On a clear night when the Moon is below the horizon, you get a feeling of great depth as you look at the heavens. Some ancient stargazers surely sensed this depth and decided that space must be a huge expanse and that Earth must be insignificant compared with the whole. For a long time, however, this idea was unacceptable. If anyone thought about such things, they kept their notions to themselves. Today’s astronomers theorize not only about what the Universe is like now but also about how it and our own Solar System came into being and evolved to its present state.
In science, there are certain things we do not know, but we develop beliefs according to what we can see and according to what we can obtain by using mathematics and logical reasoning. We might say “such and such is true” because we’ve seen it or because we’ve deduced it based on observations. If we think something is true but aren’t certain, it’s tempting to say that “such and such is likely.” But we’re still not sure.
Most of the material in this book, up until now, has been based on observed facts (except, of course, for our “mind journeys”). The diameter of Earth can be measured, as can the temperature of the solar corona. It is taken as a fact nowadays that Earth revolves around the Sun. However, the way the Solar System was formed is not known with such certainty. No human ever saw nor has any machine ever recorded that sequence of events. The best we can do is propose a hypothesis, an idea of what we think took place. Then we can make arguments, based on logic, observation, and computer modeling, so that we can come up with a theory.
When people formulate a theory, it is tempting to say that something “probably” happened in the distant past or that there is a “good chance” that such and such exists in the Milky Way galaxy. This is a logical pitfall. It is so easy to make this thought-process error that I have probably committed it somewhere in this book. (Whoops! I did it in that very sentence!)
An astronomer can say that he or she believes the Milky Way galaxy is moving away from all the other galaxies in the Universe; some will strengthen this into an opinionated statement of fact. If you say, “I think the Universe started with an explosion” or “I believe that the Universe started with an explosion,” you are within your rights. Even if you say, “The Universe began with an explosion!” your statement is logically sound, although it must be understood that this is a theory, not a proven fact. However, if you say, “The Universe probably started with an explosion,” you have committed the dreaded thought error, which, for lack of a better name, I call the probability fallacy. Whatever has been has been! Either the Universe started with an explosion, or else it did not. (There are some people who will argue that either statement can be true depending on how you define the various arrows of time, but I won’t get into that right now.)
The same holds true for other unknowns. A good example is the “probability that intelligent life exists elsewhere in the Universe.” Assuming that we have defined the meaning of the word intelligent, we can confidently say that such beings either exist or they don’t. If I say in this book that the probability of intelligent life existing elsewhere is “20 percent,” I am in effect saying something like this: “Out of 1,000 observed Universes, 200 of them have been found to have intelligent extraterrestrial life, but I don’t know which one of the 1,000 Universes I happen to live in.” This is nonsense!
When we think about how the Solar System was formed, we must keep in mind that there is a definite reality, a specific sequence of events, that took place to get us from that place where “all was dark and without form” to where we are now. Our task is to find out the truth and not to try to attach artificial “probabilities” to things that have already happened or to things that never took place at all.
Probability can be assigned to an event only on the basis of the results of observations involving a large number of samples. Additionally, probability makes sense only when talking about the future; it is irrelevant when dealing with the past or present.
There are certain theories involving so-called fuzzy truth in which some events can be considered to “sort of happen.” These theories involve degrees of reality, and in such a scenario, probability can be used to talk about events in the past, present, and future. The most common example of this kind of theory is quantum mechanics, which involves the behavior of atoms, molecules, and subatomic particles. Quantum mechanics can get so bizarre that some scientists have actually said, “If you claim to understand this theory, then you are lying.” Fortunately, we aren’t going to be dealing with anything that esoteric in this book.
When statistics is misapplied, seemingly logical reasoning can be used to support all manner of hogwash. It is done in industry all the time, especially when the intent is to get you to do something that will cause someone else to make money. Therefore, keep your “probability-fallacy radar” on. We are about to leap into territory where every good scientist needs it!
If you come across an instance where an author (including myself) slips and says that something “probably happened” or “is likely to take place,” think of it as another way of saying that the author, or scientists in general, believe or suspect that something happened or will take place.
Most ancient philosophers believed that all celestial objects were attached to concentric spheres with Earth at the center. It never crossed their minds, apparently, to inquire very much into what might lie outside those spheres. Some thought that the outermost sphere was opaque, with little holes through which an outside light shone. Some thought the stars were fires on the inside of the outermost sphere.
Eventually, observers noticed that there were some peculiarities about the concentric-sphere model of the Universe. The Sun did not always stay in the same position relative to the stars. Some of the stars moved among the stationary majority. These moving stars were called wanderers, or planets. Each planet was assigned its own sphere. The Moon had a sphere for itself, as did the Sun.
Figure 9-1 shows one of the earliest models of the Solar System out to the planet Saturn, which in ancient times was the most distant known object except for the sphere containing the stars. Earth was believed to be at the center of it all. The theory that Earth was at the center of creation has been called the geocentric theory. This theory underwent many variations, refinements, and contortions before the bearers of conventional wisdom saw fit to throw it out.
Figure 9-1. One of the earliest models of the Universe. Earth is at center. Numerical tags: 1 = Moon, 2 = Mercury, 3 = Venus, 4 = Sun, 5 = Mars, 6 = Jupiter, 7 = Saturn, 8 = stars.
Some ancient astrologers (this is what astronomers were called in the olden days) thought that since all these spheres rotated at their various speeds and on their independent axes, there must be friction among them. This friction must, they believed, create cosmic music, perhaps accompanied by the singing of angels—hence the expression “the music of the spheres.” Such a noise was considered too faint for ordinary human beings to hear. However, privileged people claimed to hear it, and they assured their contemporaries that it was beautiful. Today’s scientists will tell you that this was nonsense, imaginary at best and a bad joke at worst. Sounds made by distant celestial objects cannot reach Earth. Sound does not travel through outer space.
The model shown in Fig. 9-1 was, after a time, seen to have certain shortcomings. It failed to explain certain things. According to the simple geocentric theory, all the planets would maintain a constant and uniform motion, always in the same direction, with respect to the background of stars. However, observers noticed that the planets do not behave this way.
Once in a while, a planet appears to stop, turn around, go backwards, stop again, and then resume its normal forward motion among the stars. Some planets do this more often than others. Plotting the position of a planet over a period of weeks relative to the stars will reveal a loop (Fig. 9-2). The astronomer Ptolemy, who lived during the second century A.D., developed a model, a variant of the geocentric theory, that explained this phenomenon. It became known as the Ptolemaic model, and it endured for centuries.
According to Ptolemy’s theory, rather than following a perfectly circular orbit around the Earth, each planet was assigned an orbit consisting of two distinct components, called the deferent and the epicycle. The deferent was a perfectly circular path around Earth, but the planet in question was believed not to follow it. Instead, the planet was thought to follow the epicycle, or smaller orbit, around a point in the deferent that in turn was theorized to maintain a constant motion around Earth (Fig. 9-3). If this sounds a little strange to you, you are not alone. How can a single object orbit around a point in empty space? Doesn’t that violate some principle of physics? Of course it does! However, those principles had not yet been laid down in Ptolemy’s time, and the theory of Ptolemy did an excellent job of explaining the observations made by astronomers.
Figure 9-2. The path of a planet, as seen from Earth against the background of stars, occasionally does a “loop-the-loop.”
Figure 9-3. An example of a deferent/epicycle planetary orbit.
Ptolemy’s epicycle model was not perfect. On closer and closer scrutiny, it was found that additional epicycles within epicycles were necessary to predict the exact position of a planet at any given future moment. This process could continue ad infinitum, with smaller and smaller epicycles superimposed on one another endlessly. The believers in Ptolemy’s model began to wonder why God was so perverse when He designed the Universe. (Some scientists still ask this question.) A few people expressed cynicism and veiled sarcasm.
Whenever a scientific theory or model appears to fit observed facts, it is placed under an ongoing attack by skeptics. Their intent is not necessarily malicious. The idea is to test the theory. This is how theories are refined. Once in a while, however, a theory gets so awkward that scientists decide that it should be scrapped and that the whole business ought to start over. It’s “back to the drawing board!” This stage was not reached with regard to the Ptolemaic model until the world’s attitudes had evolved past the intellectual vacuum known as the Middle Ages.
An unfortunate Italian philosopher named Giordano Bruno was vocal about his doubts in the latter part of the sixteenth century. He was condemned by the powerful church leaders and put to death in the year 1600. A little while later, Galileo Galilei, another open skeptic, was confined to house arrest and told to be silent for expressing similar doubts. The church could not accept the idea that Earth does not sit at the center of all creation.
The “thought police” of the church held less power in northern Europe than they did in Italy. Proponents of the heliocentric (Sun-centered) theory were taken seriously in places such as Germany, France, Poland, and England.
Nicolaus Copernicus, a Polish astronomer, published a work in the early sixteenth century suggesting that the Sun, not the Earth, must be at the center of the Universe. (Remember that back in the sixteenth century the Earth, the Moon, the Sun, and the planets basically defined the entire Cosmos. No one knew what the stars were, much less how they were distributed throughout space.) The Earth, thought Copernicus, is a planet just like Mercury, Venus, or Mars insofar as its importance in the overall scheme of things. But Copernicus could not prove his theory to the complete satisfaction of the authorities in his part of the world. If the Earth is moving, asked the skeptics, why don’t we feel a constant wind from space? What force could push the Earth? Why should such a force exist?
Another astronomer, Tycho Brahe, was involved with an ongoing meticulous mapping and recording job. He kept careful records of the positions of all the planets over a period of time. Brahe had a German assistant named Johannes Kepler who eventually formulated the three fundamental rules for planetary motion, known as Kepler’s laws. Isaac Newton put it all together and finally changed mainstream thinking. The Earth had lost its exalted position, replaced by the Sun. The heliocentric theory had survived the test of time and had become the conventional wisdom.
Johannes Kepler published his famous rules of planetary motion early in the seventeenth century. They can be stated briefly as follows:
Figure 9-4. Kepler’s first and second laws. The planet’s orbit is an ellipse with the Sun at one focus, and equal areas (X) are swept out in equal periods of time (t).
Each planet follows an elliptical orbit around the Sun, with the Sun at one focus of the ellipse.
An imaginary line connecting any planet with the Sun sweeps out equal areas in equal periods of time.
For each planet, the square of its “year” (sidereal period) is directly proportional to the cube (third power) of its average distance from the Sun.
Theoretically, it is possible for a planet’s orbit to be perfectly circular. A circle is an ellipse in which both foci are at the same point. In reality, however, there is always some imperfection, so all planets follow orbits that are slightly oblong.
Kepler did not originally call his rules laws. This label was attached later by others. Kepler came up with his three principles and refined them over a period of several years. The first two rules were finalized in 1609, and the last one came out in 1618. The first two laws are illustrated in Fig. 9-4, and third law is rendered graphically in Fig. 9-5.
Figure 9-5. Kepler’s third law. The squares of planets’ orbital periods (R and S) are proportional to the cubes of their mean orbital radii (r and s).
According to the tidal theory, the Sun originally had no planets or other satellites. This theory suggests that our Sun formed alone and that the other objects, including the planets and the major asteroids, came later.
The Milky Way, the spiral-shaped galaxy in which we dwell, is believed to be 100,000 light-years across. A light-year is the distance that light travels in 1 year, approximately 9.5 trillion (9.5 × 1012) km or 5.9 trillion (5.9 × 1012) mi. Our galaxy has roughly 200 billion (2 × 1011) stars, all revolving around the nucleus like an enormous swarm of bees. According to current theories, many of the stars bob up and down, above and below the galactic plane, passing periodically through it. Some stars have highly eccentric orbits around the galactic center.
Although the stars are tiny compared with the space between them, they are in relative motion, and collisions or near misses occur once in a while simply because there are so many stars. On average, however, according to one estimate, an outright collision is an extreme rarity, taking place only about once in every 10 billion (1010) years for a typical spiral galaxy such as ours. This is almost as old as the whole Universe is believed to be! Nevertheless, those people who say that the Sun fell victim to a near catastrophe with another star cannot be discounted completely.
Suppose that another star came close enough to the Sun that it and the other star engaged in a gravitational tug-of-war. What would happen? For one thing, the paths of both stars in the Milky Way would be altered; the two stars would swing around each other. In fact, if they came close enough and the speed was not too great, they would end up in orbit around each other. Suppose, however, that the encounter was extremely close but at high speed so that the two stars did not end up in mutual orbit? They would pull matter from each other and scatter that matter into orbits around either star, where the matter would cool, condense, and form dust, rocky ice chunks, and rocks.
Given time, the particles in orbit around the Sun would coalesce into larger objects because of mutual gravitation. Eventually, several dozen spherical objects, perhaps comparable with the size of our Moon, would be created. These objects would follow all kinds of different orbits because of the chaotic way in which the matter was scattered during the original battle of the stars. The result would be frequent collisions and further coalescing. Computer models can show that the end result would be a few large, massive objects and countless tiny ones. This is, of course, the way we observe the Solar System today.
There are problems with this so-called tidal theory. If this is the way the Solar System formed, the planets would all revolve around the Sun in different planes, and their orbits would be less circular and more elongated than they are (Fig. 9-6). However, the actual state of affairs is far more orderly. The planets all lie in nearly the same plane. With the exceptions of Mercury and the Pluto-Charon system, their orbits are nearly perfect circles. All the planets revolve around the Sun in the same direction. For these reasons, few astronomers today believe that the tidal theory is an accurate representation of what happened. In addition, the fact that such catastrophes in general occur only once every several billion years, in our galaxy at least, is a good reason to doubt that this theory explains how things took place to create our Solar System.
If a star has several times the mass of the Sun, ultimately it will explode in a violent outburst called a supernova. These events leave entrails in space—clouds of gas, dust, and rocks of various sizes—in their vicinity. Such mass of debris can appear either light or dark through a telescope depending on how the light of nearby stars shines on it. The cloud is called a nebula.
Most nebulae form near the plane of our spiral-shaped Milky Way galaxy. They are clearly visible in other spiral-shaped galaxies when those galaxies present themselves edgewise to us. Some spiral galaxies are so thick with nebulae that they appear split in two when we see them from within the planes of their disks. Our Solar System is near the plane of the Milky Way, and our galaxy, like all spirals, has plenty of nebulae. This keeps the sky dark at night. If it were not for these obstructing clouds, the sky would be almost as bright when the Sun is “down” as when the Sun is “up.”
Figure 9-6. If the tidal theory were correct, the planets would have elongated orbits (gray ellipses) in various planes.
According to the nebular theory, also called the rotating-cloud theory, it is from these clouds that second-generation stars, such as our Sun, are born. Evidence suggests that the Solar System formed approximately 4.6 billion (4.6 × 109) years ago from one of these. The Earth, all the other planets, the asteroids, and the comets are all believed to have formed from a cloud produced a long time ago in one or more supernovae.
As you have already learned, the Sun takes about a month to rotate once on its axis. Because of this, it is logical to suppose that the debris cloud from which the Sun formed had some rotational momentum. Imagine a hurricane forming from the clouds in the tropics. Have you ever seen a time-lapse satellite photo of this process? Think about the eddies or whirlpools that form in the water as you pull a canoe paddle through. According to the rotating-cloud theory, the Sun formed at the center of an eddy in interstellar space.
Astronomers have shown that a cloud of debris, collapsing because of the mutual gravitation among all its particles, would develop one or more vortices, or whirlpools. Near each vortex, the matter would become aligned in a plane, creating a rotating, disk-shaped cloud. It can be demonstrated by computer modeling that the matter in such a cloud would condense into an accretion disk and thence into numerous discrete objects: a large central mass (to become the Sun) and other, relatively small masses in orbit around it (to evolve into the planets and their moons). One theory, proposed several centuries ago, took notice of this fact (without the help of computer modeling, of course) and came to the conclusion that the matter orbiting the Sun would congeal into rings before finally developing into solid planets.
Figure 9-7 is a hypothetical illustration of how the Solar System’s primordial cloud looked from a distance of about 100 astronomical units (AU). In this example, the disk is viewed at an angle, neither face-on nor edgewise, so that the nearly circular rings appear oblong. The Sun is at the center, and it is about to start up its internal nuclear-fusion furnace. The disk-shaped cloud, and in particular its rings, glow from the Sun’s increasing radiance and from the light of other nearby stars. According to the rotating-cloud theory, the particles in the rings gradually pulled themselves together over a period of millions of years into small objects called planetesimals, and these ultimately accreted into the planets. Most of the non-solar matter in the cloud found its way into the planet Jupiter; smaller amounts congealed into the other planets. As the planetesimals aggregated into larger objects, the matter in them swirled just as had the original parent cloud. This explains why the planets rotate. It also explains why most planetary moons orbit in the same sense as all the planets orbit around the sun and why most (but not all) planetary moons orbit near the plane of the planets’ orbits.
Figure 9-7. The primordial gas-and-dust cloud according to the rotating-disk theory of solar system formation.
The original version of this nebular theory is credited to two men who lived during the eighteenth and early nineteenth centuries: Immanuel Kant, a German philosopher, and Pierre-Simon Marquis de Laplace, a French astronomer and mathematician. In particular, Laplace went into detail concerning the motions of the various planets and moons. In recent decades, the nebular theory has been refined, especially in an attempt to explain why the Sun rotates only once a month and not much faster. In addition, the existence of the rings in the primordial accretion disk has been questioned. Many astronomers believe that the matter simply clumped together into larger and larger “particles,” ending up with the system of planets we now have. The asteroids in the belt between Mars and Jupiter were prevented from accreting into a planet because of the powerful gravitational influence of Jupiter.
Certain questions remain difficult to answer—in particular the extreme tilts of Venus, Uranus, and the Pluto-Charon systems on their axes. You will remember that the axis of Venus is tilted nearly 180 degrees; another way of saying this is that its rotation is retrograde. The nebular theory does not specifically forbid this, although it suggests that most planets will end up rotating in the same sense as they orbit around the Sun. In the case of Uranus, some astronomers think that it was struck by an object so massive that its rotational axis was “knocked flat” by the encounter. In that scenario, both objects were nearly shattered; in the end, however, Uranus survived, and the other object did not. The same thing may have taken place with Venus and Pluto-Charon.
The nebular theory explains why the planets orbit the Sun in a comparatively uniform manner. In addition, assuming that this theory is correct, we have good reason to believe that there are many such systems in our Milky Way galaxy, as well as in other galaxies, especially those of the spiral type with their abundant interstellar gas and dust.
Astronomers have found evidence of other planetary systems. Flat, circular clouds or rings, thought to be accretion disks, have been observed by the Hubble Space Telescope. If we actually are looking at stars with planets forming around them or in orbit around them, then it means that our Solar System is not a freak cosmic accident. If the Universe is teeming with planetary systems like ours, it is tempting to believe that there are many Earthlike planets too and that some of these planets have evolved intelligent life.
Critics of the nebular theory use the foregoing speculations against it. They say that hope drives the thinking of the proponents of the theory and that this emotion interferes with rational reasoning. If our Solar System is the only one of its kind in the whole Universe, they say, then so what? We are here to bear witness to the miracle of life on Earth simply because we are one of its products!
Just as there are theories about how the Solar System was formed, there are notions concerning its long-term future. The ultimate fate of the Solar System depends on its parent star, the Sun. Most astronomers believe that the Sun eventually will swell into a red giant, burning up or vaporizing Mercury, Venus, Earth, and Mars and perhaps blowing the gas away from Jupiter, Saturn, Uranus, and Neptune. Then the Sun will shrink down and die out like an ember in a dying fire around which living beings were once encamped. Where those creatures, our distant descendants, will be by then is a question that no theory can answer.
QuizRefer to the text if necessary. A good score is 8 correct. Answers are in the back of the book.
1. Imagine an alien star system in which Planet X has a mean orbital radius of 100 million (108) km from Star S and Planet Y has a mean orbital radius of 2 × 108 km from Star S (twice the mean orbital radius of Planet X). Suppose that the “year” for Planet X is equal to exactly one-half Earth year (0.500 yr). How long is the “year” for Planet Y?
(a) 2.000 years
(b) 1.414 years
(c) 1.000 year
(d) It can’t be figured out from this information.
2. The imperfections in Ptolemy’s theory were “corrected,” without rejecting the whole theory, by
(a) adding epicycles within epicycles until the theory fit observed facts.
(b) placing the Moon at the center of the Solar System.
(c) ignoring the distant stars.
(d) considering all the planets except Earth to orbit the Sun.
3. The planet Saturn orbits the Earth according to
(a) the geocentric theory.
(b) the heliocentric theory.
(c) Kepler’s theory.
(d) Newton’s theory.
4. Which of the following is not a good argument against the tidal theory?
(a) The planets all orbit the Sun in nearly the same plane.
(b) The planets all orbit the Sun in the same direction.
(c) Stars can never pass so close that they pull matter from each other.
(d) None of the planets have extremely elongated orbits.
5. How likely is it that beings like us exist elsewhere in the Milky Way galaxy?
(a) Not likely
(b) Somewhat likely
(c) Very likely
(d) This is a meaningless question. Either there are such beings or there aren’t.
6. Which of the following statements is implied by Kepler’s laws?
(a) A planet moves fastest in its orbit when it is farthest from the Sun.
(b) Planets far from the Sun take longer to complete their orbits than planets closer to the Sun.
(c) All the planets’ orbits lie in exactly the same plane.
(d) All the planets’ orbits are perfect circles.
7. According to the Big Bang theory of Solar System formation,
(a) The planets formed when the primordial Sun exploded, casting some of its matter into space.
(b) The planets evolved from a rotating cloud of gas and dust.
(c) The planets were formed from matter ejected from a huge solar volcano.
(d) Forget it! There is no Big Bang theory of Solar System formation.
8. According to one theory, Uranus has an axis that is tilted to such a great extent because
(a) the planet was not massive enough for its equator to align itself with the plane of its orbit.
(b) a large primordial object smashed into Uranus and tipped it over.
(c) sooner or later such a tilt will be exhibited by all the planets.
(d) the gravitational effect of Neptune pulled the axis of Uranus out of kilter.
9. For publishing his theories in the sixteenth century, Giordano Bruno was
(a) knighted by the Queen of England.
(b) made the official astronomer of the Vatican.
(d) executed.
10. When the earliest models of the Solar System were formulated, the most distant known planet was
(a) Mars.
(b) Jupiter.
(c) Saturn.
(d) Uranus.
