Lecture 10

The Formation of the Solar System

We have spent some time investigating the member objects of the solar system and outlining some of their properties.  We now have the factual basis needed to discuss possible models for the origin and evolution of the solar system.

Some constraints (i.e. pieces of evidence and things to be explained)

  1.Each planet is relatively isolated in space.  (Bode-Titius "law"????)
  2.The orbits of the planets are nearly circular. (Except
    Mercury and Pluto.)
  3.The orbits of the planets all lie in nearly the same plane.
     (Again, Mercury and Pluto are slight exceptions.)
  4.The direction in which the planets orbit the Sun (counterclockwise as
    viewed from above Earth's North Pole) is the same as the direction in
    which the Sun's rotates on its axis.
  5.The direction in which most planets rotate on their axis is roughly the
    same as the direction in which the Sun rotates on its axis.
  6.Most of the known moons revolve about their parent planets in the
    same direction that the planets rotate on their axes.
  7.Our planetary system is highly differentiated.

• The inner terrestrial planets are characterized by high densities, moderate atmospheres, slow rotation rates, and few or no moons.
• • Furthermore, they show a tendency to greater iron content toward the Sun.
• By contrast, the jovian planets are farther from the Sun, have low densities, thick atmospheres, rapid rotation rates, and many moons.
• • Furthermore, they show a tendency to greater fractional ice and methane content outward.
• The asteroid belt is also differentiated in iron, silicate and carbon content

  8.The asteroids seem to be left over "planet forming" material, hindered in this pursuit by Jupiter:

• This gives us an age and idea of content and process in the era of formation

  9.The comets are primitive, icy fragments that do not orbit in the ecliptic
    plane and reside primarily at large distances from the Sun.

• Must explain Kuiper belt and Oort cloud.

 10. The ages of the oldest rocks show a common time of origin about 4.6 billion   years ago (oldest rocks on Earth 3.9 b.y.; Moon 4.4 b.y.; Meteorites 4.6 b.y.)
 11. The cratering record shows a period of intense activity in the first billion years.
 12. Extrasolar planets and disks have recently begun being observed.
  13. The angular momentum problem ????  (Why does the Sun have so little?)

Three Models

Haphazard accumulation: in which the Sun and planets from independently and separated in space.  The Sun's gravitational field periodically "picks up" a planet.

Unique catastrophic event: in which (e.g.) the Sun and another star experience a near collision which pulls material from the Sun.  This material eventually forms the planets.

Commonplace evolution ( esp. condensation model): in which a cloud of gas and dust collapses through a "natural" evolutionary process to form the solar system (see below).

Haphazard accumulation is completely at odds with 2-7 above.

• It would require a series of such incredible coincidences to create a solar system with such patterns that this hyposthesis is easily struck down.

A near collision between star is (would be) and exceedingly rare event.

• This is at odds with the fact that we are now finding other nearby stars with planets (see below).

A collision would produce blobs with the correct rotation patterns etc,

• but the material would tend to boil off rather than condense to form planets.

The stripped material would not have the composition  to give that observed for the inner planets.

Last Model

The condensation model

The condensation model starts with a large diffuse cloud of gas and dust.

• 1 light year across (1000 Pluto distances -- so 1 billion times the present volume).
• • Density of a few thousand atoms per cc (the air in this room is about 10¹⁹ atoms per cc.)

How does the collapse begin?

There are many clouds of gas and dust in the disk of our galaxy

• Gas is primarily H and He
• Dust is higher elements cooked in stellar interiors and distributed via supernovae explosions.
• • About 10^-5 cm in size
  • Ice and silicates
  • They perform the important function of allowing the cloud to cool more efficiently via their infrared radiation.
  • Also site of some molecular formation
• In a static (non or slowly collapsing) cloud there is a balance between the gravitational self-gravity of the cloud and the internal pressure of the gas.

Collapse begins when another passing cloud or a nearby supernova explosion causes a shockwave passes through the cloud.

• The density increase from the shock will cause the self-gravity to overcome pressure and collapse will begin.

Collapse tends to flatten cloud to a disk and heat it.

Due to angular momentum conservation, the collapsing cloud will flatten into a disk.

• The nebula begins with a net angular momentum.
• As the cloud collapses the particles will collide so that motion perpendicular to the net direction of spin will cancel out.
• • But conservation of ang mom'n ensures that there is always the same net rotational motion.
  • This rotational motion will be what remains even after the other random motions cancel out.
• Think of it like the pizza dough.
• • As the cloud shrinks due to collapse it spins faster because of cons. of ang. mom. (smaller radius so speed goes up, like when a skater pulls in arms)
  • so that the cloud will flatten out like planets do due to rotation.
  • • More flattening the greater the rotation rate.

Another effect of the collapse is to heat the material of the disk (gravitational pot'l is converted to heat energy).

• The temperature rises in towards the core.
• At high enough temp. the dust will evaporate.
• As the gas cools it will re-form dust particles.
• • But now there will be a trend outward (matching the trend of temperature dropping outward).
  • • The particles that can accrete closer to the proto-sun will be richer in metals and silicates.
    • Out further the temp will drop enough to allow more volatile compounds like water (ice) to form.

The three stages of planet formation

Stage One

The dust particles act as nucleation sites for further accretion (snowballing via molecular attraction)

• As particles get bigger, they will grow even faster (via collisions and sticking).
• In this way particles will grow to sizes of 100's of km.
• Gravity then becomes significant enough to assist in the accretion process -- so even faster growth
• End with millions of randomly placed and moving planetesimals and gas remaining in disk

Stage Two
The planetesimals will then collide to form planets

• The orbits space out as a dominant body (protoplanet) develops in each region.
• Orbits circularize due to statistical process of collisions (other motions are evened out as in collapse from spherical cloud).

Stage Three
We are now about 100 million years along in the process

• The larger jovian planets have enough gravity to continue to the third stage
• They will continue to grow by sweeping up the H and He gas around them due to their large gravitational fields.

(Why are the jovians larger prior to 3?)
 

• The T rises into the centre as does the density.
• Just inside the orbit of Jupiter the T is low enough to allow ice to form.
• • This provides much more material for the outer planets to accrete in stages 1 and 2, so they are larger.
  • J and S are bigger because they are in the region of greatest density
  • • (Some say that we should always expect a Jupiter to be just beyond the ice point.)

(Where does the gas go in the inner system?)

As we will discuss in more detail later, young protostars can be observed in other regions of the disk and this gives us some idea of what occurs in the next step.

• Just prior to the turning on of the nuclear furnace within protostars they go through a phase of letting off large jets of gas (like a super solar wind) called the T Tauri phase.
• This wind clears out the surrounding the gas.
• The process is not fully understood, and has been derisively labelled the "Magic Broom"
• We have discussed this briefly as well in terms of the loss of primary atmospheres.

What about the Kuiper belt, Oort cloud and water on the inner planets?

The Kuiper belt is a remnant of the era of formation.  As well as providing some of the material for the formation of the outer planets, icy objects from this region (and further in) also experienced near collision that either

1) sent them plunging inward to collide with an inner planet (explaining water content there)
or
2) sent them flying outward creating the Oort cloud

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