Theme 7 – The Formation of
the Solar System
ASTR 101
Prof. Dave Hanes
First Impressions of the Solar System:
It’s Highly Organized!
...and yet again!
Organized inThree Senses
1.
Dynamical properties (motions) of the
constituents
2.
Physical properties
3.
Spacing and position
1. Dynamical Properties
The planetary orbits are:
- nearly circular
- all in the same direction
- all in nearly the same plane
Planetary spins (rotation) are
- almost all in the same sense as the orbital motions
- spin axes are (mostly) nearly perpendicular to the orbital
plane
Major moons
- orbit in the plane of the solar system
- rotate and orbit in the same sense as the planets
Warning!
Recognize What’s Significant
All orbiting objects must obey Kepler’s laws, whether they date
from the dawn of time or were launched into orbit around the
Sun yesterday.
That obedience tells us only that the Sun’s gravity dominates the
motions, but nothing about origins.
But is presumably significant that the planetary orbits are nearly
circular. (The orbits could, in principle, be ellipses of any
eccentricity you like.)
2. Physical Properties
There is a clear distinction between the:

Inner (terrestrial, rocky) planets: smaller, with solid
surfaces and relatively thin atmospheres
and the

Outer (Jovian, gas giant) planets: much larger, with
thick deep gaseous envelopes.
Planet
Mass
(Earth = 1)
Radius
(Earth = 1)
Average
density
Composition
Moons
?
Rings?
Mercury
0.06
0.38
5.43
Rocks, metals
0
No
Venus
0.82
0.95
5.24
Rocks, metals
0
No
Earth
1.00
1.00
5.52
Rocks, metals
1
No
Mars
0.11
0.53
3.93
Rocks, metals
2
No
Planet
Mass
(Earth = 1)
Radius
(Earth = 1)
Average
density
Composition
Moons?
Rings?
Jupiter
318
11.2
1.33
H, He, hydrogen
compounds
>60
Yes
Saturn
95.2
9.4
0.70
H, He, hydrogen
compounds
>30
Yes
Uranus
14.5
4.0
1.32
H, He, hydrogen
compounds
>20
Yes
Neptune
17.1
3.9
1.64
H, He, hydrogen
compounds
>10
Yes
Warning
Another Irrelevance!
It is no surprise that the inner planets (Mercury,
Venus,…) are currently warmer than the outer
planets (Saturn, Uranus,…). This is merely a
consequence of their proximity to the Sun.
But the warm surroundings may have had some
influence on the formation process itself, in the
distant past.
Why are the Rocky (Terrestrial)
Planets Nearest the Sun?
Do the rocky planets settle closer to the sun, with the lowdensity ones taking up orbits farther away?
NO!! Remember the astronauts in the Space Station. All
bodies ‘fall’ equivalently under gravity
If you replaced Saturn (a low-density gaseous body) with a
rocky object (say, the Earth), it would continue to orbit
in exactly the same way. The rocky object would not
‘settle’ towards the sun.
This organizational feature has a different explanation.
3. Spacing and Position
The spacing looks regular, in some sense.
Bode’s Law (1772)
[no need to memorize the numbers]
Here’s a procedure (found by trial and error):



Adopt a “starting number” = 4
For successive numbers, add 3, 6, 12, 24, 48, 96, 192,
384… in turn.
Divide by 10
What do we get?
A Good Match to the
Planets Known to Bode!
But what about the ‘missing’ entries?
(0+4)/10 = .40
Mercury
0.39
(3+4)/10 = .70
Venus
0.72
(6+4)/10 = 1.00
Earth
1.00
(12+4)/10 = 1.6
Mars
1.52
(24+4)/10 = 2.8
??
??
(48+4)/10 = 5.2
Jupiter
5.20
(96+4)/10 = 10.0
Saturn
9.54
(192+4)/10 = 19.6
??
??
(384+4)/10 = 38.8
??
??
Subsequent Discoveries
- two hits, then a miss
Uranus (1781)
Neptune (1846)
Ceres (1801)
Ceres: Not a Planet
the first of millions of asteroids found
Now being orbited by a spacecraft!
Ceres is the Largest Asteroid
[but is still very small]
Bode’s Law:
One Safe Conclusion
The orbits of the planets are not completely haphazard. The
spacing, although not uniform, is regular in some sense.
To some degree, this is inevitable! If you had two planets in
orbits that were quite close to one another, their mutual
gravitational tugs would ‘tweak’ the orbits and, over time,
lead to big changes. Planets can even migrate in this way.
For long-term stability, planetary orbits must be reasonably far
apart.
But We Are Here
The Earth itself can not have ‘migrated’ much
over the ~4.6 billion year life of the Solar
System. (If it had, the great variations in
temperature mean that we would not be here to
discuss the issue!)
On the other hand, we may be ‘lucky survivors’ –
many other planetary systems may be quite
unstable. (We will return to this.)
Solar System Formation
Three Possible Scenarios
1.
2.
Haphazard accumulation: The Sun forms first, then
later somehow collects a ‘family’ of planets and other
things
Uniquely catastrophic event: a near-collision
between two stars sparks the formation of planets.
This suggests that planetary systems might be rare.
3.
Routine formation: planets and so on form along
with the Sun. This suggests that planetary systems
should be found around many stars.
1. Haphazard Accumulation?
NO!

There is too much order and regularity in the
Solar System.

There is also no mechanism for the sun to
capture the planets. (As they fall towards it,
they pick up speed, then just race on by.)
Anyway, it doesn’t really address the question. Where did
the planets come from before capture by the Sun?
2. Near-Catastrophic Formation
[a popular theory ~1900]
Invoked the close passage of two stars, plus tidal effects.
On the Positive Side
It would explain:

Why planets orbit in the same plane

Why planets orbit in the same direction

Maybe even why the biggest planets are in the
middle of the distribution
But Near-Collisions
of Stars are Rare!
Stars move at modest
speeds, in random
directions, and are
separated by huge voids.
We expect very few ‘close
passages’ in the entire
Galaxy of 100 billion stars.
One Clear Implication
If that’s how Solar Systems form, there will
be very few of them around! We might be
unique in the whole galaxy.
Conversely: if we find evidence of lots of
planetary systems, they must form in some
other more routine way!
Anyway: Two Fatal Flaws
1.
2.
Gas pulled off a regular star would be too hot
to condense! – it would just evaporate into the
vacuum of space
There are angular momentum problems:
somehow you have to form rapidly spinning
big planets in huge orbits, but with a Sun that
is rotating very slowly. This is hard to explain.
3. The Nebular Hypothesis
[dates back to the mid-1700s]
The Solar System formed as a unit from a once-distended
cloud, or ‘nebula,’ of gas and dust.
(Dust = small particles, grains,
flakes, and chips of common
elements, ices and minerals.
Don’t visualize household dust!)
Summarized in Words
An originally distended cloud of interstellar gas
 starts to shrink under its own gravitation
 spins faster (by the conservation of angular
momentum!)
 flattens out as it does so
 gets hotter at the centre as atoms collide where
the Sun is forming
 condenses into small pebbles to start with
 these merge into larger pieces and planets
Summarized
in Pictures
…and Again
Note the timescale: from gas cloud to planets in about 100
million years. This is about 2% of the age of the Solar
System, so it all happens relatively fast!
Issues to Address
1.
2.
We still see gas clouds in space. What starts the collapse
to produce a planetary system? Why hasn’t all the
available gas in the galaxy turned into stars and planets?
Why are there planets of different sizes and compositions in
different locations? (i.e. why the organization of physical
properties in the SS?)
3.
Why are the motions, directions of rotation, etc so
organized? (i.e. why the organization of dynamical
properties in the SS?)
4.
Can we see other Solar Systems in formation?
Gas Clouds in Space
Why don’t the atoms all rush together under
gravity? (Analogous question: why doesn’t all the
air on Earth settle right down to the ground?)
Answer: the gas is warm,
and the random motions
of particles provides a
sustaining pressure.
To Trigger a Collapse
We need to cool the gas, so the supporting pressure is
reduced; or compress the cloud a bit. This could be the
result of a shock wave from a nearby supernova, as a
massive star dies in a huge explosion; once the atoms
are closer together, the force of gravity between them is
stronger.
These things happen from time to time, so there are
continuing cycles of star formation – some going on right
now.
Composition
The pre-solar-system gas cloud was


big (millions of times the volume of the
present solar system)
like the universe overall, made of
Hydrogen (2/3) and Helium (1/3), with
mere traces (a few %) of other material
What Happens as it Gets Denser?
Atoms, dust grains and so
on get packed closer
together and start to
merge and condense.
A‘Fog’ – but not just water droplets.
Atoms of all types bind together to form complex minerals
and grains, depending on the ambient temperature.
The Temperature Dependence
Near the proto-Sun, where it’s getting quite hot, only the
relatively rare heavy elements can condense. This yields small
rocky planets.
Farther out, everything condenses, yielding giant hydrogen-rich
planets.
Continued Growth
Pebbles  Planetesimals  Planets
A steady buildup to a final few larger planets
whose gravity tends to dominate their local
zones
Physical Organization Explained!
The inner planets are small and rocky because 90%+ of the original
material never condenses that close to the sun. (Only a fraction of
the available building material is used!)
The outer planets are big and gaseous because all the material
condenses there, and is also captured by the gravity of these huge
objects -- and it’s mostly H and He. (All the building material
gets used up!)
Dynamical Behaviour Explained
Grains, pebbles, rocks
and planetesimals moving
originally in oblique
directions suffer many
collisions and get ‘caught
up in traffic.’
In the end, the whole system – planets and moons – tends to
have same sense of revolution around the sun, the same senses
of rotation (spin), to be in same plane, etc. (Of course, there will
be continued gravitational interactions, with the smallest pieces
most affected. Not everything moves in circles or in the
flattened plane!)