General Astronomy
Formation of the Solar System
The Solar System
Hypothesis of the Origin of the Solar System
Early Ideas:
• The Collision Theory
– A passing star's gravity in a collision (or close miss) caused
the material of the sun to be 'pulled out' eventually
solidifying into the planets
• The Capture Theory
– The planets, and everything else in the solar system, was
simply captured when drifting about by the Sun's gravity
and eventually forced into the orbits we observe.
• The Nebular Theory
– The Sun and the planets (and everything else) condensed out
of a vast nebula composed of gas
Some Simple Observations
Before we look at the possible
scenarios, let's look at a set of simple
observations which must be explained
by any theory of formation.
If the theory cannot cope with any of
these it is in trouble.
Some Simple 'Rules': 1
Each planet is relatively isolated in space.
The planets are not bunched together
The planets exist as independent bodies at
progressively larger distances from the central Sun.
Some Simple 'Rules': 2
The orbits of the planets are nearly circular.
Exception: Mercury
While all planet's orbits are ellipses, they are nearly circular
(unlike most comets and some asteroids)
Mercury
Earth
In each case, the blue object is a
perfect circle for reference
Some Simple 'Rules': 3
The orbits of the planets all lie
nearly in the same plane.
The planes swept out by the planet’s orbits are
accurately aligned to within a few degrees.
(Mercury is a slight exception)
Mercury
7.004 Degrees
Venus
3.394
Earth
0.0
Mars
1.850
Jupiter
1.305
Saturn
2.490
Uranus
0.773
Neptune
1.774
Some Simple 'Rules': 4
The direction in which the planets orbit the Sun is
the same as the direction in which the Sun rotates
on its axis.
Virtually all large-scale motions in the solar system are in
the same plane and in the same direction.
(Other than some comet orbits)
Some Simple 'Rules': 5
The direction in which most planets rotate is
roughly the same as the direction in which the
Sun rotates.
Exceptions: Venus, Uranus
Venus rotates retrograde (very slowly)
Uranus is 'tipped' over so that it depends on
how you measure the angle of inclination as to
the direction it rotates
For example, Suppose that the planet is rotating such
that the top of the page is "in" and the bottom is "out"
Then if the North Pole is to the left, the rotation is prograde;
if the North is to the right, then it rotates retrograde
Some Simple 'Rules': 6
The direction in which most of the major moons
revolve about their parent planet is the same as
the direction in which the planets rotate on their
axes.
There are exceptions amongst the smaller moons
Some Simple 'Rules': 7
The planetary system is highly-differentiated.
The inner planets are characterized by high densities,
moderate atmospheres, slow rotation rates and few or no
moons or ring systems
The outer planets are characterized by low densities,
thick atmospheres, rapid rotation rates, many moons
and ring systems.
Some Simple 'Rules': 8
Asteroids are very old and exhibit a range of
properties not characteristic of either the terrestrial
or jovian planets or their moons.
Asteroids share, in rough terms, the bulk orbital
properties of the planets. However, they appear to be
made of primitive, unevolved material.
Similarly, meteorites are the oldest rocks known.
Some Simple 'Rules': 9
Comets are primitive, icy fragments
that do not necessarily orbit in the
ecliptic plane.
Most reside primarily at large distances
from the Sun in the Kuiper Belt or the
Oort Cloud.
The Collision (Tidal) Theory
A passing star's gravity in a collision (or a close
miss) caused the material of the sun to be
'pulled out' eventually solidifying into the
planets.
Could the Sun lose enough material to form all the rest of the
solar system?
Easily. The Sun has over 98% of all the material in the system
This makes a solar system a rather freak occurrence.
There are problems with several of our observations.
Particularly 1, 7, 8, and 9 (isolation, differentiation, comets and
asteroids)
Tidal theories
• 1794: G. L. Leclerc, Comte de Buffon
– suggests material that formed the planets was
‘ripped’ from the Sun by a colliding comet
• 1917: James Jeans proposed ‘modern’ tidal
theory involving the interaction between the Sun
(all ready formed) and a massive star
Motion of filaments
Sun
Massive star
Filaments ‘dragged’ from Sun
through gravitational interaction
The Capture Theory
The planets, and everything else in the solar
system, was simply captured when drifting
about by the Sun's gravity and eventually
forced into the orbits we observe.
Again, the same observations give problems:
Why the spacing between planets?
Why are they grouped as they are?
Why the general prograde directions of revolution and
rotation?
Capture the Oort Cloud??
Capture
• 1964: M. Woolfson presents details of the
Capture Theory
– variant of the Tidal Theory
– tidal interaction between the Sun and a
collapsing, low-density proto-star.
Depleted proto-star
Material captured
by Sun
Sun
Proto-star moves on
a hyperbolic orbit
Tidally distorted
proto-star
The Solar Nebula Theory
• 1755: Immanuel Kant suggests that the
solar system formed from a collapsing
cloud of gas
• 1796: P. S. de Laplace discusses the idea
of a collapsing nebula of gas and includes
the effect of rotation
– this is the basic solar nebula model
The Nebular Theory
The Sun and the planets (and everything else) condensed out of
a vast nebula composed of gas
Nebular Theory
This does better.
In fact it explains nearly everything
except for why the inner and
outer planets are different.
In this particular, it fails completely
Origin of the Solar System
Each of our ideas have failed.
What are we missing?
There are two features which would certainly
be present in the early nebula which we did
not take into consideration
Heat and Dust
Temperature in the solar nebula
Planet
Distance (AU) Temperature (K)
Mercury
0.387
1400
Venus
0.723
900
Earth
1.000
600
Mars
1.524
400
Jupiter
5.203
200
At about 3AU the temperature falls to 300K
(Freezing point of H2O)
Formation of planetesimals:
Distance from the Sun and temperature play the key role
• Inner planets formed from high-density metal oxides
• Outer planets formed from low-density ices
Condensation of Different Chemicals
Temperature
Accretion
• After condensation comes accretion
– the growth of grains through collisions and
‘sticking’
– this is the real planet building process
• accretion proceeds in two ways
– growth by collision due to the geometric cross
section
• direct impacts upon the ‘seed’ grain
– growth by collision due to gravitational
attraction
• sweeping-up of material from a region much
larger than the grain diameter
The coalescing of planetesimals into protoplanets
Gravitational instabilities led to clumping
The T Tauri Phase
• Much of the remnant solar nebula gas is cleared by
a strong wind that develops when the proto-Sun
undergoes what is known as a T Tauri phase
– named after the prototype: variable star ‘T’ in
the constellation of Taurus
– this phase occurs before a star initiates steady
nuclear burning in its core and lasts for about
107 years
Clearing the nebula
• Radiation pressure from the protosolar
radiation
• The solar wind: flow of ionized gas
• The sweeping up of debris by the planets.
Heavy bombardment ~ 4 billion yr ago!
• Ejection of remains to the outskirts of
the Solar system in close encounters with
planets.
Clearing the Nebula
Remains of the protostellar nebula were cleared away by:
• Radiation pressure of the sun • Sweeping-up of space debris by planets
• Solar wind
• Ejection by close encounters with planets
Surfaces of the Moon and Mercury show
evidence for heavy bombardment by asteroids.
Remaining Problems
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Venus’ retrograde rotation
Uranus’ tilt
The Sun’s angular momentum
The Earth’s Moon
Retrograde orbits of various moons
The Earth’s Moon
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Moon has low average density - hence no iron core.
Moon's surface rocks have no water.
Moon formed at least 4.4 billion years ago.
Moon's orbital plane inclined to plane of Earth's equator.
Moon is an overly large satellite.
No other terrestrial planet has “natural” moons
Tilt of Earth’s axis.
The Earth’s Moon
• Fission Theory - Moon thrown off by
young rapidly spinning Earth
– Requires identical compositions
– Explains low Moon density
– Cannot explain Moon's inclined orbit
• Accretion Theory - Moon and Earth
formed together
– Similar but not identical compositions
– Why do Mercury & Venus not have moons?
• Capture Theory - Moon formed
elsewhere and captured by Earth
– Requires totally different compositions
– Earth's gravity accelerates incoming object
making capture more difficult
The Earth’s Moon
• Giant Impact Theory
– Mars-size planetesimal collides with
Earth
– Metal sinks to Earth’s core, volatiles lost,
mantle material “splashed” into space
– Mantle material coalesces into Moon
– Moon spirals away from Earth over time
Odd Motions and Angles
• As in the case of Earth’s Moon, collisions can
produce both Venus’ retrograde rotation and
Uranus’ tilt
– During the Era of Heavy Bombardment
collisions were common
• Retrograde orbits of various moons can be
explained by capture phenomenon
• Pluto’s problems disappear if it is a KBO
The Angular Momentum Problem
• The Sun does not have enough angular
momentum!
– As the gas condensed and pulled
together, the spin should have become
faster
– Instead most of the solar system’s
angular momentum is in the planets, not
the Sun
– Perhaps magnetic braking?