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Learning Activity 1bc: Nebular Theory
Unit A: Earth’s Place in the Universe
Learning Targets:
1b)
I can describe the nebular theory for the formation and structure of the solar system (DOK1)
1c)
I can make a claim about the formation of the solar system and support it with evidence
(DOK2-3)
Directions:
1. Read one or more descriptions of the nebular theory attached to this paper in order
to create a FLOW MAP* that shows the sequence of events that formed our solar
system. As you make your FLOW MAP, record where you found your information by
including an article number and paragraph letter for each box and arrow in your
FLOW MAP.
A FLOW MAP is a type of thinking tool that helps you answer the following
questions: What happened? What is the sequence of events? It looks like this:
As you fill in the information, consider the following:
 place nouns inside of the boxes and verbs on the arrows.
As you read and produce your flow map, consider the following questions. Take notes
and participate in a class discussion of these two questions:
2. What evidence do scientists have to support this theory?
3. What makes the nebular theory a theory and not a law? Is the nebular theory
widely accepted in the scientific community? If not, what are some of the problems
with the theory? How do you know?
After the class discussion, record your best thinking about the answers to questions 2
and 3.
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Reading 1: Origin of the Solar System
by Jerry Coffey on July 18, 2009
http://www.universetoday.com/34729/origin-ofthe-solar-system/
1A - The theory that explains the origin of the
Solar System is a little bit different than the Big
Bang that is used to explain the beginning of the
Universe. The beginning of the Solar System are
explained by nebular hypothesis. It has been
revised several times and the modern incarnation
is called the Solar Nebular Disk Model (SNDM) or
simply the Solar Nebular Model. The basic theory
is explained below.
The famous "Pillars of Creation" in
the Eagle Nebula. Credit: NASA/STScI
1B - The nebular hypothesis states that stars are formed in clouds of interstellar gas. Each of these
clouds is filled with ice and rock. The cloud thought to have formed this Solar System is generally
referred to as the solar nebulae. Astronomers have proven that turbulence and agitation will cause
these clouds to collapse, heating them up. This collapse and heating does not happen overnight; it
takes thousands of years. The superheated gas and dust are pulled together to form a star. More gas
causes the star to grow, while some of the material forms a solid ball. Once these solid balls are large
enough, their gravity will attract more material. Eventually, these balls become large enough to be
planetary cores. The objects that separate from the star early on become comets. This process takes
millions of years. Overtime, each core accretes varying amounts of material and is shaped by impact
events. This explains why each planet is different from the other.
1C - The nebular hypothesis was first introduced in 1734 by Emanuel Swedenborg. It fell out of use
for quite a while, but was revived several decades ago with a few revisions. Technology in the last 30
years have called it into question of late. Questions abound: why are the planets not on the same
ecliptic plane, why are extrasolar planets so different from the ones in this Solar System, and why do
some clouds not collapse are among them
1D - As technology improves and astronomers are able to more closely study objects in the outer
Solar System and discover new objects outside of our Solar System, the ability of the nebular theory
to explain the origin of the Solar System comes under more intense fire. Changes have been made to
attempt to explain some of the theory’s discrepancies, but all of the questions have not been
answered yet.
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Reading 2: Nebular Theory
by Matt Williams on November 5, 2010
http://www.universetoday.com/77525/nebular-theory/
2A - When being asked the tough
questions about why things are the way
they are (usually by our own children!)
one of the toughest questions to answer
is, how did we get here on planet Earth?
Well, if you were to answer this question
scientifically, chances are it would unleash
a whole sleuth of questions. And to be
able to answer them, you would first have
to be familiar with a little something
known as Nebular Theory. This is the
theory of how not only our own solar
system, but all star systems were formed.
Although still a theoretical model, it is the
most widely accepted scientific
explanation of how stars came to emerge
from the cosmos.
Solar System Montage
2B - This nebular hypothesis was first developed in the 18th century by Emanuel Swedenborg,
Immanuel Kant, and Pierre-Simon Laplace. Kant argued that gaseous clouds—nebulae, which slowly
rotate, gradually collapse and flatten due to gravity and eventually form stars and planets. Laplace’s
proposed a similar model in which a protosolar cloud (a nebular cloud) contracted and cooled,
flattening and shedding rings of material in the process which later collapsed to form the planets.
Over the course of the 20th century, this model came to be challenged by a number of theorists who
proposed numerous models in an attempt to replace it. However, none of these attempts were
successful and it was not until the 1970’s with Soviet astronomer Victor Safronov that the modern
(and widely accepted) Solar Nebular Disk Model (SNDM) came into being.
2C - According to this model, our star system was formed 4.568 billion years ago when a small part of
a giant molecular cloud experienced a gravitational collapse. Most of the collapsing mass collected in
the center forming the Sun while the rest flattened into a protoplanetary disk, out of which the
planets, moons, asteroids, and other small Solar System bodies formed. Since that time, our system
has evolved considerably due to collisions between objects, planetary migration and the capturing of
extra-solar objects by our own system. While originally applied only to our own Solar System, the
SNDM has since been used by theorists to explain star formation throughout the known universe.
2D - While this remains the most widely accepted theory, alternative models still exist. Since the
dawn of the space age in the 1950s and the discovery of extrasolar planets in the 1990s, all of these
models have been both challenged and refined to account for new observations.
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Reading 3: Solar Nebula Theory
by Jerry Coffey on September 2, 2010
http://www.universetoday.com/72589/solar-nebula-theory/
3A - The solar nebula theory, also known as the
nebular hypothesis, is the most widely accepted
model explaining the formation and evolution of
our solar system. It was originally applied to our
solar system only but, this method of planetary
system formation is now thought to hold true
throughout the universe. The modern variant that
is most accepted is the Solar Nebular Disk Model
(SNDM).
Proto-Planetary System
in the Orion Nebula
3B - According to SNDM stars form in massive, dense clouds of molecular hydrogen called giant
molecular clouds(GMC). These clouds are gravitationally unstable, and matter coalesces into smaller,
denser clumps inside. These clouds collapse and form stars. This can give birth to planets under the
right circumstances, which are not fully understood. So, the formation of planetary systems is
thought to be a natural result of star formation. The process is thought to take at least 100 million
years.
3C - According to SNDM, rocky planets form in the inner part of the protoplanetary disk where the
temperature is high enough to prevent condensation of water ice and other substances into grains.
This results in the coagulation of purely rocky grains and later into the formation of rocky
planetesimals. After small planetesimals have formed, runaway accretion begins. Growth accelerates
as mass accumulates. This leads to the growth of larger bodies by the destruction of smaller bodies.
This lasts between 10,000 and 100,000 years and ends when the largest bodies exceed approximately
1,000 km in diameter. Next, oligarch accretion begins. Several hundred of the largest
bodies(oligarchs), continue to accrete planetesimals. Only the oligarchs grow. Occasionally, oligarchs
impact each other and form a larger body. The final result of the oligarchic stage is the formation of
about 100 bodies between the size of the Moon and Mars. Last is the merger phase. This begins when
the oligarchs become massive enough to perturb each other causing their orbits to become chaotic.
This lasts for 10 to 100 million years and forms a limited number of Earth sized bodies. Some of the
oligarchs are thought to have brought water to Earth. The resulting rocky planets eventually settle
into nearly stable orbits.
3 D - Scientists are still trying work out how to apply the solar nebula theory to the formation of giant
planets. It is great to see that there are still challenges for our scientists. Unanswered questions mean
unexplored frontiers.
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Reading 4: Theory of Planetary System Formation
by Patrick Cassen of NASA-Ames Research Center
http://astrobiology.arc.nasa.gov/workshops/1996/astrobiology/speakers/cassen/cassen_abstract.html
4A - Observations and theoretical considerations support the idea that the Solar System formed by
the collapse of tenuous interstellar matter to a disk of gas and dust (the primitive solar nebula), from
which the Sun and other components separated under the action of dissipative forces and by the
coagulation of solid material. Thus, planets are understood to be contemporaneous by-products of
star formation. Because the circumstellar disks of new stars are easier to observe than mature
planetary systems, the possibility arises that the nature and variety of planets might be studied from
observations of the conditions of their birth. A useful theory of planetary system formation would
therefore relate the properties of circumstellar disks both to the initial conditions of star formation
and to the consequent properties of planets to those of the disk. Although the broad outlines of such
a theory are in place, many aspects are either untested, controversial, or otherwise unresolved; even
the degree to which such a comprehensive theory is possible remains unknown.
4B - The main features of the theory adopted by most researchers are as follows. A small fraction of
the material in cold, interstellar clouds becomes gravitationally unstable to collapse, either by gradual
evolution or by a sudden compression induced by some disturbance. Collapse occurs rapidly enough
(within a million years) to preclude the loss of much of the cloud's angular momentum, so the
collapsed configuration takes the form of a centrifugally supported disk, in which most of the original
angular momentum is retained. Dissipative forces within the disk promote the accretion of most of
the material to its center to form a star; lesser fractions of material spread out in the remaining disk
or are ejected back into space by energetic processes near the star. Solid objects grow in the disk due
to the coagulation of dust and ice grains (both interstellar survivors and newly condensed). Although
gas dynamic forces may cause most of these objects to be accreted into the growing star, or even
ejected in the wind, some become large enough for gravitational forces to dominate their motions, at
which point they become potential planetary system survivors. Growth to planetary size occurs
through collisions made possible by the perturbations of orbits induced by mutual gravitational
scattering and perhaps by collective interactions with nebular gas.
4C - In the case of the Solar System, the outer planets apparently grew fast enough to capture and
retain substantial amounts of hydrogen and helium from the solar nebula. The final planetary masses
were probably attained within 100 million years, although the cleanup of debris persisted longer. In
the context of this theory, the asteroid belt is the result of a failed planet, the accumulation of which
was frustrated by the gravitational influence of Jupiter. Comets are believed to be planetesimals
accumulated in the Uranus-Neptune region, scattered to great distances by encounters with the
outer planets.
4D - Several components of the theory enjoy solid support from observations and/or rigorous
theoretical analysis. For instance, the basic tenets of star formation theory have been verified by
observations which have recently detected direct evidence for collapsing gas in star-forming regions
and established the prevalence of circumstellar disks around young stars. The idea that disk evolution
proceeds through the accretion of material onto the young star is confirmed by measurements of the
accretion luminosity. The fact that dissipative angular momentum transport in a disk leads to
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distributions of mass and angular momentum characteristic of a star and planetary system has an
unequivocal physical basis. Numerical simulations have demonstrated that the characteristics of the
terrestrial planets are typical of the configurations ultimately attained by a swarm of dynamically
evolving planetesimals possessing the total mass and angular momentum of the planets they
eventually form.
4E - Nevertheless, there are major gaps at every step in the theory, and recent developments have
emphasized the need to remain undogmatic about even the most entrenched assumptions. For
instance, it has been commonly supposed that the distribution of mass in the present Solar System is
not a perverse distortion of that of the primitive solar nebula in the planet-forming era. Although it
was acknowledged that the distribution of sedimented solids need not be exactly the same as that of
the gas (the latter being the predominant component), the discovery of Jupiter-mass planets very
close to their parent stars has focussed attention on the theoretically identified possibility of
extensive planetary migration: the radial motion of a planet due to its gravitational interaction with
its parent disk. A consequence of this phenomenon could be the effective obliteration of evidence of
the original mass distribution in the disk. (In a similar vein, the recent suggestion that meteoritic
material was thermally processed near the Sun and subsequently ejected to great distances in the
nebula contains the implicit and radical notion that even the distribution of coagulated solids had
little to do with the original distribution of mass in the nebula.) Thus, understanding the precise way
in which planetary material becomes decoupled from nebular gas is central to interpretations of disk
properties in terms of potential planetary systems.
4F - In addition to these issues, one can identify several other specific, unresolved questions of the
most fundamental order, relevant to the characteristics of planetary systems: (1) How are the
conditions that lead to the birth of a single star surrounded by a disk distinguished from those that
lead to the formation of a multiple star system? (2) What are the specific mechanisms responsible for
the transport of angular momentum in circumstellar disks, and how efficiently do they act? (3) What
processes determine the mass of a gas-rich planet? (4) What determines the ultimate fate of nebular
gas that is not incorporated into planets? (5) What processes govern the final distributions of volatile
substances among the planets? For most of these questions, definitive observational tests will be
difficult to identify. Well-designed, computationally intense studies, which incorporate dissipative
processes in a quantitatively realistic way, will be required to address them.
(This abstract is based in part on the article "Origin of the Solar System," by P. Cassen and D. S.
Woolum.)
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