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. 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 repeat of Chapter 5: Planetary Geology, by James Bell III, 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. A complete listing of all planetary missions can be accessed on this NASA Goddard web site. 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. 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 his AST121 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 Space Science (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 this Missions site.
Books that treat planetary remote sensing as part of a larger review of Planetology include a now out-of-print text by this Tutorial's author (Nicholas M. Short), Planetary Geology, 1975, Prentice-Hall Publ., still in libraries, Murray, Malin, and Greeley's Earthlike Planets, W.H. Freeman & Co., 1981, and Billy P. Glass's Introduction to Planetary Geology, 1982, Cambridge University, Press. More recent are Planetary Landscapes by R. Greeley, 1985, Allen & Unwin, The Planetary System, by D. Morrison & T. Owen, Addison-Wesley Publ., 1988, and Exploring the Planets by W.K. Hamblin and E.H. Christiansen, MacMillan, 1990.
The writer strongly recommends Lonely Planets: The Natural Philosophy of Alien Life, by astrobiologist David Grinspoon, Harper Collins Publ. published in March, 2003 that covers much of what we knew by that time about planets in the Solar System. Its main purpose, however, is to review the conditions for and likelihood of life (from very primitive to advanced thinking creatures) in our galaxy and beyond - throughout the Universe. Although a bit wordy and redundant, this book explores almost all facets of whether Earth is unique (he thinks not!!) and under what circumstances planets containing living organisms (or once living, now extinct) can develop.
Scientists who have spent at least part of their careers studying the planetary bodies of the Solar System are called Planetologists. The majority of these are also Geologists, although some are instead Astronomers and Physicists.
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.
We concentrate in Section 19 almost entirely on the planetary bodies of the Solar System. Other planetary systems, around different stars, have been discovered discovered in recent years, as described on page 20-11.
The central body controlling these planetary bodies is the Sun. (For background information on the Sun, check Sol and/or Sun)). Despite the large size of some of the solar planets relative, say, to Earth, these planetary bodies orbiting the Sun taken together, plus also asteroids and comets, contain only 0.14% of the mass of the Sun (99.86%). They do, however, contain most of the angular momentum of all bodies (including the Sun) in the Solar System.To set a framework for our survey of the Solar System's inhabitants (exclusive of the Sun - that star is treated at the top of page 20-5a), which we will consider as the "Planetary System" despite the recent discovery of more than 200 extrasolar planets (again, Section 20), look first at the illustration below, which shows the relative sizes of the nine planets, with a small segment of the Sun shown to the far left, so that its size relative to the planets is scaled (the distances between them are not actual - each planet is just placed at the same distance next to its neighbors)(this diagram dates before the 2006 demotion of Pluto within our Solar System).
In late 2003, announcement was made by a group of astronomers of discovery of what they claim to be a possible Solar System planet, which they named Sedna, that lies astride Neptune's orbital path. This is an isolated small body (about 1250 to 1800 km; 750 - 1100 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. 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.
Actually, another planet had been imaged in 2003 but astronomers at Cal Tech did not recognize it as such until 2006 when three images taken 90 minutes apart were inspected and analyzed. This is what they saw:
The circled bright object in the left image seems to be one body. But in the second image, two bodies are resolved. The upper body moves again in the third image, taken 90 minutes later. The lower body proved to be a distant star. The upper body was shown to be within our Solar System. It was named 2003 UB313. After its acceptance as a planetary object, it was given the name Eris. At the 2003 sighting time, Eris lay a distance of 97 Astronomical Units. (An Astronomical Unit [A.U.] is defined as the distance between Earth's center and the Sun's center, approximately 151 million kilometers or 93,000,000 miles.) Continuing observations show it to have a highly elliptical orbit so that its closest approach to the Sun will place it at a distance of 38 A.U. This orbit is inclined 44° from the ecliptic. Eris will take 560 years to complete one revolution around the Sun. Eris is one of several larger spherical objects that are within the Kuiper belt of asteroids. Most of those are irregular in shape. So, is the sphericity of Eris enough to qualify it as a planet, along with Sedna and others.
As a momentary digression, this plot is an example of how remote sensing - in the form of gathering reflectances as a function of wavelength - is used in planetary studies. The plot shows the spectral reflectance curve for Pluto and for Eris. Both are nearly identical, suggesting that Eris has a surface composition similar to Pluto.
The Sedna and Eris discoveries along with detection of other similar bodies have reopened one of the great debates in Solar Planetology: Is Pluto really a planet? Its size is the smallest (2288 km diameter) of the 9 traditional ones stated in textbooks. Purests consider it not worth of planet status; some believe it, like Sedna, is an escaped moon. The controversy was the centerpiece of a debate at the 2006 meeting of the International Astronomical Union (IAU) in Prague. A committee there produced this simplified definition of a planet (applicable to any planetary system, not just that of the Solar System):
A planet is a celestial body that (1) is in orbit around a star but is neither a star nor a satellite of a planet; (2) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes (usually after melting) a hydrostatic equilibrium (nearly round) shape, and (3) has swept up most of the smaller solid bodies (e.g., asteroidal "chunks") in the spatial region around its orbit.
Item 3 is the new criterion that is to be used in defining a true planet. A key word is "most". Both Earth and Mars have some asteroidal-sized rock chunks within their orbital zones but these bodies are in toto only a very small fraction of the mass of the parent planets. Pluto has much more such bodies still in its orbital zone, so it has failed this third criterion.
Item 3 leads to another parameter that clearly separates the Main Planets from smaller bodies. This is done by calculating the ratio of the mass of the parent planet to the mass of all small body materials (excepting moons) in the orbital zone of the planet; this ratio is represented by the Greek letter μ. This plots as follows:
The mass ratios show that the 8 Solar System planets all have μ values greater than about 7000; the three spherical bodies Pluto, Ceres, and Eris all have values less 1. On this basis, the reason the IAU now contends that there are only eight major planets is obvious.
This reclassification might seem straightforward, but exceptions may cause confusion, e.g., Pluto's moon Charon. But Charon lies beyond the barycenter for the Pluto-Charon pairing. The barycenter is the center of gravity for a two body system that has one body orbiting the other. The Moon remains a satellite despite its size because of the Earth-Moon barycenter still residing within the larger Earth. In time (billions of years), as the Moon recedes the barycenter will migrate outward beyond the Earth, so that, technically, the Moon would become a planet in the sense of Charon. The large satellites of Jupiter, Saturn, Uranus, and Neptune all associate with barycenters located within their parent planet.
So, what did the IAU decide: They first mulled over the "strawman" concept that as of 2006 there might be 12 planets in the Solar System, with the three new ones (2003 UB313 is now named Eris) shown in this diagram. They firmly decided at this meeting against these three as 'full blown' planets, keeping them as large members of the Kuiper Asteroid Belt.
The latest thinking is that there should be three categories: 1) planets; 2) dwarf planets; 3) solar system objects. Dwarf planets must not be satellites of larger planets. Dwarf planets must be round, whereas solar system objects must be irregular in shape. Dwarf planets that are icy can be as small as 200 km in diameter; if they are rocky, the smallest size that is round is about 400 km. As of 2010 about 50 round dwarfs have been found in the Solar System.
A number of small round bodies have now been found in the Kuiper (asteroidal) Belt and possibly the Oort (cometary) Cloud (page 19-22). They are part of a vast collection of large to small objects - most non-spherical - orbiting the Sun. According to the IAU definition, some smaller round ones may be eventually raised to the dwarf planet level. Here are some current candidates:
The IAU voted on August 24, 2006 to further redefine planets by size - these falling into Main (large) and Dwarf (small) categories. This enabled Pluto to retain some planet status by becoming a Dwarf Planet along with Ceres and Xena. Pluto's moon Charon remained just that - a satellite. But as in any democratic procedure, those on the losing side will continue to disagree.
This diagram depicts some of the Dwarf planets in terms of their relative sizes:
Since the Dwarf planets are small, good photo images of them are not easily acquired. Most are depicted from observations that are translated into "artist's concept", visual representations of the 'best guess' of their appearance. Below are some of these planets - read their captions to determine if each one is an actual image or an artist's concept:
The question "What is a Planet" has been addressed in the January, 2007 Scientific American article by that name as written by Steven Soter.
There seems little doubt that Earth is still the premier planet among those now discovered (within and beyond the Solar System). It is (so far) unique in having life (but not organic molecules) as a main characteristic. In the Solar System, Earth is alone in having 1) widespread water oceans, and 2) abundant vegetation. It is the presence of trees, grasses, and other vegetative cover that distinguishes Earth from the other solar planets; but from the standpoint of remote sensing this absence on the other planets makes it possible to examine them in terms of the geologic processes that led to their development (on Earth, extensive vegetation usually masks the underlying geology, as is typical in the eastern United States).
This subsection ends with the illustration below, which shows the full disc appearance of each of the four Inner or Terrestrial Planets scaled to their actual relative sizes, of which Earth is the most important (or you wouldn't be reading this):
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.
* Included in the Table despite its reclassification as a Dwarf Planet.
Note: For the number of satellites; that numeral left of R refers to those satellite that are nearly spherical - that left of IR refers to irregular shaped satellites (see page 19-14).
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):
The above table lists the distances of the planets from the Sun. This diagram shows these distances in terms of the orbits of the planets:
As will be described later, beyond Neptune and Pluto are Sedna and the Kuiper Belt (asteroidal bodies) and Oort Cloud (mostly comets). This diagram indicates the extreme distances of these latter features in relation to the nine planets:
(A brief mention is made here of an interesting hypothesis maded by Dr. Harold Levinson of the Southwestern Research Institute: He postulates that in the early Solar System the planets had orbits different from the present. The Giant Planets Jupiter and Saturn had orbits such that on very infrequent occasions they would approach each other so that their massive gravitational interactions caused them to start a process called resonance. This eventually forced them to readjust their positions and also hurled Uranus and Neptune further out. About 3.9 billion years ago, the resonance also perturbed the asteroid belt objects, forcing many of these towards the Inner planets. The Moon, formed by then, underwent a cataclysmic bombardment [see subsection on the Moon] during which most of its craters were formed [extensive cratering also took place on Earth but almost all of these have been obliterated by subsequent erosion]).
The densities of the planets is one characteristic parameter. Here is a histogram that compares this property:
The Inner planets all show rocky materials at their surfaces (all but Mercury have atmospheres). Much of the density components depends on the relative sizes of the core, mantle (if present), and crust. The next two illustrations show the measured (or estimated) size of each component:
The next table summarizes orbital parameters and atmospheric characteristics of the solar planets.
The orbital inclination is a measure of the departure of a planet's orbital plane, in degrees, from the orbital plane of Earth around the Sun (Sol) which defines the ecliptic, a plane containing both the Sun and the Earth's orbit (see page 20-5, near the top).
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). The ancients perceived the "Universe" (for them, mostly the known planets, and other points of light called stars) as a set of concentrically nesting spheres that had the Sun on one sphere; all spheres rotated around the Earth at different rates. The "map" below is one version of the geocentric Solar System as envisioned in late Medieval times; note the descriptors are in Latin:
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 protege) 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). Kepler's second law is derived as follows (see figure below):
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.
Another, rather curious relationship was put forth by Johann Titius in 1766, with later modification and promotion by Johann Bode. To formulate it, look at this sequence:
Now, add 0.4 to each N term, giving this new sequence:
The actual positions (in A.U.s) of the planets are: Mercury = 0.39; Venus = 0.72; Earth = 1.0; Mars = 1.5; Asteroid Belt = ~2.8; Jupiter = 5.2; Saturn = 9.5; Uranus = 19.2; Neptune = 30.0; Pluto = 39.5.
For each successive planet we doubled the previous N and added 0.4; for Mars this yields 1.6 and for Neptune this results in 38.4 + 0.4 = 38.8. Remarkably, this set of numbers is closely matched by the actual distances as Astronomical Units for all the planets except Neptune which lies at 30.07 A.U. Pluto (no longer a planet), however, lies at 39.4 close to the 38.8 value. The value 77.2 suggests a "missing" planet at that distance. No obvious physical reason has yet been found for the Titius-Bode "rule", nor is the Neptune anomaly readily explanable (it may relate to an interaction with Pluto). But one consequence was a 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.
The more general subject of celestial motions, or also referred to as celestial mechanics, is rather complex - beyond the scope of the Tutorial here (some aspects are treated near the top of page 20-2). But a few ideas relating to the Sun's and the planets' motions relative to Earth as the observing platform are introduced here.These motions with respect to the Earth and to the Sun's path across the sky can be plotted on the Celestial Sphere - usually displayed as either the hemisphere north or the hemisphere south of the Equator or one's local horizon, for each case showing the expanse of celestial objects above the horizon in all directions. Thus from any location on one or the other hemispheres, there is a a local observational hemisphere on which the motions of the Sun, the Moon, the planets, and the stars appear to move as the Earth rotates each day, causing each body or point of light to trace an arcuate path across the hemisphere (best seen at night). An axis around which the heavenly bodies seem to move in circular arcs is imagined to pass from Earth to the hemisphere, with the North Star (Polaris) being very close to the point where the celestial axis would penetrate the (hemi-)Sphere; by convention, the Earth's rotational axis is placed to coincide with the celestial axis, and the Earth' North Pole is near the celestial North Pole. The position of the North Star, and hence all other celestial bodies, will vary with latitude and with season of the year. At different times in a year different distinctive groupings of stars - known as constellations (discussed again on page 20-2) appear in the same part of the Sphere at the same time of night, are illustrated here:
One approach to understanding celestial motions is to trace the pathway of the Sun across the Sphere as it varies seasonally and with location on the Earth's spherical surface. First, we introduce the concept of the solar ecliptic. This is the plane defined by the elliptical trace of the Earth's annual orbital path around the Sun (the orbital traces of the other planets approximately follow the ecliptic as well; Mercury at 7° and Pluto at 17° have the largest departures from the orbital plane as defined by Earth's path). A corollary: the ecliptic is also the Sun's annual path among the stars as viewed from Earth. In this frame of reference, the Earth, and also six other planets rotate in the same direction - counterclockwise (prograde) as viewed from above (as from the celestial North Pole); Venus and Uranus have retrograde (clockwise) rotations. None move in circular orbits; each follows an elliptical path, usually with small ellipticity, such that it will be varying distances to the Sun with points of closest and farthest approaches.
While all the planets orbit on or near the ecliptic, their rotational axis is tilted to varying degrees relative to the ecliptic plane (or to a line normal to [90°] the plane). Only Mercury's axis is at 0° with respect to this normal. Venus and Pluto have their north rotational pole tilted more than 90°, hence the top of the axis lies beyond (south of) the ecliptic. The Earth's axis is tilted 23.5° from the a line at right angles to (normal to) the ecliptic.
Two diagrams will indicate the seasonal and the latitudinal effects of the Sun's movement across the celestial hemisphere:
In the above diagram, the (unspecified) latitude of observation, along with Earth's axial tilt, is responsible for the celestial North Pole being about 60° down towards Earth's Equator (in this projection, an 'ellipse' marked by E, W, N, and S). The highest the Sun will reach is at Summer Solstice (~June 21); the plot of a daily path then across the sky at that latitude is a partial circle (arc) shown in red that has the greatest length, thus resulting in the maximum amount of sunshine on that day. That arc begins at sunrise (after dawn) and ends at sunset. The smallest arc is at Winter Solstice (December 21), shown in green; at that time the daylight is the least - several hours less than 12. At the Spring (~March 21) and Fall (~September 21) Equinoxes - blue line - the arc is at a median length, and the sunrise-to-sunset duration is about 12 hours.
The second diagram has a different orientation of the equator, and a set of seasonal Sun paths, for a location at 41° North. Here the Sun becomes almost vertical (would cross the zenith on the Sphere) on June 21. Thus, the Sun path will vary systematically with latitude (the effect of the Earth's axial tilt is built in). This can be plotted on a diagram:
(1) the sunrise direction changes
(2) the sunset direction changes
(3) the length of daylight changes
(4) the height of the Sun at noon above the southern horizon changes
Thus, the two Celestial Sphere diagrams show that the Sun follows arcuate patterns (of varying lengths depending on time of year) across the sky as seen at any location during the day. Stars, on the other hand, will follow circular patterns, centered on the Celestial North or South Poles, over much of the Sphere. Planets, being part of the Solar group circuiting the Sun, will also move during a given night, but their paths over months and the seasons will appear as though they "wander" through the background of stars and constellations. Their shifts are not equal per terrestrial time period and are seen from Earth as influenced by their relative distances from the Sun and their locations in their elliptical orbits relative to earth observers.