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On this page, links to other relevant planetary Websites are emplaced. A pitch is made to the reader to consider taking a look at the extensive review of Cosmology in Section 21 as a background to this Section - preferably switching to it before moving on to the planets. A table lists the major facts and parameters pertaining to the solar planets. Some of the characteristics of the motions and distribution of the planets are described in terms of the historical contributions by Copernicus, Brahe, Kepler, and Newton.

Links to Planetary Sites

Planetary exploration has become one of incredible and vast accomplishments, in which huge amounts of data have now accumulated. Much of it has relied exclusively or largely on remote sensing. This Section is one of the longest in the 27 units of the Tutorial. The intention is to provide a thumbnail view of the major missions to the planets, but with the restriction that we show almost exclusively just surface images; usually no more than three to ten representatives of each mission. Despite the importance of learning about planetary atmospheres, we will not say much about the results of remote sensing of these gaseous envelopes nor do we discuss in any detail facts and conjectures about planetary interiors.

There are many sources of additional images and descriptive information. Among the best of these currently online is a replica of Chapter 5: Planetary Geology, by James Bell, Bruce Campbell, and Mark Robinson, in the 3rd Edition of the Manual of Remote Sensing: Earth Sciences Volume, 1996, at Marswatch. This lengthy and detailed review focuses on remote sensing approaches to planetary exploration. Its one drawback is a sparsity of images (compared with this Section 19 overview). An excellent chronological survey of the history of space exploration is found at the planetscapes website. Another site worth visiting is the Home Page of the Jet Propulsion Laboratory (JPL) where you can get addresses to visit other sites dealing with terrestrial and planetary space programs. JPL has recently selected choice images of the planets from various missions in a special Web site called the Planetary Photojournal, which you can access at USGS. Some images used in this section come from that source. Another NASA source is the National Space Science Data Center NSSDC. Two other exceptional Home Pages are The Nine Planets, by Bill Arnett of the Lunar and Planetary Laboratory, University of Arizona (LPL) and Views of the Solar System, by C.J..Hamilton of the Los Alamos National Laboratory (Spaceart). Dr. J. Schombert of the University of Oregon offers three courses on Planets, Astronomy, and Cosmology that he has put on the Web; the first of these - The Solar System - is accessed at hisAST121 site. The NASA Headquarters Space Sciences Directorate maintains an excellent Site that summarizes the major findings in both planetary and cosmological realms during the latest 9 to 12 months that can be accessed at this site (see its lists, especially News). Many solar system missions were managed and conducted by the Jet Propulsion Laboratory; descriptions of Past, Current, and Future missions are obtaining by clicking on any of interest at thisMissions site.

Books that treat planetary remote sensing include the aforementioned one by Billy Glass of the University of Delaware, and a now out-of-print text by this Tutorial's author (Nicholas M. Short, Planetary Geology, 1975, Prentice-Hall Publ.), still in libraries. More recent are Planetary Landscapes by R. Greeley, 1985, Allen & Unwin, and Exploring the Planets by W.K. Hamblin and E.H. Christiansen, MacMillan, 1990.

Before we start our tour of the planets, you may wish to review some of the main principles and concepts of astronomy. If so, please skip to Section 20, which is a comprehensive review of this subject as it is subsumed into the closely related field of Cosmology. In that Section, the point is made that remote sensing is by far the main tool or means by which scientists have learned about stars, galaxies, and intergalactic material in the Universe and in so doing have arrived at an ever more maturing understanding of the origin and development of the entire Cosmos.

Planetary Parameters and Aspects of their Motions

We concentrate in this Section almost entirely on the planetary bodies of the solar system (for information on the Sun, check Sol and/or Sun) , excluding Earth. To set a framework for our survey, look first at the illustration below, which shows the relative sizes of the nine planets of our solar system (the distances between them are not to scale). From their appearance, how many can you name?

Illustration showing the relative sizes of the nine planets of our solar system.

19-1: Using their appearance, how many of the above planets can you name? ANSWER

Consult the table below, which summarizes the principal characteristics and properties of the nine planets. We list them from top to bottom in the same sequence as those shown from left to right in the above illustration. To simplify, we do not include the names of the principal satellites orbiting some of these planets, but we cover them in a listing below the table.

DENSITY (gm/cm)3

AU = Astronomical Unit, which is the mean distance (approx. 150 million kilometers, or 93 million miles) from the Sun to Earth

Names of principal satellites (smaller ones omitted):

In October of 2000, announcement was made by a group of astronomers of discovery of what they claim to be a possible tenth planet. Orbiting the Sun beyond Jupiter but inside Pluto's orbit, this is an isolated small body (about 500 km; 300 miles in diameter) that is spherical. Such a shape is suggestive of melting and reorganization into a round mass. According to the American Astronomical Society rules, this size is below the lower limit agreed to be the smallest a body can be to be named a planet. Much remains unsettled before the limit is revised to include this new body. It appears to be a new class - between irregular asteroids, which can be larger, and the solar planet sizes; some rounded "moons" are in this size range which could explain it as an escaped Neptunian satellite but a mechanism to remove it from Neptune's orbital family into its own solar orbit is yet to be proposed.

The table above shows that the four planets closest to the Sun are small compared with those beyond Mars. These are the Inner or Terrestrial (like Earth, with rocky material at their surfaces) planets. From Jupiter through Neptune, the planets are much larger (the Outer or Giant group) and have surfaces that are all gas (Pluto, the exception, may be a "maverick", possibly being an escaped satellite). Nearly all planetary satellites are either rocky or a mix of rock and ice (one, Saturn's Titan, has a thin atmosphere). The four inner planets and Jupiter and Saturn were known since ancient times; Uranus was discovered in 1781, Neptune in 1846, and Pluto in 1930. The Sun-orbiting planets are recognized by astronomical observations because they move relative to the background stars (the ancients called them "wanderers").

Despite the efforts of pre-Renaissance astronomers (e.g., the Greek, Ptolemy, living in 2nd Century Alexandria, and later Arab observers) to develop a legitimate model of the Solar System, the frame of reference put the Earth at the center of the System (geocentric model). This was replaced in 1543 (date of publication) by the heliocentric model, based on work by the Polish scholar and priest Nicolaus Copernicus, who postulated that the Earth rotates and the planets revolve around the Sun. This Copernican model was largely ignored for decades, mainly from philosophical/theological objections, until observations by Tycho Brahe in the 17th Century supported the Sun-centered scheme (which, unfortunately, he rejected after conducting a flawed experiment). Galileo also made vital observations through one of the first telescopes; his discovery of satellites around Jupiter confirmed the notions of bodies revolving around a central body. General acceptance by the scientists of the times was still slow but the laws of planetary motion enunciated by Johannes Kepler (Tycho's protoge) and motion in general by Isaac Newton finally led to such overwhelming evidence that scientists and other thinkers and eventually the Church acceded to this reality.

Kepler deduced from the patterns of motion that the planets revolving around the Sun did not follow precise circles but instead followed elliptical paths with the Sun at one of the two foci that characterize an ellipse. The ellipses defined by him and later astronomers were only slight departures from circularity, except for Mercury (strongly elliptical) and Pluto (which periodically crosses the ellipse traced by Neptune). His second law is derived as follows (see figure below):

Diagram illustrating Kepler's Second Law; see text.

Start with a line from the Sun to a planet at any locus. e.g., a, along its orbital path. After it had moved some distance a-a' along the path, it will define some given area A for the time in transit, For another segment elsewhere along the orbit, a different pattern - area B - ensues as it traverses the distance b-b'. Now, if the elapsed time between orbital transits from positions a to a' and b to b' are specified to be the same, the areas in the patterns will be equal (A = B). The law can thus be stated: Imaginary lines from the Sun to any planet sweep out equal areas in equal elapsed time intervals during different stages in the planet's revolution. Since the distance a-a' is shorter than b-b', it follows that the velocity (distance/time) of the planet moving through b-b' is greater than the speed through a-a'; in other words, planets move faster when closer to the Sun. Separate arguments based on Newtonian mechanics show that the velocities of the planets decrease progressively outward from the Sun.

(As an aside which applies both to the planets and to orbiting satellites [like Landsat], the velocity needed to achieve and maintain orbit is a balance between the forward motion vector of the moving body and the gravity vector pulling it towards its parent body [whose mass is assumed to be at its center]; thus the tendency to move away tangentially is offset by gravitational force such that as the parent, e.g., Earth, rotates such that seemingly its surface falls away from the tangential line, in fact the satellite (or planet) is pulled downward just enough to maintain the same distance to the center of mass, describing a path that produces a circular orbit [or is modified to some degree of ellipticity], even as its momentum [mv; v varies for the elliptical case] keeps it in that orbit. Like the planets, the velocity needed to get and keep an Earth-orbiting satellite in place decreases outward. Landsat moves much faster [~26,600 km/hr], and with a period [time to complete one orbit] of 103 minutes, than does a geostationary satellite. The latter, when placed at 22,300 miles [36,235 km] above ground, moves slower [24 hours to complete an orbit] over a much longer orbital path at an orbital velocity of ~11000 km/hr; when inserted so as to move parallel and over the equator, the geostationary satellite moves forward at the same speed as its nadir point on the equator and thus is stationary [no relative movement] with respect to that point on the Earth's surface.)

Kepler discovered a third relationship affecting the paths of the planets. If the orbital period P of a planet (third column in the table above) is plotted on log-log graph paper against its distance R (second column) from the Sun (taken as equal to the semi-major axis of the path ellipse), then the result is as appears below. The mathematical expression for the equation representing the resulting line is P2 = R3, the mathematical statement of Kepler's third law.

Kepler's Third Law, as determined by the equation of the line shown in this log-log plot of P vs R.

Another, rather curious relationship was put forth by Johann Titius in 1766, with later modification and promotion by Johann Bode. To formulate it, start with Mercury and assign it the value N = 0 and add 0.4 to it (yielding 0.4). Next assign to Venus N = 0.3 and add 0.4 (giving 0.7). Now double 0.3 (0.6) and add 0.4 (= 1.0). Fo each successive planet double the previous N and add 0.4; for Mars this yields 1.6 and for Neptune this results in 38.4 + 0.4 = 38.8. This set of numbers if closely matched by the actual distances as Astronomical Units for all the planets, except Neptune which lies at 30.07 A.U. Pluto, however, lies at 39.4 close to the 38.8 value. No physical reason has yet been found for the Titius-Bode "rule", nor is the Neptune anomaly explanable. But one consequence is its prediction that some planetary body should exist at A.U = 2.8. None was known at that time but the later discovery of the asteroid belt at 2.8 fulfilled the prediction. That gap is evident in the figure above, as is the anomalous position of Neptune.

We have said nothing on this page, or elsewhere in this Section, about the origin of the planets and the development of a Solar System. This is treated in some detail on page 20-11.. Nor do we consider in this Section the Earth itself, since that has been the subject of most of the preceding Sections. We will start our planetary tour with the Earth's sole satellite, the Moon.


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Primary Author: Nicholas M. Short, Sr. email: