Heliocentric, Tidal and Nebular Theory Help (page 4)
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:
- 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.
The Tidal Theory
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.
Relative Motions Of The Stars
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 × 10 12 ) km or 5.9 trillion (5.9 × 10 12 ) mi. Our galaxy has roughly 200 billion (2 × 10 11 ) 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 (10 10 ) 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.
The Nebular Theory
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 .
The Stuff Of Stars
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.”
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 × 10 9 ) 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.
The Accretion Disk
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.
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.
Are We Too Hope-driven?
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!
Decline And Death
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.
Practice problems of this concept can be found at: Evolution of Solar System Practice Problems
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