General Astronomy

Formation of the Solar System

The Solar SystemHypothesis of the Origin of the Solar SystemEarly 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

The planets are not bunched together

The planets exist as independent bodies at progressively larger distances from the central Sun.

Each planet is relatively isolated in space.

Some Simple 'Rules': 2The 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)

Earth MercuryIn each case, the blue object is a perfect circle for reference

Some Simple 'Rules': 3The orbits of the planets all lie

nearly in the same plane.

(Mercury is a slight exception)

The planes swept out by the planet’s orbits are accurately aligned to within a few degrees.

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

Virtually all large-scale motions in the solar system are in the same plane and in the same direction.

The direction in which the planets orbit the Sun is the same as the direction in which the Sun rotates on its axis.

(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) TheoryA 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

Massive starSun

Filaments ‘dragged’ from Sunthrough gravitational interaction

Motion of filaments

The Capture TheoryThe 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.

Sun

Proto-star moves ona hyperbolic orbit

Material captured by Sun

Depleted proto-star

Tidally distortedproto-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 1400Venus 0.723 900Earth 1.000 600Mars 1.524 400Jupiter 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 NebulaRemains of the protostellar nebula were cleared away by:

• Radiation pressure of the sun

• Solar wind

• Sweeping-up of space debris by planets

• Ejection by close encounters with planets

Surfaces of the Moon and Mercury show evidence for heavy bombardment by asteroids.

Remaining Problems

• Venus’ retrograde rotation• Uranus’ tilt• The Sun’s angular momentum• The Earth’s Moon• Retrograde orbits of various moons

The Earth’s Moon

• 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

objectmaking 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?