DECIPHERING THE UNIVERSE
Writer’s comment: I have had difficulty in the past with geology because studies of the Earth never really captured my interest, and I was hoping that Geology 113, which deals with the solar system, would interest me more. Early in the quarter, my hopes were confirmed. Captivated by Professor Kellogg’s passion for the material, I realized that geology opens doors that help us understand the universe. For this essay, I chose to write about the origin of the solar system because I am fascinated with origins. And fortunately, Larry Guenther, the teaching assistant to whom I submitted my topic for approval, believes that essays are more interesting to read when the topic is personally interesting to the writer. So I owe many thanks for whatever success this essay achieves to Louise Kellogg and Larry Guenther, both of whom spurred my newfound interest in geology.
Instructor’s comment: Geology 113 is a General Education course that introduces students to the planets, the sun, and the small objects of the solar system. The term paper assignment was called “Comparative Planetary Geology”; most students compared and contrasted geologic features on several planets, and then evaluated theories about the evolution of these features. Robert Payawal chose the largest possible topic, the origin of the solar system itself. His lively essay on the current hypotheses for the origin of the solar system draws on a variety of observations and illustrates the use of the scientific method to evaluate the hypotheses. His paper was a delight to read and could be a model for future students.
—Louise Kellogg, Geology Department
Within the past few decades, scientists have sought a theory to explain the creation of the universe. The renowned scientist Albert Einstein called this theory the Unified Field Theory. The theory is based on the assumption that creation could be explained with a simple, beautiful equation. Although this paper does not discuss the discovery of this theory, it explores a closely related concept: the creation of the solar system. Studies of both the universe and solar system are based upon similar evidence gathered throughout history. This paper presents different hypotheses regarding the creation of the solar system to show how evidence applied through comparative planetology supports or contradicts these hypotheses and why studying the solar system is important.
Scientists have gathered clues to answer the question, How did the solar system form? As science evolved, new techniques for collecting data and enhanced equipment offered a new set of clues. By observing the skies, charting the stars, and patiently tracking minute movements of stellar objects millions of kilometers away, scientists realized that the universe is a dynamic place. Although many features of our solar system are still unexplained, three hypotheses can account for its major features—the Nebular Hypothesis, the Planetesimal Hypothesis, and the Dust Cloud Hypothesis. This paper will focus on these hypotheses to show how comparative planetology can determine the validity of a hypothesis. This process reveals that hypotheses can be widely accepted even if they do not explain all of the observable data.
The Nebular Hypothesis
The Nebular Hypothesis was first proposed by Pierre Simon, Marquis de Laplace in 1796. His proposal for the creation of the solar system was a general hypothesis that did not take into account the movements of planets beyond Saturn, the angular momenta of the planets and the sun, or the compositions of the outer planets; these data were unknown to astronomers at this time because of limited technology (Huffer, et al., 1967). An updated variant of this hypothesis is explained by Eric H. Christiansen and W. Kenneth Hamblin (1995): The Nebular Hypothesis assumes that during the primordial existence of the universe, a nebulous cloud of gas (primordial gases, mostly helium and hydrogen) collapsed due to gravitation. As the nebula collapsed, atoms collided with other atoms, which, in turn, created heat. The heat from the interior increased gas pressure, which slowed the gravitational collapse. Meanwhile, rising temperatures at the center created a protostar that radiated light. While the nebula slowly contracted, it began to spin. As the material was pulled to the interior, gravity and centripetal forces caused the nebula to spin faster. Centrifugal forces prevented a disk of material from moving toward the interior; the center became more dense. The disk created the foundation for the plane of the ecliptic (the plane upon which most of the planets and their satellites revolve). Within the nebula, a T-Tauri phase occurred, a phase in which the protostar sent forth “irregular outbursts of light” and “strong magnetic fields” (p. 27). These emanations are thought to have blown away much of the cosmic dust in the nebula. Eventually, the protostar increased in temperature (triggering nuclear fusion of hydrogen) and stabilized to form the sun at the center of our system.
The second stage of this process involved differentiation and condensation of material in the solar system. It is believed that during this stage planets formed. Differentiation entails the separation of material into “physically and chemically unlike products,” while condensation involves the formation of solids from the gaseous nebula (Christiansen and Hamblin, p. 28). Given that gases of different materials will condense at different temperatures, and assuming that the interior of the nebula was warmer than the exterior, scientists predicted, correctly, that refractory materials will be found nearer to the sun while volatiles will be found at greater distances from the sun. (Materials that condense at high temperatures are called refractory, while those that condense at low temperatures are called volatiles.) By means of this principle, scientists can predict the occurrences of materials found on planets and asteroids: Mercury will have refractory materials, for example, while Jupiter will have volatiles (Christiansen and Hamblin, 1995).
During condensation and differentiation, low-impact collisions (or accretion) formed young planets (or planetesimals) around the sun. The orbiting planetesimals began to spin and created mini-nebulae which, in turn, created satellites through similar condensation and accretionary processes. In summarizing the way the Nebular Hypothesis explains the creation of our solar system, Christiansen and Hamblin (1995) observe that “In short, elemental material of our solar system evolved from gas to dust to clots in co-orbiting streams that eventually accreted to form planets” (p. 30).
The Planetesimal Theory
In 1900, Thomas C. Chamberlain and Forest Ray Moulton, unsatisfied with the Nebular Hypothesis, attempted to explain the creation of the solar system as the passing of a star close to our sun (Huffer, et al., 1967). The pair based their hypothesis on the assumption that the sun was unchanged from the sun of the present. They speculated that a high-velocity star passed very close to our sun in a hyperbolic fashion, pulling two bolts of material from the sun. Great explosions ensued, creating ten more such bolts, each given a motion in the same plane and orbit as the passing star. Lighter materials in the bolts, such as hydrogen and helium, were dragged from the interior of the bolts, and heavier materials fell back toward the sun due to gravitation. The bolts were then given forward momenta due to the pull of the star (Huffer, et al., 1967).
The heavier materials coalesced into planetesimals in the same fashion as described in the Nebular Hypothesis; accretion formed planetesimals that orbited the sun. Protoplanets formed after atmospheres adhered to the planetesimals due to gravity. These protoplanets approached close to the sun, and, by similar forces, bolts of material were pulled from the protoplanets, forming satellites (Huffer, et al., 1967).
In effect, the sun, no different from our sun today, lost a small amount of material because of the pull of a passing star. That material then condensed to create planets and their satellites.
The Dust Cloud Hypothesis
In about 1946, the presence on photographic plates of dark anomalies in front of certain nebulae spurred interest in the existence of so-called “globules,” or stars that are in the process of forming (Huffer, et al., 1967). Before 1946, these globules—actually discovered years before by Edward Emerson Barnard—had been discounted as mere defects in photographic plates, though they were accepted as viable stellar manifestations. Scientists believed that these globules were about 250 times the size of our solar system, contained as much mass as our sun, and were dense enough to block out light. Furthermore, it was believed that the particles in these globules were being pushed by outside radiation pressure from stars (Huffer, et al., 1967). As a result, they should have been forming and rotating quite slowly. In the final stages globules would collapse and contract.
In light of these phenomena, Fred L. Whipple developed the Dust Cloud Hypothesis in 1946. He explained that a dust cloud and protosun, which formed from the contraction of the globule, either developed smaller clouds or captured stray clouds. The smaller clouds spiraled toward the center of the dust cloud and accreted to form planetesimals that developed an orbit. Further collisions created primordial planets. Like the Nebular Hypothesis, the Dust Cloud Hypothesis acknowledges denser material in the center of the cloud and explains the loss of lighter material by assuming that a process comparable to the T-Tauri phase occurred (Huffer, et al., 1967).
The major processes in this hypothesis are slow formation and rotation of a star from a dark globule, the eventual collapse of that globule into a dust cloud and protosun, the formation or capture of small clouds, the formation of these small clouds into planetesimals and primordial planets, and the loss of primordial gases by a process similar to the T-Tauri phase.
THE COSMIC EVIDENCE
The universe has left many clues that the scientist must gather and apply to hypotheses for verification. Collectively, these clues comprise the data with which scientists can decipher the universe. Once collected, these clues are analyzed through comparative planetology to determine the structure, composition, and movements of the solar system. This information can validate or invalidate hypotheses that attempt to explain the creation of the solar system. Three aspects of comparative planetology are relevant in assessing the three hypotheses—Laplace’s perspective of the solar system, the studies of carbonaceous chondrites, and the observation of angular momentum.
Laplace’s Four Miracles
Pierre Simon, Marquis de Laplace, the first to propose the Nebular Hypothesis, based his hypothesis on four simple observations of the solar system: (l) the thirty planetary bodies known during the late eighteenth century all revolved in an east to west manner; (2) the known planets (out to Saturn) all rotated in the same east to west manner; (3) the planetary bodies all revolve on about the same plane; and (4) the planets and moons revolve in nearly circular ellipses (Thiel, 1957). These four observations, or “Four Miracles,” had been known to astronomers for centuries, but it was not until Laplace compiled these observations that they were worked into a possible hypothesis for explaining the creation of the solar system. Even today, any hypothesis must account for these four observations in order to be credible.
Carbonaceous chondrites are meteorites that provide valuable clues about the age and composition of the solar system. Since their formation from the materials within the primordial nebula, these meteorites have remained relatively unchanged for billions of years. Evidence for this is seen in their structure. First, from spectrographic analysis scientists have determined that this class of meteorite contains material similar to the sun’s (without the primordial gases). Second, there is evidence that the meteorites had not undergone melting because the material within is largely undifferentiated; heat or melting would have segregated the materials. Because of these characteristics, scientists have been able to establish the age of the solar system at about 4.6 billion years. Furthermore, scientists have determined that the planets probably accreted from the same primordial material as these carbonaceous chondrites (Christiansen and Hamblin, 1995). These meteorites have given scientists insight into the age and early materials of the solar system.
Angular momentum is a large factor when determining the creation of the solar system because it reveals a pattern of motion that hypotheses must be able to explain. Angular momentum is the “quantity of motion due to the rotation or revolution”; this quantity is also dependent on the mass of an object. This interdependence between motion and mass is seen in the equation:
angular momentum = mwr = mr2w, “where m is the mass, r is the distance from the center of motion, v is the linear velocity, and w is the angular velocity, the number of turns in a unit of time” (Huffer, et al., 1967, p. 232). Thus, the more massive an object, the greater the angular momentum it should have.
Although the relationship between mass and angular momentum predicts that the sun (which contains 99.8 percent of the mass in the solar system) should have the greatest angular momentum in the solar system, Chamberlain and Moulton (founders of the Planetesimal Hypothesis) calculated the sun’s angular momentum at only 2 percent; the majority of angular momentum in the solar system is found in the large planets, Jupiter and Saturn (Huffer, et al., 1967). This oddity is particularly important because any hypothesis must be able to explain the peculiar distribution of angular momenta among bodies in the solar system.
TESTING THE HYPOTHESES
The true hypothesis for the creation of the solar system will explain all of the observations described above. Currently, none of the three hypotheses presented is able to explain all of the information gathered from the solar system. However, the most acceptable hypothesis is the one that best explains the most information. It should account, at least, for the following data:
1. Planetary bodies revolve in an east to west manner.
2. Planets (with the exception of Uranus) rotate in an east to west manner.
3. Planets revolve (roughly) on the plane of the ecliptic.
4. Planets and their satellites revolve in nearly circular ellipses.
5. The solar system is approximately 4.6 billion years old.
6. Much of the primordial gases were swept away from the solar system.
7. Planets were formed from the same material as carbonaceous chondrites.
8. The sun has a very small angular momentum in proportion to its size.
Table 1 “grades” each hypothesis according to its ability to account for each of these pieces of information. (See following page.)
Table 1. The strengths and weaknesses of three hypotheses that seek to explain the origins of our solar system, evaluated according to their ability to account for eight criteria. The grading scale ranges from A to I.(a)
|1. East-west revolution||C||C||C|
|2. East-west rotation||C||C||C|
|3. Revolves on plane of ecliptic||A-||B-||D+|
|4. Nearly circular ellipse||B-||C||B|
|5. Age of solar system||C||I||C|
|6. Primordial gases gone||A||A||A|
|7. Planetary materials like cc(b)||A||F||C|
|8. Small momentum of sun||F||C||A|
(a) A = excellent explanation
C = average explanation
F = bad explanation, or contradiction
(b) cc = carbonaceous chondrites
B = good explanation
D = poor explanation
I = provides no explanation
The Nebular Hypothesis
The Nebular Hypothesis can explain both the revolution and rotation of planets in an east to west fashion because it entails the spinning of the nebula. However, since Uranus has an anomalous rotation, this hypothesis cannot fully explain the data. Because the hypothesis describes the creation of the disk where planets formed, it accurately explains the plane of the ecliptic and nearly circular revolutions of planets caused by the spinning nebula. The hypothesis also fits within the time-scheme of the solar system because “the evolution from a stellar nebula to hydrogen-burning star may take only 100,000 years” (Christiansen and Hamblin, 1995, p. 28). Additionally, primordial gases could have been swept away by a T-Tauri phase, as this hypothesis suggests. It could also explain how the material in the primordial nebula accreted to form planets, thereby explaining the similarity of material in the sun and planets to carbonaceous chondrites. The Nebular Hypothesis fails to explain why the sun has only 2 percent of the angular momentum of the solar system, however, even though it has the most mass and was formed in the center of the nebula.
The Planetesimal Hypothesis
The Planetesimal Hypothesis can explain the east to west manner and rotation as well as the Nebular Hypothesis if the star pulled the bolts in an east to west manner. Furthermore, the passing star can also explain why the planets and their satellites revolve on the plane of the ecliptic and why they revolve in circular ellipses because the hypothesis suggests that the star started the planets revolving in the same motion and plane. In explaining the age of the solar system, this hypothesis is neither accurate nor inaccurate because there is no date of occurrence. It can explain the sweeping of primordial gases from the solar system by suggesting that the lighter gases were pulled by the passing star, while heavier elements fell back to the sun because of gravitation. Its proposal for the way planetary material is formed is questionable: If planets were born from material dragged from the sun, planets would contain materials similar to those found in carbonaceous chondrites. However, Lyman Spitzer, Jr., observed that because the molecules used to form planets would be about one-million Kelvins in temperature, the particles would disperse instead of condensing into planets (Huffer, et al., 1967). Thus, the hypothesis fails to explain the presence of materials in planets similar to carbonaceous chondrites. The Planetesimal Hypothesis can explain, however, why the sun has such a small angular momentum and the planets have so much: If the star began the spinning, the outer planets would spin faster than the inner planets. Nevertheless, this model was criticized by Henry Norris Russell, who observed that the distribution of angular momentum proposed by the hypothesis was incorrect because it meant that the inner planets would be larger than the outer planets (Huffer, et al., 1967). In this view, the Planetesimal Hypothesis fails to explain the angular momentum of bodies in the solar system.
The Dust Cloud Hypothesis
This hypothesis can explain how the rotation and revolution of the planetary bodies came about, but it does not satisfactorily explain revolutions on the plane of the ecliptic because the smaller clouds could have coalesced into planets in random areas of the larger cloud, creating several different planes of revolution. The Dust Cloud Hypothesis does, however, explain the orbits of a nearly circular ellipse by the slow rotation of the cloud. The process the hypothesis describes falls within the age range of the formation of the solar system because it estimates that a dark globule would collapse in about 100 million years (Huffer, et al., 1967). This theory also makes use of a T-Tauri phase to explain how the gases were swept from the solar system. Furthermore, it can be inferred that if everything originated from the same cloud, cosmic bodies would share a similar composition. Thus, this hypothesis supports the evidence provided by carbonaceous chondrites. Finally, the hypothesis satisfactorily explains the small angular momentum of the sun if the dust cloud created the motion of cosmic bodies and if the dust cloud had a very slow spin. However, this hypothesis cannot explain the great angular momenta of the planets.
It appears that the Nebular Hypothesis is a more viable possibility than the other two hypotheses for five reasons: First, according to the Planetesimal Hypothesis, it appears as if our solar system occurred by chance: It is too coincidental that a star would pass close enough to the sun (without colliding) to pull those bolts. Second, one could question how the sun could remain unchanged from today if ten bolts of material were pulled from it by a passing star. Third, the Dust Cloud Hypothesis does not address the randomness of how the planets coalesced; it is unlikely that the planets formed on the same plane by chance. Fourth, it seems that the Dust Cloud Hypothesis could be more acceptable if it explained the movement of smaller clouds (did they move in a circular ellipse?). Finally, the Nebular Hypothesis seems to explain the majority of the data better than both the Planetesimal and Dust Cloud hypotheses.
Though not all hypotheses adequately explain all of the available data, it is clear that the hypothesis that endures will explain a good part of the data. Whether the answer is attainable is unknown. However, scientists working together on these questions can eventually teach us more about ourselves. For instance, if scientists can discover how the solar system was created, they can generalize about how life began on Earth and can then look for clues in space for solar systems of similar origins to find other worlds and other forms of life.
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Thiel, Rudolf (1957). And There Was Light. New York: New American Library of World Literature.