Our Solar System

[tammie_c]

THE SOLAR NEBULAR HYPOTHESIS

The solar nebular hypothesis states that the solar system formed from the collapse of an interstellar gas cloud.

Properties of the cloud:
Large and covered a wide area, and it slowly rotated – means that it had some angular momentum
This explains the motions of the planets because if the whole cloud rotated in the same direction, then obviously the planets formed from this cloud would rotate in the same direction as well
Composed of mostly hydrogen and helium, and some heavier elements (found in terrestrial planets and asteroids, etc.)
This explains why there are two types of planets – terrestrial and jovian – because there are gases that make up the jovian planets, and also some heavier elements that make up the terrestrial planets

What happened to the cloud?

“The collapse is triggered by an increase in density, and driven by gravity.”

During the collapse:
- Angular momentum causes the rotation rate to increase
- The centre heats up
- The disk of gas and dust flattens, and particles of the cloud lose energy as they collide with each other

The nebular cloud collapsed under its own gravity. It heats up as it compresses, and dust vaporizes. The centre compresses and becomes a protostar, while the rest of the gas orbits around the protostar. Gas flows inwards and becomes part of the star, but the gas is still rotating. The force from this rotation of the gas in the star prevents some gases from reaching the star, and it forms an accretion disk. Energy is radiated away from this disk and it cools. The gas orbiting the star cools enough for metal, rock and ice to condense into solid particles, like dust. Metals condense at approximately the same time that the accretion disk forms, and rock condenses later.

Dust particles collide and stick to each other, forming larger particles. They soon become the size of small asteroids. These objects grow large enough to have their own gravitational force and they pull in smaller particles. This causes growth to be very rapid. The size of these objects, called planetessimals, depends on the distance from the Sun, density, and composition. The planetessimals in the inner solar system are a large asteroid to lunar size, whereas the planetessimals in the outer solar system are 1-15 times Earth's size. From this, we can see that the planetessimals were larger when they were further away from the Sun. The difference in sizes between the region where Mars is and where Jupiter is was quite large.

Ice cannot condense near the Sun, so it is a vapour, and not a solid, and therefore not accretable. More massive planetessimals had a greater gravitational force and it pulled in nebular gases and became a gas giant. Planetessimals that were too small to have a strong gravitational force would stay rocky and/or icy. Planetessimals collided and merged with each other to become more and more massive. Eventually, only 8 planets were left, each with a stable orbit around the Sun.

However, planets can be modified by the last collision. There were many planetessimals in the early solar system, and they collided with the larger bodies, causing unusual situations, such as the tipping over of Uranus and Earth's large moon.

Accretion:
The process in which planetessimals, and sometimes planets, are formed.

1) Gas particles and dust stick together due to the electrostatic attraction.
2) They continue to stick to each other and grow larger and larger.
3) As they grow larger, their gravity was strong enough to attract particles (in addition to the electrostatic attraction) and the growth sped up.
4) Growth continues.
5) It grows large enough for gravity to pull all the contents of the planetessimal into a spherical shape.
6) As it grows, gravity becomes stronger, and the growth speeds up and it takes all of the material around it in. It is similar to the snowball effect.

The Jovian planets were formed through the process of accretion. Their masses were so large that the gravity of the planet became stronger than that of the Sun’s at the point where it was (gravity is stronger near the Sun and it gradually grows weaker as it moves out). The material around the planet started to orbit the planet instead of the Sun. The force of gravity pulled in nebular gases and other materials. It accumulated and the Jovian planets were formed.


What is the frost line?

The frost line is an imaginary line that separates the jovian planets from the terrestrial planets. It is also known as the snow line. This imaginary line refers to the point at which hydrogen compounds such as water, ammonia, and methane are able to condense into solids. The area from this line to the sun is too hot for these compounds to condense, but beyond this line, it is cool enough for these compounds to condense. Beyond this line, planets can form from the condensed material using the process of accretion. The frost line lies in between the orbits of Mars and Jupiter.


The two main types of planets

Terrestrial planets: the inner planets that are composed of mostly rocks and heavier elements compared to the jovian planets. Terrestrial planets are composed of solid material, unlike the jovian planets. They are also called rocky planets, and are described as being “Earth-like”. Their structures are very similar, with a core made up of mostly iron, surrounded by a mantle. There are four terrestrial planets in our solar system: Mercury, Venus, Earth, and Mars.

Jovian planets: also known as a gas giant. They are composed of gases and non-solid materials. One is not able to land on a jovian planet like we land on terrestrial planets. It may have a solid core, but the rest of it is composed mostly of hydrogen and helium, with small amounts of methane, water, and ammonia. The four jovian planets in our solar system are: Jupiter, Saturn, Uranus, and Neptune.

Why inner planets are terrestrial and outer planets are gas giants:

- The Sun’s gravitational force pulls heavier, denser materials closer to itself
- These materials are the ones that form terrestrial planets
- The original orbits of the terrestrial planets come closer to the Sun and they are pulled in and they fall into smaller orbits
- Near the Sun, inside the frost line (from the Sun to the frost line), only heavier elements and rocky material can condense and form planets
* anything else has to move beyond the frost line for it to be cool enough for it to condense
- The jovian planets formed first, which drew a majority of the gaseous material due to the gravitational force and it left behind the heavier elements and rocky material


Asteroids and comets

Asteroids are made up of mostly rock and metal, and very little ice, whereas comets (found in the Kuiper belt) are made up of mostly frozen materials (ices), and are often referred to as “dirty snowballs.”

What are the asteroid belt and the Kuiper belt?

Asteroid belt: the area in which irregularly shaped bodies called asteroids are found. It is found approximately in between the orbits of Mars and Jupiter. The asteroid belt is composed of mostly rock and metal.

Kuiper belt: it is similar to the asteroid belt, and is found beyond the orbit of Neptune. It is much larger than the asteroid belt. Unlike the asteroid belt, the Kuiper belt is composed of mostly frozen materials such as methane, ammonia, and water. Pluto, once considered a planet, is now considered a dwarf planet which resides in the Kuiper belt. It is the largest body in the Kuiper belt. Comets are found in the Kuiper belt.

What caused the asteroid belt to form where it is?

Scientists and astronomers believe that the asteroid belt is the remainder of material that never formed into planets. The reason why these materials did not come together to form into a planet is because Jupiter is so massive. Because it is so massive, its gravitational force is very strong. It is strong enough to cause the planetessimals and other larger objects to collide and shatter. A new planet could not form because Jupiter’s gravity is so great that it overpowers and alters the gravity around it, and therefore planetessimals cannot merge together due to their own gravitation force. As a result, all this shattered material became asteroids and they orbited around the Sun in the same direction as all the planets.
Gravity from Jupiter is so strong that planets could not form around it – REMEMBER: planets form when their gravitational force is strong enough to overpower that of the Sun’s, but the small planetessimals in the area between Jupiter and Mars were too close to Jupiter to overpower Jupiter’s gravitational force.


How does the solar nebular hypothesis account for all the characteristics of the solar system?

A good solar hypothesis needs to account for four main characteristics of the solar system.

This is why the solar nebular hypothesis is the model that the astronomers consider the best. It explains all four characteristics very well, as opposed to other theories where some were either unexplained or were explained poorly.


The four terrestrial planets

There are 4 terrestrial planets in our solar system: Mercury, Venus, Earth, and Mars.

Works Cited:

http://slides.diigo.com/list/linzel/assignment-4-_-our-solar-system
I used the webslide to find most of the information about the solar nebular hypothesis.

http://blogs.answersingenesis.org/museum/pictures/Panel-6-Solar-System.jpg
I found the image of the solar system here.

http://www.nasa.gov/worldbook/index.html
I found information about the 4 terrestrial planets here.

http://en.wikipedia.org/wiki/Asteroid_belt
I used this webpage to find details about the asteroid belt.

http://science.howstuffworks.com/asteroid-belt2.htm
I used this webpage to find information about why the asteroid belt is located between the orbits of Mars and Jupiter.

http://en.wikipedia.org/wiki/Kuiper_belt
I found details about the Kuiper belt on this webpage.

http://en.wikipedia.org/wiki/Frost_line
I found information about the frost line here.

http://en.wikipedia.org/wiki/Rocky_planet
I used this webpage to find information about terrestrial planets.

http://en.wikipedia.org/wiki/Gas_giants
I used this site to find information about jovian planets.

http://en.wikipedia.org/wiki/Mars
I found details about Mars here.

http://en.wikipedia.org/wiki/Earth
I found details about Earth here.

http://en.wikipedia.org/wiki/Venus
I found details about Venus here.

http://en.wikipedia.org/wiki/Mercury_%28planet%29
I found details about Mercury here.


*Note: please click on the tables and diagram to make them full-sized. Thank you!!