The Formation of the
Solar System
Model Requirements
• Each planet is relatively isolated in space.
• The orbits of the planets are nearly circular.
• The orbits of the planets all lie in nearly the same
plane.
• The direction in which the planets orbit the sun
(counterclockwise as viewed from above Earth’s
north pole) is the same as the direction which the sun
rotates on its axis.
• 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 (not Venus, Uranus, and
Pluto).
Model Requirements
• The direction in which most of the known moons revolve
about their parent planet is the same as the direction in
which the planet rotates on its axis.
• Our planetary system is highly differentiated (terrestrial
and jovian planets).
• Asteroids are very old and exhibit a range of properties
not characteristic of either the terrestrial or the jovian
planets or their moons.
• The Kuiper belt is a collection of asteroid-sized icy bodies
orbiting beyond Neptune.
• The Oort cloud comets are primitive, icy fragments that do
not orbit in the plane of the ecliptic and reside primarily at
large distances from the sun.
Nebular Contraction
• Cloud of interstellar dust and
gas - a nebula, begins to
contract (for whatever reason)
under its own gravity.
• As it contracts, it becomes
denser and hotter, eventually
forming a star at its center.
• As it contracts, the cloud spins
faster and faster forming a
flattened pancake-shaped disk
(due to angular momentum).
Nebular Contraction
• The flattened-pancake is usually referred to as
the solar nebula since it will form our solar
system.
• The idea that planets form from such a disk is
called the “nebular theory.”
• We have seen such disks formed in other
systems.
• The old nebular theory is wrong as we now know
clumps of matter would not form from the gas as
they would have dispersed and not formed
planets.
Newborn Solar System?
Condensation Theory
• The current theory, condensation theory, is
built on the nebular theory.
• Key ingredient - interstellar dust in the solar
nebula.
• The dust acts as condensation nuclei
(microscopic platforms to which other atoms
can attach) and helps the cloud cool enough
for condensation to occur in the first place.
Planet Formation
• According to condensation theory, the planets formed in 3
distinct stages. First 2 apply to all planets, 3rd applies only to
the Jovian worlds.
• Stage one
– Dust grains in the solar nebula formed condensation nuclei.
These clumps then stick to other clumps, causing the clumps
to grow in size rapidly.
– The process of accretion (gradual growth of objects by
collision and sticking) created objects a few hundred km
across.
– At the end of the first stage, solar system consisted of
hydrogen and helium gas and millions of planetesimals
(objects size of small moons having gravitational fields just
strong enough to affect their neighbors).
Planet Formation
• Stage two
– Gravitational forces between planetesimals caused
them to collide and merge, forming larger and larger
objects.
– Because larger objects have stronger gravitational
pulls, eventually almost all of the planetesimal material
was swept up into a few large “protoplanets”
(accumulations of matter that would eventually evolve
into the planets we know today).
– The asteroids and comets originated as collisions
between planetesimals and protoplanets sent out small
chunks of material that escaped capture.
Planet Formation
• After 100 million years, we
have
– Nine protoplanets.
– Dozens of protomoons.
– A glowing protosun at the
center.
• Roughly a billion years were
required to “sweep” the
system clear of interplanetary
“trash.” This is a period of
intense meteoritic
bombardment whose effects
on the moon and elsewhere
are still evident today.
Making the Jovian Planets
• There are two conflicting views on how the Jovian
planets formed.
• View one
– 4 largest protoplanets became massive enough to
enter a 3rd stage of evolution - sweeping up large
amounts of gas directly from the solar nebula.
• View two
– Giant planets formed through instabilities in the
cool outer regions of the solar nebula - mimicking
on small scales the collapse of the initial
interstellar cloud.
Making the Jovian Planets
• Many of the Jovian
moons probably also
formed by accretion.
Some of the smaller
moons may be captured
planetesimals.
• Eventually, the sun blew
away any remaining
gas between the
planets, which is why
we don’t see it today
(the outer planets must
have formed before the
nebular gas dispersed!).
The Differentiation of the Solar
System
• The closer to the protosun, the hotter the temperature. The
temperature determines what could form where and when.
(Note that as the solar nebula contracted due to gravity, it
heated up as it flattened into a disk.)
• In the innermost regions (Mercury), only metallic grains could
form due to the high temperature.
• At 1 AU, rocky, silicate grains could form.
• Beyond 3 or 4 AU, water ice could exist, and so on.
• More and more matter could condense out at greater and
greater distances from the sun.
• Further out, water vapor, ammonia, and methane could
condense into solid form, creating the cores of the Jovian
planets.
Which of the following types of planets w would
form in the early Solar system at locations hot
enough for liquid water to boil into gas?
a. Rocky terrestrial
b. Jovian gas giant
Lecture Tutorial: Temperature and
Formation of our Solar System (p. 103)
• Work with a partner!
• Read the instructions and questions
carefully.
• Discuss the concepts and your answers
with one another. Take time to
understand it now!!!!
• Come to a consensus answer you both
agree on.
• If you get stuck or are not sure of your
answer, ask me or another group.
The standard model of solar system formation offers
what explanation for the different compositions of the
terrestrial and Jovian planets?
1.
2.
3.
4.
During the condensation, the heavier elements tend to sink
near the Sun and only provided enough material to build the
relatively small terrestrial planets
During the collapse of the gaseous nebula, most of the
material tended to collect far from the Sun because of the
large centrifugal forces, which provided the necessary material
to build the large Jovian planets
The large gravitational forces of Jupiter tended to prevent
planet formation in the inner solar system and eventually
attracted most of the material to the Jovian
The terrestrial planets were formed near the Sun Where
because of the high temperatures only heavier elements were
able to condense.
The standard model of solar system formation offers
what explanation for the different compositions of the
terrestrial and Jovian planets?
4. The terrestrial planets were formed near the Sun Where because
of the high temperatures only heavier elements were able to
condense.
Which planet formed closest to the Sun at
temperatures below thee freezing point of
water?
1.
2.
3.
4.
5.
Mercury
Venus
Mars
Jupiter
Saturn
Which planet formed closest to the Sun at
temperatures below thee freezing point of
water?
Jupiter
Astronomers have discovered massive gas giants like
Jupiter orbiting stars at closer than .7 AU (Venus’ orbit)
Why don’t astronomers believe that these gas giants
originally formed at these locations?
1. The planet’s gravity would have been too large to
form that close to the star
2. Their orbital periods are too long
3. A young star’s solar wind would have blown the
planet’s farther away.
4. The temperature in the early nebula was too high at
these distances
Astronomers have discovered massive gas giants like
Jupiter orbiting stars at closer than .7 AU (Venus’ orbit)
Why don’t astronomers believe that these gas giants
originally formed at these locations?
4. The temperature in the early nebula was too high at
these distances
Asteroids and Comets
• Planetesimals beyond the orbit of Mars failed to accumulate into
a protoplanet due to the large gravitational field of Jupiter
constantly disturbing their motion. These are in the asteroid belt
and also include the Trojan asteroids.
• Planetesimals further out were “kicked” into outer orbits and
form the Oort cloud.
• Most planetesimals formed beyond Neptune are still there and
make up the Kuiper belt.
• The condensation theory could not account for the water and
other volatile gases found on Earth and elsewhere.
• Comets, containing both water and other volatile gases,
bombarded the inner planets after they were formed. Thus, the
water on Earth originated in comets.
• Activity: Space Objects
Random Encounters in the
Solar Nebula
• Random collision of planetesimals and other bodies
are allowed within the current condensation theory.
• These random collisions can be used to explain
everything from Venus’ slow retrograde motion (due
to two protoplanets of comparable mass colliding
nearly head-on) to the formation of Earth’s moon.
Detecting Extrasolar Planets
• More than 170 extrasolar planets have been
found in more than 145 separate systems.
• We generally can’t observe any of the
extrasolar planets directly.
• As a planet orbits a star, gravitationally pulling
one way and then the other, the star
“wobbles” slightly - we can measure this
wobble and determine the mass of the planet.
Detecting Extrasolar Planets
• More than 170 extrasolar planets have been
found in more than 145 separate systems.
• We generally can’t observe any of the
extrasolar planets directly.
• As a planet orbits a star, gravitationally pulling
one way and then the other, the star
“wobbles” slightly - we can measure this
wobble and determine the mass of the planet.
Detecting
Extrasolar Planets
Lecture Tutorial: Motion of Extrasolar
Planets (p. 117)
• Work with a partner!
• Read the instructions and questions
carefully.
• Discuss the concepts and your answers
with one another. Take time to
understand it now!!!!
• Come to a consensus answer you both
agree on.
• If you get stuck or are not sure of your
answer, ask me or another group.
Planetary Properties
• 5 % of nearby stars surveyed so far show signs of
extrasolar planets. About a dozen of these systems
contain more than one “observed” planet.
• These planets are usually Jupiter-sized and nearby
their parent stars - called “hot Jupiters.”
• These planets often appear to be the sole large body
in their system.
• The above two facts are due to a selection effect - the
effects of smaller planets and those located at larger
distances from their parent stars are difficult to detect.
Is Our Solar System Unusual?
• Planetary systems are quite common.
• Those systems discovered so far do not look like our
own.
• We have explanations for how Jupiter-like planets
can wind up close to the parent star (compared to our
system), but we don’t yet know if that is the norm (or
whether it’s more common for Jupiter-like planets to
be farther out as in our solar system).