Solar System Formation Notes

Planets

1. There are two types of planets: Terrestrial and Jovian.  Terrestrial o Small o Dense o Low mass o Rocky o Close to the sun o Few moons  Jovian o Large o Low density o Massive o Gas o Far from the Sun o Lots of moons 2. Temperature is the key to whether a planet will be terrestrial or Jovian. Terrestrial planets form where it’s hot and Jovian planets where its cold. 3. By definition, a planet has to pass these two tests:  Has to be large enough to be spherical in shape.  Has to be massive enough (heavy enough) not to be pulled out of its orbit by another body.

Dwarf Planets

1. Pluto is round, so it’s large enough, but its orbit has been altered by Neptune, so it’s not considered a planet. We call it a dwarf planet. 2. There are thousands (and probably hundreds of thousands) of dwarf planets. In odd orbits beyond Neptune.

Comets

1. Irregular, icy bodies beyond the dwarf planets. 2. These objects get pulled by the Suns gravity and melt as they approach the Sun. 3. Comets have collided with all of the planets of our solar system many times. 4. Our water comes from cometary collisions billions of years ago.

Asteroids

1. Irregular shaped bodies of rock between Mars and Jupiter. 2. Left over planetesimals from the early solar system. 3. They never formed into a body because of Jupiter’s gravitational pull and their distance from the Sun (they cooled two quickly).

Meteors

1. Pieces of an asteroid, planet or comet that heats up in the Earth’s atmosphere due to friction. 2. Most are very small (grain of sand)and never hit the ground. 3. Larger ones do hit and we call them meteorites.

Age of the Solar System

1. Believed to be about 4.6 billion years old. 2. Based on samples of lunar rocks, meteorites and earth rocks.

Solar System Formation

1. Solar Nebula: collection of cold gases (mostly hydrogen and helium along with heavier elements) that contract to form the Sun and the planets. 2. Condensation: gas turns to a liquid due to temperature. Close to the Sun it becomes blobs of molten iron, further out around the Earth’s orbit, silicate rocks, and the area around Jupiter it becomes ice. 3. Accretion: the blobs of liquid collide and stick together. These collisions are the beginning of planet building. 4. Planetesimals: These are the building blocks of planets. The larger blobs of liquid attract more of the smaller ones due to their higher gravitational pull. The planetesimals collide become larger still. The largest ones will win out to become planets. 5. Differentiation: When impacts occur, the heavier elements like iron sink to the center and the lighter elements float to the surface. This creates different layers and gives the body an iron core. 6. Protoplanets: These are the largest molten bodies sweeping out the smaller material around the Sun. They will become planets. Typically there are no large collisions left (although it can still happen at this stage). 7. Radiation Pressure: The Sun ignites for the first time and a shock wave of energy pours out blasting away the remaining dust and gas and leaving behind the planets to slowly cool. 8. New research shows that Jovian planets could not have formed this way- there wasn’t enough time for them to collect all of the gases they need for their large size. Today’s belief is that the Jovian planets formed directly bypassing accretion to make its core.

Exceptions

1. The Moon Triton around Neptune revolves the wrong way: it was a dwarf planet captured by Neptune. 2. Uranus is tilted on its side: a large collision blasted the gas planet apart and it reformed with a different tilt. 3. The Earth’s Moon is too large: A collision with another protoplanet tilted the Earth giving us seasons and ripping a piece of the Earth off to create our Moon. 4. Venus is upside down: A collision may have toppled the planet 180 degrees.

Exoplanets

1. Planets that are outside the solar system. We’ve already discovered some 2,500 of them. 2. Found mostly Jovian planets because they are larger. 3. Most of the Jovian planets were found in terrestrial orbits where they could not have formed. The current idea is that they formed in the colder areas and were pulled into their inner solar system by a passing star or other object. 4. We have found earth-like planets around other stars as well. Five of them are in the goldilocks zone- not too hot and not to cold, a place where water can exist as a solid, liquid and a gas at the same time.