Download Formation of the Solar System

Formation of the Solar System
Here’s a quick overview of the layout of the solar system:
Motion of the planets in their orbits:
+
Aside from planets, there are also:
asteroids littered
about, but
primarily in the
asteroid belt
between Mars
and Jupiter
comets, which
mostly reside in
the Kuiper Belt
(just beyond the
orbit of Pluto, at
about 40 AU)
and the Oort
cloud (probably
at about 100
AU), which has
not yet been
observed.
Any theory which explains the formation of the solar system must at least
account for 4 main challenges:
Large bodies in the solar system have
orderly motion.
Planets fall into two main catelgories.
Swarms of asteroids and comets
populate the solar system.
There are several important exceptions
to these trends.
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.
Small, rocky terrestrial planets near
the Sun and large, hydrogen-rich
jovian (gas planets) planets farther
out. The jovian planets have many
moons and rings made of rock and ice.
Asteroids are concentrated in the
asteroid belt, and comets populate the
regions known as the Kuiper belt and
the Oort cloud.
Planets with unusual tilts, very large
moons, or moons with unusual orbits.
The Solar Nebula Hypothesis
Based on these observations, astronomers think that the best model for how the
solar system formed is from the collapse of an interstellar gas cloud. There were
other ideas about the formation of the solar system, but they didn’t fit these four
important characteristics.
There is other evidence that we are on the right track. We now see
protoplanetary disks around other stars.
What were the properties of the cloud to begin with?
 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…
 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…
Elements heavier than lithium are formed only in stars!
Evolution of the Solar Nebula
The collapse is triggered by an increase in density, and driven by gravity.
What do we mean by “collapse”?
During the collapse…
1. The cloud’s rotation rate increases, due to conservation of angular
momentum.
2. The cloud heats up, as compressing the gas cause the particles to speed
up, increasing the temperature of the gas and dust particles.
3. 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.
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.
Building Planets
Condensation
The nebula heats up during the collapse. The densest, hottest part of the nebula
is at the center. As a result of this, all material very near the protosun existed in
a gaseous state. As you move outward, the nebula is cooler. At different radii,
the temperature is low enough for certain materials to condense.
Why are there two types of planets, terrestrial and Jovian
A. The force of gravity due to the massive Sun draws the heavier, dense
material of the terrestrial planets closer.
B. Initial orbits of the terrestrial planets bring them closer to the Sun where they
fall into smaller orbits.
C. Near the Sun, only heavy elements and rocky material can condense from the
solar nebula.
D. Jovian planets form first and draw much of the gaseous material to them via
gravity, leaving only the heavier elements and rocky material behind.
So beyond the frost line, which lies between the orbits of Mars and Jupiter,
temperatures had dropped enough for ices such as water, ammonia and
methane to condense. Notice that these ices are hydrogen rich, since there was
plenty of hydrogen to go around out there.
Small eddies formed in the disk material, but since the gas and dust particles
moved in almost parallel, near-circular orbits, they collided at low velocities.
Instead of bouncing off each other or smashing each other, they were able to
stick together through electrostatic forces to form planetesimals. The larger
planetesimals were able to attract other planetesimals through gravity and
increase in size. This process is called accretion.
Accretion
How to grow planetesimals:
 Initially small particles of gas and dust were able to stick together via their
electrostatic attraction.
 As they grew larger, their gravity began to be strong enough to attract
particles as well, and their growth accelerated.
 Once large enough, gravity pulls the planetesimal into a spherical shape.
 Once a planetesimal 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 last two challenges
Once we understand the process of accretion, the solutions to the last two
challenges follow naturally.
Asteroids and comets are leftover planetessimals of terrestrial and Jovian
planets. The nebular theory predicts that their compositions should be quite
different, which they are: asteroids are mostly rocky with very small amounts of
ices, comets are “dirty snowballs”.
The early solar system must have been full of planetessimals, so that there was
a period of heavy bombardment during which impacts were very common. We
have direct evidence that some of these impacts involved large bodies, which
may have led to the exceptional situations in our solar system (e.g. the tipping
over of Uranus, the formation of Earth’s large moon).
Age of the Solar System
How do we know the age of our solar system?
We use a technique called radioactive dating.
We can apply this to many different samples:
 Earth rocks
 Moon rocks
 Meteorites
Some meteorites have not changed since they were formed via accretion, and
provide the most reliable age of the solar system, 4.6 billion years.
Compared to the Universe (10-15 billion years), that is not very old.
Summary of steps for building a Solar System (a.k.a. the nebular theory):
1. Collapse of the nebula and formation of the protoplanetary disk and protosun.
2. Condensation of planetessimals.
3. Accretion of planetessimals to form planet seeds.
4. Formation of Jovian planets through nebular capture.
5. The solar wind of young Sun clears away the remaining gas.