| Overview of the Solar System |
| Before we begin a detailed discussion of the components of the Solar System, it is useful to get an overview of the general properties and features of the Solar System. We begin by surveying properties such as size, mass, density, and then discuss the orbits of the planets. Finally, we discuss a theory for the origin of the Solar System that is capable of accounting for these observations. |
| | The Origin of the Solar System |
| | The Nebular Hypothesis in its original form was proposed by Kant and Laplace in the 18th century. The initial steps are indicated in the following figures. |
| | Condensation of Protosun and Protoplanets |
| | As the nebula collapses further, instabilities in the collapsing, rotating cloud cause local regions to begin to contract gravitationally. These local regions of condensation will become the Sun and the planets, as well as their moons and other debris in the Solar System. |
| | While they are still condensing, the incipient Sun and planets are called the protosun and protoplanets, respectively. |
| | The Spinning Nebula Flattens |
| | Because of the competing forces associated with gravity, gas pressure, and rotation, the contracting nebula begins to flatten into a spinning pancake shape with a bulge at the center, as illustrated in the following figure. |
| | Collapsing Clouds of Gas and Dust |
| | A great cloud of gas and dust (called a nebula) begins to collapse because the gravitational forces that would like to collapse it overcome the forces associated with gas pressure that would like to expand it (the initial collapse might be triggered by a variety of perturbations---a supernova blast wave, density waves in spiral galaxies, etc.). |
| | It is unlikely that such a nebula would be created with no angular momentum, so it is probably initially spinning slowly. Because of conservation of angular momentum, the cloud spins faster as it contracts. |
| | Evidence for the Nebular Hypothesis |
| | Because of the original angular momentum and subsequent evolution of the collapsing nebula, this hypothesis provides a natural explanation for some basic facts about the Solar System: the orbits of the planets lie nearly in a plane with the sun at the center (let's neglect the slight eccentricity of the planetary orbits to simplify the discussion), the planets all revolve in the same direction, and the planets mostly rotate in the same direction with rotation axes nearly perpindicular to the orbital plane. |
| | The nebular hypothesis explains many of the basic features of the Solar System, but we still do not understand fully how all the details are accounted for by this hypothesis. As we discuss in the next section, we now have some direct observational evidence in support of the nebular hypothesis. |
| | Revolution and Rotation of Planets |
| | As discovered by Kepler, the planets orbit on ellipses with the Sun at one focus. In addition, the planets all revolve in the same direction on their orbits (direct orbital motion). Let's now consider the orbits of the planets in more detail. |
| | Here is the inner solar system constructed with the Solar System Live software. |
| | The sizes and shapes of the orbits are realistic, as is the relative positions of the planets for the date in the Fall, 1996, when the plot was constructed. The sizes of the planetary symbols are not to scale; the planets would be too small to see at this scale as more than dots of light. Notice the eccentricity of the orbits for Mercury and, to a lesser degree, Mars. From this perspective (which corresponds to looking down on the Northern hemisphere of the Earth), the planets all revolve in a counter-clockwise sense, as indicated by the arrow. |
| | Here are the present positions (to scale) of planets in the inner solar system. In this plot, the portion of orbit in blue is above and the portion in green is below the plane of the ecliptic. As noted in conjunction with Kepler's Third Law, motion of the innermost planets is much faster than that of the outermost; |
| | this animation illustrates realistic motion of the inner solar system. |
| | The preceding views represent a "top" or North perspective. Here is a side perspective of the inner Solar System showing the tilt of the planetary orbits with respect to the plane of the ecliptic. |
| | In this figure the white portion of the orbit is above the ecliptic plane and the yellow portion is below. Notice that the orbits of the inner planets are nearly, but not quite, in the same plane. The orbit of Mercury, in addition to being the most eccentric, has the largest tilt (7 degrees) with respect to the ecliptic plane. |
| | Here is the entire Solar System to scale for the orbits, also in the Fall, 1996: |
| | Notice the enormous amount of empty space in the outer Solar System. To show the entire Solar System to scale, the inner Solar System becomes so compressed that the planet orbits almost appear to run together. The very large eccentricity of Pluto's orbit is also obvious. |
| | Here are the present positions (top view, to scale) of all planets in the Solar System. As above, the portion of orbit in blue is above the plane of the ecliptic; portion in green is below the plane of the ecliptic. |
| | The following figure shows the full Solar System to scale from a side view to illustrate the tilt of the orbits. |
| | Notice that Pluto's orbit is highly tilted (17 degrees) relative to the plane of the ecliptic. |
| | Here is the present position (side view, to scale) of all planets in the Solar System. The portion of the orbit in blue is above the plane of the ecliptic; portion in green is below the plane of the ecliptic. View is from 20 degrees above ecliptic plane to emphasize the tilt of orbits. |
| | Here is the average separation of the planets from the Sun (in astronomical units) displayed in graphical form, |
| | and here are the eccentricities of the planetary orbits |
| | Binary Star Systems Versus Planetary Systems |
| | Our Solar System may not be the norm for stars in the Universe. The observational evidence is that most stars are parts of multiple star systems, not single stars like our Sun. |
| | If Jupiter Had Become a Star . . . |
| | We note in this connection that if Jupiter had been about 100 times more massive than it is, it would have formed a star instead of a planet. Thus, maybe the Solar System very nearly became a binary star system instead of a single star with planets. We may speculate that in that case the Earth might not even exist, or even if it existed would be in an orbit giving surface conditions not favorable to the evolution of life. |
| | Formation of Binary Star Systems |
| | The most common occurrence of stars appears to be as parts of binary (two-star) systems. This suggests an alternative to the nebular hypothesis illustrated in the following figure. |
| | Although planets might still form in such binary systems by a similar mechanism as discussed before, it is an open question whether they would have stable orbits that would keep them bound in the system without running into the stars. Another question, assuming such planets were on stable orbits, is whether they could have temperature ranges favorable for the formation of life. |
| | Solar Systems in the Making? |
| | The nebular hypothesis for the origin of our Solar System has been bolstered by a variety of recent observations that look very much like star and planetary systems in various stages of formation. |
| | More Star-Forming Regions |
| | Many other star-forming regions are known. In addition to the Eagle Nebula discussed below, here are images and discussion of |
| | In each of these examples there is strong evidence that stars are being born in the region shown in the image; presumably, at least in some of the cases, attendant solar systems are being formed also. |
| | Star-forming regions in the Trapezium (the central region of the Orion Nebula). |
| | Star formation in the Lagoon Nebula, with possible evidence for tornadoes 1/2 light year in length |
| | Star-forming region in the nebula NGC 604 (in the galaxy M33). |
| | Recent Hubble Space Telescope observations shed considerable light on the birth of stars and associated planetary systems. The following image shows regions in the Orion Nebula where solar systems may be forming. |
| | The Orion Nebula is approximately 1500 light years from Earth. It is visible to the naked eye as the middle "star" in the sword of the constellation Orion. These images were taken with the Wide Field Planetary Camera 2 of the Hubble Space Telescope (C.R. O'Dell, Rice University). Details of the images show several protoplanetary disks ( proplyds ), including a single dark disk surrounding a central star. |
| | Star Formation in the Eagle Nebula |
| | The following images show examples in the Eagle Nebula of regions where stars (and possibly solar systems) appear to be forming. |
| | Planets Around other Stars |
| | In recent years rather conclusive evidence has accumulated for planets orbiting other stars. This evidence comes from the gravitational perturbations exerted on the star by the unseen companion planet that can be exposed by very accurate measurement of the radial velocity of the star (see the related discussion of detecting unseen companions in binary star systems). These measurements require that variations in the radial velocity of order 10 meters per second be detected relative to a total radial velocity typically of order 10-100 kilometers per second. Here is more information about the newly discovered planets, and here is an online Extrasolar Planets Encyclopedia. |
| | Conservation of Angular Momentum |
| | Our theory for the origin of the Solar System is a very old one with some modern innovations called the Nebular Hypothesis. A crucial ingredient in the nebular hypothesis is the conservation of angular momentum. |
| | Objects executing motion around a point possess a quantity called angular momentum. This is an important physical quantity because all experimental evidence indicates that angular momentum is rigorously conserved in our Universe: it can be transferred, but it cannot be created or destroyed. For the simple case of a small mass executing uniform circular motion around a much larger mass (so that we can neglect the effect of the center of mass) the amount of angular momentum takes a simple form. As the adjacent figure illustrates the magnitude of the angular momentum in this case is L = mvr, where L is the angular momentum, m is the mass of the small object, v is the magnitude of its velocity, and r is the separation between the objects. |
| | Ice Skaters and Angular Momentum |
| | This formula indicates one important physical consequence of angular momentum: because the above formula can be rearranged to give v = L/(mr) and L is a constant for an isolated system, the velocity v and the separation r are inversely correlated. Thus, conservation of angular momentum demands that a decrease in the separation r be accompanied by an increase in the velocity v, and vice versa. This important concept carries over to more complicated systems: generally, for rotating bodies, if their radii decrease they must spin faster in order to conserve angular momentum. This concept is familiar intuitively to the ice skater who spins faster when the arms are drawn in, and slower when the arms are extended; although most ice skaters don't think about it explictly, this method of spin control is nothing but an invocation of the law of angular momentum conservation. |
| | There are many popular misconceptions concerning the size and scale of objects in the Solar System. These mostly have to do with a failure to realize the relative radii of planets and the Sun, and the failure to appreciate how large the outer solar system is relative to the inner solar system. |
| | The Relative Radii of the Sun and Planets |
| | The Sun and the gas giant planets like Jupiter are by far the largest objects in the Solar System. The other planets are small specks on this scale, as the following figure illustrates. |
| | Indeed, on this scale the smaller planets like Pluto and Mercury are barely visible. |
| | The masses of the planets are also concentrated in the Gas Giant planets Jupiter, Saturn, Uranus, and Neptune, as the following graph indicates. |
| | However, the large mass of these planets comes from their absolute sizes, not their densities. The inner planets are by far the most dense, as the following graph indicates: |
| | This distribution of masses and densities in the Solar System is a key observation that a theory of the origin of the Solar System must explain. |
| | Here is an astrophysical calculator that will display basic astronomical constants and solar system data at the touch of a button, and also allow calculations using these quantities. |
| | Overview of the Solar System |
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