tiny seeds of solid metal, rock or ice. These tiny seeds existed because of a process called condensation. Condensation is where atoms or molecules in a gas bond and solidify into solid or liquid; it happens when the temperature gets low enough. Different materials condense at different temperatures. We see an example of condensation in the formation of snowflakes in clouds on Earth.

As mentioned before, 98% of the solar nebula was hydrogen and helium gas. These compounds do not condense in space. Hydrogen *compounds*, however, such as water, methane, and ammonia can solidify into ices at low temperatures (below about 150 Kelvin) while Rock and Metal begin to condense at temperatures below 1600 Kelvin.

Thus, seeds of rock and metal formed into terrestrial planets in the warm, inner regions of the swirling disk.

In the colder, outer regions, beyond what is known as the frost line, the Jovian planets formed from a combination of hydrogen compounds and rock and metal. Since hydrogen compounds were much more abundant in the gas cloud than rock and metal, the Jovian planets grew much larger than the inner planets.

The remaining majority of hydrogen and helium gas in the nebula was cleared out by high energy radiation from the Sun and solar wind, ending the era of planet formation.


[SLIDE 3]

The process where small seeds grew into planets is called accretion. Microscopic solid particles that had condensed from the gas of the nebula gradually combined into larger particles, eventually growing large enough to attract each other through gravity and forming into planetesimals, which literally means "pieces of planets."

Where do asteroids and comets come from? Asteroids and comets are leftover planetesimals from the time of planet formation. Asteroids are the *rocky* leftover planetesimals of the inner solar system, while comets are the *icy* leftover planetesimals of the outer solar system.

During the first few hundred million years, a period called "heavy bombardment," these asteroids and comets collided with planets continuously, resulting in the impact craters visible on many moons and planets.


[SLIDE 4]

The Jovian planets have many moons because they were large enough to capture passing planetesimals. The Earth's moon, however, is a mystery because there is no way a small planet like Earth could capture an object as large as our moon.

The leading hypothesis is that the Moon formed as the result of a giant impact between Earth and a huge planetesimal. A large Mars-size object struck our planet, tilting its axis, and blasting Earth's outer layers into orbit, where the material accreted to form the Moon.

This hypothesis is supported by the fact that the Moon's composition matches the Earth's outer layers and the Moon has smaller amounts of vaporized ingredients such as water than Earth. These two features support the giant impact theory.

Giant impacts may also explain other exceptions to the trends in our solar system, such as the sideways tilt of Uranus's axis and Venus's slow and backward rotation.