Solar System Formation

Where to Start?

• When trying to figure out how the solar system was created, we had a few clues...

• 1st Clue: All of the planets orbit the sun in the same direction.

2

• 2nd Clue: Nearly all of the planets rotate in (pretty much) the same direction (counter-clockwise when viewed from above the North Pole), with the exception of Uranus and Venus (and Pluto, which isn't a planet).

• The angle this axis makes with a vertical arrow is called the orbital tilt or obliquity of the planet.

3

• 3rd Clue: All of the planets are (more or less) in the same plane, making the solar system a pretty flat disc.

• Mercury has the largest deviation... off by an angle of about 7o (and Pluto is off by about 17o).

• This angle is called the inclination of the planet’s orbit. • It is the angle that the orbital plane of the planet makes

with the ecliptic (which is pretty much the disc of the solar system, or the plane of the Sun's equator).

• Note that (as in most cases) Pluto doesn’t really seem to be a team player.

4

So What’s the Theory of How This Formed?

5

Start with a Solar Nebula

• Solar system formation begins with a huge cloud of gas & dust called a solar nebula.

• For an idea of the size of this cloud, think of this... – Pluto is (on average) about 40 AUs from the Sun. – The original solar nebula that formed our solar system

had a radius of about 10,000 AUs. – This would have given it a volume about 15 million times

larger than a sphere that would encompass our entire solar system now.

– If out solar system was the size of a basketball, the solar nebula that formed it would have been the size of the basketball stadium.

– Despite it's huge size, it's total mass wasn't much larger than that of our Sun now.

6

• The cloud is cold - about -260oC.

• Absolute zero (the coldest possible temperature that can ever be reached) is -273oC.

• Temperature can be thought of as the average amount of thermal energy per unit volume. Since there are so few particles per cubic meter, the temperatures are extremely low.

7

• The gas making up the cloud is almost entirely hydrogen & helium – the 2 most abundant elements in the universe.

• There are some heavier elements (carbon, oxygen, iron, etc) that come from old, dead stars.

8

Relative Elemental Abundances in the Universe

9

• The type of dust that we’re talking about in a nebula is very small. It’s not like dust in your house – more like particles of cigarette smoke.

• Particles in the cloud are extremely small – the largest might be the size of a spec of sand.

A particle of interstellar dust, approximately 1 micrometre

(or micron) in size.

1000 m = 1 mm

10

Interplanetary Dust Particles (IDP)

11

A Solar Nebula

In general, nebulae are some of the most beautiful

features in nature. 12

The Collapse Begins

• All the dust and gas particles exerts a gravitational force on all of the other particles, according to Newton's Law of Universal Gravitation. – See end of slides for more detail on this if interested.

• This causes the whole cloud to contract towards the centre.

13

• At the centre, there will be a relatively dense region of the cloud, which we call a protosun.

• This is the hot, dense part of the cloud that will eventually turn into a our Sun.

• The planets form from the rest of the cloud farther out, which only accounts for about 0.1% of the entire mass of the original cloud.

14

The Kelvin Temperature

Scale

To find the Celsius temperature from the Kelvin,

just subtract 273.

Note: 0 K is the coldest anything can ever get.

15

Heating Up The Protosun

• As the particles in the cloud move towards each other (and ultimately the centre) they increased in speed.

• Upon reaching the centre they collide with each other, converting their kinetic energy (energy of motion) into thermal energy.

• The temperature in the protosun quickly rises to thousands of Kelvin.

• After about 100 million years, the centre of the protosun increases to a few million Kelvin, while the surface is still about the same temperature it was before (a few thousand Kelvin).

16

Fusion Begins

• At the high temperatures and pressures at the core of the protosun, particles are very close together and moving at extremely high speeds.

• This allows subatomic particles to slam together, and begin a nuclear chain reaction.

17

• Note that this is nuclear fusion (small atoms fusing to form bigger ones), while nuclear reactors on Earth use fission (big atoms breaking into smaller parts).

Fusion Fission

18

A Star is Born!

• When the fusion begins to occur, the core of the protosun becomes a massive nuclear generator, and we now consider it to be a true star.

Fusion!

Protosun / Protostar + Fusion = Sun / Star 19

We've explained how the Sun (a star) formed.

Now what about planets?

20

Rotation

• If there is no initial rotation in the solar nebula, then all material will fall inward and become part of the sun.

• If however, there is a slight rotation in the cloud, then the material will continue to spin faster and faster as in falls inward (due to the conservation of angular momentum).

21

• As it spins, the randomly shaped cloud tends to flatten out into a flat disc.

• This is the same as a ball of dough flattening as it is spun by a pizza chef... just not as tasty.

22

• The particles in the cloud will begin to experience a centrifugal force which makes them want to pull away from the protosun.

• This is happening long before the sun itself is created.

• At some point, this centrifugal force outwards will balance the gravitational force inwards, and the particle will happily orbit the protosun.

23

• This disc of particles is why all the planets formed in essentially the same plane, and orbit in the same direction.

24

An artist’s illustration of a protoplanetary disc.

25

An actual photo of a protoplanetary disc found in the Orion nebula.

26

Growing Planets

• Chucks start to form (because of gravity).

• From the dust comes chunks the size of golf balls... those come together to form rocks the size of houses... then mountains. The bigger they get, the bigger their gravitational pull, and the more they attract.

27

• The largest chunks become planetesimals (which are similar to most asteroids found now, with diameters of about 10 km).

• These planetesimals begin to collide with each other and form larger bodies... protoplanets, perhaps the size of our moon.

28

• Finally, these protoplanets either collide with other protoplanets and merge, or were far enough away and stay alone. As the last of the small chunks got eaten up, the planets were born.

• This entire process of smaller chunks gathering to form larger chunks is called accretion. The disc of gas that started it all is also known as an accretion disc.

29

Accretion

30

Different Types of Planets

• Although all the planets formed because of accretion, they did so a little differently depending on how far they were from the Sun (since the temperatures vary greatly).

• The accretion process is still not finished 4.6 billion years later! We still see planets (including Earth) gathering dust and rocks from space.

31

• Near the centre of the solar system, the temperatures are relatively high (you'll get quite the sunburn tanning on Mercury), but the temperatures farther out are a lot lower. Near the outer edge of the cloud, temperatures drop off to about 50 K (-220oC).

• This means that the inner solar system is hot enough that things like water (H2O), methane gas (CH4), and ammonia (NH3) to exist as liquids or gases, while farther out they exist as solid ice. We tend to think of ice as solid water, but it often just refers to the solid form of anything that's normally a liquid or gas at room temperature.

32

• This temperature difference is one of the reasons why the inner planets are all rocky (made of heavy stuff) and the outer planets are made of lighter stuff, like gases.

33

Mercury Venus Earth Mars

Inner Planets (Rocky)

34

Outer Planets (Gas Giants)

35

One Last Thing...

• We know that heavy elements (basically anything other than hydrogen and helium) exits in extremely rare amounts in interplanetary space.

• But look at the planets... especially the inner ones. Lots and lots of heavy elements!

• Where did that material come from?

36

37

• Heavy elements are the byproduct of nuclear fusion (that occurs inside of stars).

• Nearly everything on the inner planets (and a good portion of the outer planets as well) came from the core of a star... one that exploded in a violent supernova long ago.

38

Nuclear Fusion (making bigger atoms)

39

Supernova (an exploding star)

40

• It is believed that the accretion of the disc that made our solar system may have been given a jump start by the material passing through from a nearby supernova.

• This could have given the disc its initial rotation that allowed the planets to form.

41

Making a Solar

System

42

43

44

Solar System Objects (The Sun, Planets, Moons, Asteroids and Comets)

45

Newton's Law of Universal Gravitation (You are not required to know this!)

• All bodies of mass "m" are attracted to all other bodies of mass "M" (both in kg) by a force "F" (in Newtons) based on their masses and the distance between them, "r" (in metres) according to the equation...

– where G is the Universal Gravitational Constant

• G = 6.67 x 10-11 Nm2/kg2. 46