• A cloud of interstellar gas and/or dust (the "solar nebula")
  is disturbed and collapses under its own gravity. The disturbance
  could be, for example, the shock wave from a nearby supernova.
  
  

  
  

• As the cloud collapses, it heats up and compresses
  in the center. It heats enough for the dust to
  vaporize. The initial collapse is supposed to take
  less than 100,000 years.
  
  

  
  

• The center compresses enough to become a protostar
  and the rest of the gas orbits/flows around it. Most
  of that gas flows inward and adds to the mass of the
  forming star, but the gas is rotating. The centrifugal
  force from that prevents some of the gas from reaching
  the forming star. Instead, it forms an "accretion disk"
  around the star. The disk radiates away its energy and
  cools off.

• The gas cools off enough for the metal, rock and (far
  enough from the forming star) ice to condense out into
  tiny particles. (i.e. some of the gas turns back into dust).
  The metals condense almost as soon as the accretion disk
  forms (4.55-4.56 billion years ago according to isotope
  measurements of certain meteors);
  the rock condenses a bit later (between
  4.4 and 4.55 billion years ago).
  
  

  
  

• The dust particles collide with each other
  and form into larger particles. This goes on
  until the particles get to the size of boulders
  or small asteroids.
  
  

  
  

• Run away growth. Once the larger of these
  particles get big enough to have a nontrivial
  gravity, their growth accelerates. Their gravity
  (even if it's very small) gives them an edge over
  smaller particles; it pulls in more, smaller particles,
  and very quickly, the large objects have accumulated
  all of the solid matter close to their own orbit.
  How big they get depends on their distance from
  the star and the density and composition of the
  protoplanetary nebula. In the solar system, the
  theories say that this is large asteroid to lunar
  size in the inner solar system, and one to fifteen
  times the Earth's size
  in the outer solar system. There
  would have been a big jump in size somewhere
  between the current orbits of
  Mars and Jupiter:
  the energy from the Sun would have kept
  ice a vapor at closer distances, so the solid,
  accretable matter would become much more common
  beyond a critical distance from the Sun. The
  accretion of these "planetesimals" is believed
  to take a few hundred thousand to about twenty
  million years, with the outermost taking the longest
  to form.

• At this point, the solar system is composed
  only of solid, protoplanetary bodies and gas
  giants. The "planetesimals" would slowly collide
  with each other and become more massive.
  
  

  
  

• Eventually, after ten to a hundred million years,
  you end up with ten or so planets, in stable orbits,
  and that's a solar system. These planets and their surfaces
  may be heavily modified by the last, big
  collision they experience (e.g. the largely
  metal composition of Mercury or the
  Moon).

Large bodies in the solar
system have orderly motion. 

All planets and most
satellites have nearly circular orbits, in the same direction, and nearly in
the same plane.  The Sun and most of
the planets rotate in the same directions as well.

Planets fall into two main catelgories.

Small, rocky terrestrial planets near the Sun and
large, hydrogen-rich jovian
planets farther out.  The jovian planets have many moons and rings made of rock and
ice.

Swarms of asteroids and
comets populate the solar system.

Asteroids are concentrated
in the asteroid belt, and comets populate the regions known as the Kuiper belt and the Oort cloud.

There are several important
exceptions to these trends.

Planets with unusual tilts,
very large moons, or moons with unusual orbits.

Large and diffuse, slowly rotating – it had some angular momentum.

This
property of the cloud primarily accounts for the motions of the planets. Why?
Think about momentum and energy…

<!--[if !supportLists]-->·     
<!--[endif]-->Composed primarily of hydrogen and helium, but there must have been some heavier elements,
including metals, since we find them in the terrestrial planets for example.

This
property of the cloud will dictate how the planets ended up in two main
types.  Think about phases of matter…

The collapse is triggered by
an increase in density, and driven by gravity.

 What do we mean by “collapse”? 

During the
collapse…

<!--[if !supportLists]-->1.  <!--[endif]-->The cloud’s
rotation rate increases, due to
conservation of angular momentum.

<!--[if !supportLists]-->2.  <!--[endif]-->The cloud
heats up, as compressing the gas
cause the particles to speed up, increasing the
temperature of the gas and dust particles.

<!--[if !supportLists]-->3.  <!--[endif]-->The disk of
gas and dust flattens, as collisions
between the particles of the cloud to lose energy in the direction perpendicular
to the cloud’s rotation.

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So we can now account for one
of the four challenges:  the orderly
nature of the orbits of planets in the solar system is due to conservation of
energy and angular momentum during the collapse of the gas cloud from which
they formed. 

The direction of rotation is dictated by the angular momentum of the
cloud. 

The inclination of the planets is due to the flatness of the nebular disk after
collapse.

Why are there two types of planets, terrestrial and
Jovian?

<!--[if !supportLists]-->A.  <!--[endif]-->The force of gravity due to the massive Sun draws the
heavier, dense material of the terrestrial planets closer.

<!--[if !supportLists]-->B.  <!--[endif]-->Initial orbits of the terrestrial planets bring them
closer to the Sun where they fall into smaller orbits.

<!--[if !supportLists]-->C.  <!--[endif]-->Near the Sun, only heavy elements and rocky material
can condense from the solar nebula.

<!--[if !supportLists]-->D.  <!--[endif]-->Jovian planets form first and draw much of the gaseous
material to them via gravity, leaving only the heavier elements and rocky
material behind.

Accretion

How to grow planetessimals:

<!--[if !supportLists]-->·     
<!--[endif]-->Initially small
particles of gas and dust were able to stick together via their electrostatic
attraction.

<!--[if !supportLists]-->·     
<!--[endif]-->As they grew larger,
their gravity began to be strong enough to attract particles as well, and their
growth accelerated.

<!--[if !supportLists]-->·     
<!--[endif]-->Once large
enough, gravity pulls the planetessimal into a
spherical shape.

<!--[if !supportLists]-->·     
<!--[endif]-->Once a planetessimal reaches a certain size (around 1 km) this
process really takes off and it begins to gravitationally dominate everything
around it.

We can now account for the
second important challenge of explaining our solar system, the division of
planets into two basic types.

Rocky, metallic material of
the terrestrial planets could condense nearer to the Sun than the ices.   Hydrogen and helium gas remained gaseous
throughout the solar system.

Forming the Jovian
planets through nebular capture

Once accretion finished
building the seeds of the Jovian planets, their large
masses meant that their gravity was strong enough to accumulate large amounts
of the remaining nebular gases – i.e.  the force of gravity of the planet was stronger than that
from the Sun at that point, so that the gas went from orbiting the sun to
orbiting the planet.

This process proceeded in
basically the same way as the nebular collapse which formed the solar system,
forming similar disks of material around the Jovian
planets.  Some of the material
contributed to the planet, and some to satellite systems through accretion.

The Solar Wind
blows out the Remaining Nebular Gases

The solar wind is composed of
charged particles from the Sun’s hot (millions of degrees K) corona which carry
the Sun’s magnetic field. 

We see evidence (T Tauri stars) that this solar wind is very strong in young
stars.  Radiation pressure from the young
sun is also important – photons have momentum. 

These effects work to clear
out the remaining gases, before they cool enough for ices to condense in the
inner solar system.

• The celestial objects are bright points of light while the Earth is an
  immense, nonluminous sphere of mud and rock. Modern astronomers now know that the stars are
  objects like our Sun but very far away and the planets are just reflecting
  sunlight.
  
• The Greeks saw little change in the heavens---the stars are the same night after night.
  In contrast to this, they saw the Earth as the home of birth, change, and destruction. They
  believed that the celestial bodies have an immutable
  regularity that is never achieved on the corruptible Earth. Today astronomers know that
  stars are born and eventually die (some quite spectacularly!)---the length of
  their lifetimes are much more than a human lifetime so they appear unchanging.
  Also, modern astronomers know that the stars do change positions with respect to each other
  over, but without a telescope, it takes hundreds of years to notice the
  slow changes.
  
• Finally, our senses show that the Earth appears to be stationary!
  Air, clouds, birds, and other things unattached to the ground are not left
  behind as they would be if the Earth was moving. There should be a strong wind
  if the Earth were spinning as suggested by some radicals. There is no strong wind.
  If the Earth were moving, then anyone jumping from a high point would
  hit the Earth far behind from the point where the leap began. Furthermore, they knew
  that things can be flung off an object that is spinning rapidly. The observation that
  rocks, trees, and people are not hurled off the Earth proved to them that the Earth was
  not moving. Today we have the
  understanding of inertia and forces that explains why this does not happen even
  though the Earth is spinning and orbiting the Sun. That understanding, though, developed
  about 2000 years after Plato.

The model he chose was one
developed by another follower of Plato, Eudoxus. The planets and stars were on concentric
crystalline spheres centered on the Earth. Each planet, the Sun, and the Moon were on
their own sphere. The stars were placed on the largest sphere surrounding all of the rest.

Aristotle chose this model because most popular and observational evidence supported it
and his physics and theory of motion necessitated a geocentric
(Earth-centered) universe. In his theory of motion, things naturally move to the
center of the Earth and
the only way to deviate from that is to have a force applied to the object.
So a ball
thrown parallel to the ground must have a force continually pushing it along.
This idea was unchallenged for almost two thousand years until Galileo showed
experimentally that things will not move or change their motion unless a force
is applied. Also, the crystalline spheres model agreed with the Pythagorean paradigm of
uniform, circular motion (see the previous
section).

• Major axis---the length of the longest dimension of an ellipse.
  
• Semi-major axis---one half of the major axis and equal to the distance
  from the center of the ellipse to one end of the ellipse. It is also the average
  distance of a planet from the Sun at one focus.
  
• Minor axis---the length of the shortest dimension of an ellipse.
  
• Perihelion---point on a planet's orbit that is closest to the Sun. It
  is on the major axis.
  
• Aphelion---point on a planet orbit that is farthest from the Sun. It
  is on the major axis directly opposite the perihelion point. The aphelion +
  perihelion = the major axis.
  
• Focus---one of two special points along the major axis such that the
  distance
  between it and any point on the ellipse + the distance between the other focus
  and the same point on the ellipse is always the same value. The Sun is at one of
  the two foci (nothing is at the other one). The Sun is NOT at the center of the
  orbit!

Galileo Galilei (1564--1642 C.E.) was
the first person we know of that used the telescope for astronomical
observations (starting in 1609). The telescope was originally used as a naval
tool to assess the strength
of the opponent's fleet from a great distance. He found many new things when
he looked through his telescope:

1. The superior light-gathering power of his telescope over the naked eye
   enabled him to see many,
   many new fainter stars that were never seen before. This made Bruno's argument more
   plausible.
   
2. The superior resolution and magnification over the naked eye enabled him to
   see pits and craters on the Moon and spots on the Sun. This meant that the
   Earth is not only place of change and decay!
   
   
   
   

• With his superior ``eye'' he discovered four moons orbiting Jupiter. These
  four moons (Io, Europa, Ganymede, and
  Callisto) are called the Galilean satellites in his honor. In this system
  Galileo saw a mini-model of the heliocentric system. The moons are not
  moving around the Earth but are centered on Jupiter. Perhaps other objects,
  including the planets, do not move around the Earth.
  
• He also made the important discovery that Venus goes through a complete set
  of phases. The gibbous and full phases
  of Venus are impossible in the Ptolemaic model but possible in Copernican model
  (and Tychonic model too!). In the Ptolemaic model Venus was always
  approximately between us and the Sun and was never found further away from the
  Earth than the Sun. Because of this geometry, Venus should always be in a
  crescent, new, or quarter phase. The only way to arrange Venus to make a
  gibbous or full phase is to have it orbiting the Sun so that, with respect
  to our viewpoint, Venus could get on the other side of, or behind, the Sun
  further away from us than the Sun. This was
  possible only if Venus orbited the Sun (see the figure at the end of the
  planetary motions section
  of the previous chapter).