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Models for the Origin of Planetary Systems

This "crash course" on Cosmology closes with the following brief synopsis on the methods and results of astronomers' search for other planetary systems and consideration of the origin of our own Solar System, plus a quick look at several of the latest, provocative, and especially exciting theories (or brash speculations) on the presence of other intellectual beings in our Universe that might be "out there".

Two hallmarks in particular distinguish planets from stars they orbit: First, they usually show marked differences in composition, being either gas balls with other elements besides hydrogen that have rocky cores or they are dominantly rock with many elements making up usually silicate minerals. Second, they are significantly smaller in diameter (hence volume) than their parent star. This SOHO image of a part of the Sun, with a solar prominence, illustrates this size difference, as displayed by adding a drawn sphere the size of the Earth to allow comparison:

A SOHO image of part of the Sun, with a filled circle representing the size of the Earth.

This huge size difference between the Earth and its normal G star Sun is humbling in its stark truth. The great disparity in size also makes it clear how difficult it will be to find Earth-sized planets around nearby stars - and even more of a technology challenge as astronomers entertain hopes of finding stars elsewhere in the Milky Way galaxy and galaxies beyond.

It is natural for humans to wonder if there is life elsewhere in the Milky Way, and by implication in other galaxies. The starting point in searching for life is to prove the existence of other planets and inventory their characteristics. In the last decade the hunt for planetary systems has intensified. The first extrasolar planet was found in 1995 orbiting the star 51 Pegasi. As of June 2003), planetary bodies have now been detected in at least 87 other stellar (~solar) systems; as of July 2, 2003 a current count of 127 individual planets have been associated with these systems. Analysis of one such system - Upsilon Andromedae -indicates it to have 3 planets (a second triple star system was recently discovered); 8 other stars have 2 planets. The U. Andromedae system consists of Giant (probably gas-ball) planets (much smaller ones presently cannot be detected), all within orbits occupied by the four small terrestrial planets in our Solar System:

The Upsilon Andromedae System of Giant planets around a star in the M.W. galaxy; their relative locations are compared with those of the 4 inner planets of the Solar System.

So far, no planet has actually been seen, being too small to be detected optically using present day technology, but the existence of such low-luminosity bodies can be deduced from their interactions with their parent star. Almost all discovered so far are large - Jupiter-sized or greater - and are gas balls. Several are extremely close to their star (at distances less than Mercury's orbit). Many planets have much more elliptical orbits than those moving in the Solar System. It is hypothesized that most planetary systems will consist of multiple planets but the smaller ones are presently still invisible to us.

A high point in the current search for planets occurred on June, 2002 when several groups of astronomers announced jointly the discovery of up to 20 new planets - including at least two of Jupiter size - in the Milky Way galaxy, whose stellar parents reside at distances ranging from 10.5 to 202 light years from Earth. The closest in has been provisionally named Epsilon Eridani b (its star is the one actually cited in the Star Trek series as the location around which Mr. Spock's home planet, Vulcan, was orbiting). The rate of planet discovery seems to be accelerating. With it is the growing belief by cosmologists that planets could well exist in the billions, i.e., they are the inevitable result of processes that take place when most stars are born. Thus, planets may well turn out to be the norm - the expected, and perhaps the most significant products of nebular collapse in stellar evolution. The proponents of SETI (Search for Extraterrestrial Life; seeking primarily radio signals that have non-random character [perhaps some form of mathematical organization]) have been galvanized by these recent discoveries. (The writer is convinced that it is only a matter of time - probably during the 21st Century - until contact with other intelligent beings is achieved.) We will return to the SETI idea near the bottom of this page.

Current search for extra-solar planets is restricted to the Milky Way and galaxies close enough to Earth for an individual star to be resolved to the extent that changes in its motion can be measured. Gravitation attractions from orbiting planetary bodies cause the central star to wobble. This is the basis for the three methods currently being used to detect anomalies in a star's behavior that lead to the inference of one or more orbiting bodies.

The method that has so far been the most successful in locating (invisible) planetary bodies is called the radial-velocity technique. A component of a star's wobble will potentially lie in the direction on-line to the Earth as an observatory. This to and fro (forward-backward) motion causes slight variations in the apparent velocity of light. That, in turn, gives rise to small but measurable Doppler shifts in the frequencies of light radiation from specific excited elements, as expressed by lateral displacements of their spectral lines. From the wobble magnitude and period, the approximate orbit and mass of its presumed cause - the orbiting planet - can be calculated. This method is sensitive to wobble velocities as low as 2 meters per second. (Jupiter, for example, causes a wobble velocity of 12.5 m/sec imposed on light radial from the Sun. Generally, this method, applied to nearby stars, will detect mainly large planets close to their star but in March of 2000, two planets about Saturn-size (1/3rd the mass of Jupiter) were found.

The second method, astrometry, also relies on wobble but depends on measuring side to side displacements by direct observation through periodic sightings. This determination of relative shifts can be done on photographic plates taken of part of the sky at different times, commonly using the same telescope. But, considerable improvement shift resolution results when two telescopes are positioned apart and joined electronically. This permits application of interferometry such that the two telescopes act as though they were one large one. Resolution as sharp as 20 millionths of an arc second (an arc second is 1/3600ths of an arc degree on the sky hemisphere [0° at the sealevel horizon; 90° at the zenith near Polaris in the northern hemisphere]). The Keck interferometric telescope in Hawaii will soon be operational. This should facilitate detection of even smaller planets in nearby space or large planets in stars 10s of light years away.

As we have seen with the HST images in this Section, resolution (and clarity) is significantly improved by operating a telescope in space, above the distorting atmosphere. The use of two telescopes mounted on a single boom but separated by meters allows the interferometric method to work on space-quality imagery. SIM (Space Interferometry Mission) is a NASA probe slated to fly during or after 2005. This will lead to a millionth of an arcsecond resolution, capable of inferring the presence of planets just larger than Earth-size or of large planets with far out orbits.

A third method is called transit photometry. When a planet's orbit takes it on a path where it passes across the face of the star under observation, the body will block out a small amount of radiation (usually visible light) for the period of transit. Sensitive detectors can note the slight diminution (up to about 2%). To distinguish this from a "transient event" of other origin, the astronomer needs to establish some regularity (reproducible at fixed intervals) of the drop in radiation, which will depend on the nature of the orbit (ellipticity; distance, etc.) Depending on planet size and proximity, the drop in stellar luminosity will be a few percent or less (an accurate determination helps to establish the planet's actual size). Such an effect was first noted in 1999 when a giant planet (earlier found by the radial-velocity method) passed in front of a star (HD 209468) whose light intensity underwent a drop of 1.5%. In a June, 2002 meeting, two other groups using telescopes in Chile have reported 3 and 13 possible transit detections, but these observations have yet to be confirmed independently.

This promising transit approach will be used in the recently approved Kepler mission (launch in 2006) in which a space telescope would be pointed on the same areas in the stellar field for up to four years, integrating brightness measurements to single out variations as small as 0.01% (capable of detecting Earth mass-sized planets). Up to 100,000 stars can be conveniently monitored over time and individuals with multiple planets should reveal the relative number of stars that possess planetary systems. Two other non-U.S. missions, COROT and Eddington, are in the planning stage. Thus, as detectors improve and instruments become spaceborne above the atmosphere, sometime in the next 10-20 years Earth-sized planets should become detectable by measuring drops in stellar light curves owing to transits of large planet(s) across their parent star.

A fourth method has just had its first success in June of 2002. Examine this pair of images, shown first as a photo negative plate and then in color:

The <i>winking</i> star method of planet detection

KH 15D observed in color

This is called the eclipsed or "winking" star method. In the left image (a photo negative), a Milky Way star KH 15D (2400 l.y. away; about the size of the Sun) is visible behind a much closer (or larger) star. In the right image it is totally absent, a condition lasting for about 18 days, and then it reappears. This on-off cycle occurs every 48 days. The eclipsing body could not be another star nor is it likely to be a huge (star-size) planet. The interpretation is that there is a cloud of asteroids and dust in a smeared-out clump orbiting KH 15D which block the starlight when a clump passes across the parent star; speculation considers that there may already be one or more planets formed from this debris. There may actually be two clumps (symmetrical pairing) at opposite positions in a single orbit; this has yet to be confirmed. A further anomaly: examination of photographic plates taken many years ago (although at limited intervals) does not detect this on-off phenomenon.

A fifth method is still in the experimental planning stage. The Terrestrial Planet Finder program at Princeton University is developing a special type of "cats-eye" mirror that will greatly reduce the effect of the luminous parent star. When this technology is deemed ready, they hope to persuade NASA or some other agency to use mirrors on several telescope-bearing satellite flying in formation and spaced to utilize the principle of interferometry to improve resolution and to detect small planetary objects.

The ultimate dream is to directly visualize individual planets. This may be possible using several HST type spacecraft flying in formation ("clusters") with separations of a few hundred meters to hundreds of kilometers. In one mode, data will be combined using interferometric principles. Light from the central star can be blocked out by specialized image processing, leaving a residue of low luminosity orbiting bodies detectable by resolution- and radiation-sensitive interferometry. Both NASA and ESA each have in the planning stage such a mission (called The Terrestrial Planet Finder and Darwin, respectively).

Statistically speaking, the number of such planetary systems should extend into the millions within individual galaxies and the billions when the whole Universe is considered. It would logically be likely therefore that non- or weakly-self-luminous bodies, i.e. planets, are the norm orbiting around a central star for at least some of the size classes on the Main Sequence of the Hertzsprung-Russell diagram. As such stars proceed through their developmental stages (before they leave the Main Sequence), planets seem the inevitable outcome of the formational processes involved in star-making.

Two scientists, C. Lineweaver and D. Grether of the University of New South Wales in Australia, have recently published a study that relies on reasonable probabilities to estimate the number of planets just in the Milky Way. They argue that, of the approximately 300 billion stars they calculate to be the total population of the M.W, about 10% or 30 billion consist of stars similar to our Sun and most likely to have favorable conditions for planet formation. Assuming that, of these, at least 10% will produce giant, Jupiter-like planets; thus their earlier number estimates 3 billion giant planets. Such large planets would almost certainly be accompanied by smaller ones formed out of the materials (they call "space junk") associate with the parent star. These giants help in the collection process that leads to smaller companions. But, more importantly, the giants serve as the principal attractors that gravitationally pull comets and asteroids into them (remember the Shoemaker-Levy event discussed in Section 19) and thus function as "protectors" of the small planets by minimizing the impacts these receive. Now, in a more recent presentation at the 2003 International Astronomy Union in Sydney, Australia they have raised their estimate to perhaps as much as 30 billion giant planets and a similar number of earthllike planets. This bodes well for future hunts as observational technology improves. Although this may seem "wildly optimistic", the likelihood of life on planets (see below) continues to rise dramatically with the increases in estimates of planetary occurrences - especially if one presumes that planets are the norm around stars in size ranges no greater than 10 solar masses.

For a while, astronomers assumed that most stars with planets would be relatively small - Sun-sized to perhaps 10 solar masses. These stars last for billions of years and thus favor the eventuality of life if planets developed during the stellar formation process. Now, several notably larger stars in the Milky Way have been found to have large planets. So, planetary formation is a function of process primarily and may have little to do with how long its star can survive. But the really big stars, even with planets, would burn their fuel and destruct long before evolution would likely foster even primitive organic matter.

The expectation is that planets have formed over most of the time that stars have developed in galaxies. One star pair (one a pulsar; the other a white dwarf) in a globular cluster within the Milky Way some 5600 l.y. away (in the constellation Scorpius), has been shown to be perhaps as old as 13 billion years. This is based on the sparcity of elements of atomic numbers higher than helium. The large planet (4 times the diameter of Jupiter) now associated with it, is almost certainly the same age since prevailing theory holds planets to form roughly at the same time as parent stars. It verified as a true planet, it must be a hydrogen/helium gas ball similar to Jupiter. It is likely devoid of life owing to the turbulent history reconstructed for this star pairing and to the absence of life-forming elements. Even if some form of primitive life did form on it or associated planets, those would have perished in which harsh conditions prevailed in its later years. But the chief implication of this observation (reported in July 2003) is that planetary formation can be traced to the early days of the Universe and, as carbon accumulated from the many early supernova explosions, some planets may have developed life of some type(s) since the first few billion years of cosmic time or even earlier.

While astronomers exercise caution about conclusions that specify the number of earthlike planets to be expected from their estimation procedure, they do propose that that number should be in the millions. Whether such planets also harbor life is much harder to pin down numerically but their statistical approach suggests that a significant fraction of the earthlike planets would possess the proper conditions. How much of that is intelligent life is still guesswork. The current sample of 1 (us!) is the only data point. But if the reality is actually a much larger number, then, purely from statistical logic, we should expect that some of these intelligent civilizations should be more advanced than ours. Why we have yet to "hear" from them remains uncertain (but now SETI improves the chances for this) unless there is some fundamental reason that makes space travel, even from nearby stars, very difficult.

Now, let's turn to consideration of the ways in which planets form. For planets in general, terrestrial and gas envelope types, dust must be present in sufficient quantities to collect as cores or to comprise the main body of the planet. (As a corollary: if gases dominate, they must remain below temperatures at which fusion can start). There is plenty of dust in galaxies, mixed with gases from which stars emerge. The source of the dust has been somewhat problematic but a prime candidate is exploding stars large enough to synthesize silicon, oxygen and other heavier elements. A recent observation strongly supports this:

A supernova 11000 light years away in the Milky Way.

This 300 year old supernova is hard to see in visible light because of a superabundance of dust. When imaged in submillimeter light, the above pattern stands out. The brightest areas are advancing gases with large quantities of dust that give off light at these wavelengths.

One essential requirement for a planetary system to develop is that it forms during the organization of a central star (possible exception: a captured planet, probably rare). Also critical is the availability of dust and gas. The processes involved can be somewhat varied but are sensitive to a relatively narrow range of conditions. The sequence of formative events probably begins during the T Tauri stage of developing stars in which the conditions are favorable. These stars have notable dust clouds (nebulae) that can be monitored in the infrared. Some evidence indicates the clouds will begin to reorganize their tiny particles into large clots, which can grow to planet size, in about 3 million years. But, detection of cold nebular material at longer wavelengths suggests the dust can take 10 million years or more to build up any planets that may result.

Another important factor, recently reported, is that stars which have a relatively high content of iron (from gases enriched by repeated mixing of supernova explosions over time) will have a much greater likelihood of producing planets. Iron is a measure of the metallicity of a star (page 20-7). The iron may be needed to develop planetary nuclei (most ending in planet cores) that help in the gravitation attraction that drives accretion through collisions and infall. Stars with three times the iron content of the Sun have an estimated chance of having planets set at 20% (This comes from a study of 754 nearby stars in the current (on-going) inventory of which 61 have detected planets (this amount to a probability of about 8% for all stars of mass less than 10 times the Sun having auxiliary planetary bodies). The results of this study are depicted in this graphical diagram:

Metallicity diagram of Fe distribution in 754 nearby stars.

The paradigm summarizing the processes involved in the formation of the Sun and its planets probably applies (with variations) to most other planetary systems in general. The first realistic notion of how planets form was proposed by Pierre Laplace in the 18th Century. In its modern version, both stars and their planets are considered to evolve from individual clots or densifications within larger nebular (cloudlike) concentrations dominantly of molecular hydrogen mixed with some silicate dust particles that spread throughout the protogalaxies and persisted even as these galaxies matured. In younger stars, much of the hydrogen and the heavier elements are derived from nova/supernova explosions that have dispersed them as interstellar matter that then may initiate clouds or mix with earlier clouds. Such nebulae are rather common throughout the Universe, as is continually being confirmed by new observations with the Hubble Space Telescope.

One of the best studied and, in itself spectacular, is the Orion Nebula, seen here:

HST view of part of the Orion galaxy, showing billowy nebular matter in process of the clumping of material that will generate new stars.

Below are three views of nebular materials associated with the famed Eagle Nebula: Top = full display of the M16 (Eagle) nebula (note the dark dust areas; the white dots are stars lying outside this nebula); Center = part of Eagle nebula, showing top of the Horsehead pillar of dust and gases from which stars and planets may eventually evolve (a few stars already being evident), made by combining three film exposures through the Kuevn Telescope at the European Southern Observatory ; Bottom = details of the temperature variations in the dust making up the solid particulates in the Eagle nebula as imaged by the European Space Agency's (ESA) Infrared Space Observatory (ISO) at two thermal infrared wavelengths, in which red is hot and blue is cooler (about - 100° C):

M16 - the Eagle nebula

Detail of the top of the Horsehead Pillar, in the M16 Eagle Nebula.

ISO thermal image of part of the Eagle nebula; blue made from 7.7 µm exposure; red from 14.5 µm band.

As individual stars start to develop within these gas and dust clouds, in many instances that dust will organize into a protoplanetary disk (see third figure below). The NICMOS infrared camera on the Hubble Space Telescope has observed a prime example of this stage, in which the glowing gases moving into the central region where a protostar is building up are cut by a band of light-absorbing dust that is most likely disk shaped (can't be verified from the side view in this image):

Gases heated to incandescence that are presumably collecting into a central star; the dark band cutting across the bright field is almost certainly a protoplanetary disk from which planets may emerge as the star becomes fully organized.

An Earth-based telescope has captured this view of a nearby star (nicknamed the "flying Saucer"), again with a girdling disk of dust (and an as yet unexplained anomaly in that the upper half of the image is redder than the bluer lower half):

Disk of dust around a star, in a photograph made through the European Space Observatory's New Technology Telescope.

Examination of these gas and dust clouds by HST has led to the discovery of small clumps or knots of organized gas-dust enrichment within the protoplanetary disks called Propylids found in the neighborhood of a parent star. This may be a more advanced stage of concentration that results in a new star with an envelope of gas-dust suited to accretion that produces planets. Three such propylids are evident in this image of the nebula associated with the Orion galactic cluster (go back to the top image in the three above to try to spot for a conspicuous propylid).

Propylids in Orion.

Other individuals, at least one possibly as recently formed as 100,000 years ago, found during the Orion study (see page 20-2 for a view of the entire young nebula in the Orion group) look like this in closer views:

Additional protoplanetary disks in Orion.

Propylids are vulnerable to being destroyed by UV radiation from massive, young nearby stars. It is surprising therefore that many propylids (some shown below) have survived in the Carina Nebula, which has numerous UV-emitting stars. Other factors must be involved

A sampling of propylids in the Carina Nebula.

Development of planetary cores, from which full larger planets then form, is a race against time. Examine this model:

Model of protoplanetary disc evolution (credit Ann Field)

As the gas and dust cloud forms, particles of solids begin to clump. However, the cloud is threatened by stellar winds and UV radiation from the parent star that remove most gas and some dust. Evidence suggests that most propylids are blown away before a planet grows large enough to survive, implying that the planet formation process may be less efficient and common than had been thought during the last decade. If the planetary cores do build up fast enough, they will survive the expulsion of the bulk of the gas/dust. This phase of planet formation occurs typically in a time frame of just 100,000 years or so; it is estimated that 90% of such clouds are dissipated before significant planetary cores can form. Planet accretion leading to survival is estimated to take up to 10,000,000 years.

Most galaxies began during the first half of the Universe and contained a large number of massive stars that formed early in galactic histories. These galaxies have continuously been evolving through the eons as supernovae synthesized elements (see page 20-7) and dispersed them, and new, mostly Main Sequence stars, chemically enriched with elements of higher atomic number, have continued to form well into younger times up through the present. The smaller stars have lasted much longer and are probably the preferred sizes suited to planetary formation and survival. Even today stars are developing from the gases contained in remaining nebular material, so new planets could still be forming. What is not yet known is the percentage of stars in a galaxy that actually have planetary systems. The low number so far found does not necessarily represent an indication of sparsity, since planets are so small and most low in luminosity (mainly from reflected light) relative to their parent star that present direct observations will produce extremely low numbers, thus subjecting our current conclusions that planets may be rare to a misleading bias. Some models of star formation from gaseous nebula suggest that a fraction of the gases, dust, and free molecules is trapped in orbit without infalling to the central star and can organize in planets of various sizes, distances, and composition in a manner similar to our Solar System. If a valid argument, this says that planetary bodies are commonplace throughout the Universe.

A telescope observation, reported in April, 1998, records the sighting (through the Keck II telescope on Mauna Kea, Hawaii) of what is interpreted to be another "solar" system around star HR 4796 (about 220 light years away). This image (at a resolution in which individual pixels stand out), taken in the IR, shows this central star (yellow white) surrounded by a lenticular (in an oblique view), flattened disk of gases and solid matter (glowing hot [reds] in the infrared):

Keck II IR image of a possible solar system around star HR4796.

The diameter of the lens is about 200 A.U. No evidence of individual planets can be made out but the discoverers judge this feature, which has caused quite a stir of interest, to be an emerging planetary system in a "young adult" stage of development. It will certainly be a target for more detailed HST observations.

More recently, the Hubble Space Telescope has imaged star HR4796A, in our galaxy, which shows both a disk and irregular dust and gas clouds. This disk is interpreted to be in a more advanced (mature) stage of development than the protodisks shown above. Although not discernible, there may be planets already in the evolving gas/dust cloud which is made visible by the star's light.

Illuminated dust and gas in the disk surrounding state HR4796A; HST image.

Star HD1569A, in the Milky Way galaxy, also shows wide disk of dust glowing from reflected light emanating from the black objects (their light blocked out by the coronagraph on Hubble's High Resolution Camera) which astronomers have identified as a triple star complex. This disk may be in an earlier stage of organization than shown in the preceding image, but it too may eventually lead to planets.

Glowing disk around ternary star complex HD1569A.

Theory indicates that, in the earlier stages of planetary formation, some number of broad rings should develop at various distances around the central star. One or more of these would appear as torus like glowing collections of dust and gases. At least two stars with this feature were imaged by HST and reported in January of 1999. Here is one of the star-ring systems:

 Ring of nebular material around Star H141516, which some claim is similar to conditions that could lead to planet formation; HST image.

Visible is a bright ring at some considerable distance out from the tiny parent star (white dot) and a more diffuse, darker mass extending beyond, both features occupying a flattened disc. In this instance, there are no rings close in (analogous to the regions occupied by the inner planets of the Solar System). The white circle is added by the astronomers to mark a boundary; the broad black cross (X) is an optical artifact. This star is about 350 light years from Earth.

A recent image, made from data detected at 1.3 mm by a French radio telescope, may have caught the formation of two large clots of matter likely to eventually contract into giant planets. These occur in the ring around the central star Vega, 25 light years away (in the Constellation Lyra). Here is an image based on observations made by D. Wilner and D. Aguilar of Harvard's Smithsonian Center for Astrophysics. (Note: this image has been enhanced artificially as an artist's rendition.)

The ring of gas and dust around the star Vega, with the yellow clots being possible sites of planetary formation.

Drs. C. Chen and M. Jura, Univ. of California-Berkeley have detected (monitoring infrared radiation) a ring of dust which seems to contain asteroid-like bodies around Zeta Leporis, a star some 70 l.y. from Earth. The ring is much closer (~2.5 A.U.) to its parent star than the distances found for other recently observed or inferred planetary bodies around stars. They propose this ring to be the precursor of eventual formation, by collision of asteroidal bodies, of rocky planets analogous to those of the Solar System. These bodies, form from smaller particles (dust) condensed from the gas-particle cloud associated with the forming star. Much of that dust can move inward towards the star by a process called Poynting-Robertson drag. This is caused by radiation from the parent star being absorbed and re-radiated differentially, leading to a Doppler effect (here, the energy of emission in the direction of dust motion is at shorter wavelengths [more energetic] and thus by retro-action slows the particles) that promotes drift of the dust towards the star.

The following is a generally accepted model (called "core accretion") for establishing a planetary system: A nebula is subject to gravitational irregularities and other perturbations that cause free-fall collapse to numerous clots around which surrounding gases and particulates usually adopt a disklike form. The influence of gravity, which builds up progressively as clots enlarge, is the prime driving force promoting both planet and star (Sun) formation. In some instances, shock waves from a supernova can cause interstellar matter to initiate collapse and compress into protostars and debris orbiting them. Matter is also redistributed along magnetic field lines by magnetohydrodyamic processes. The main phases of planetary formation extend over about 10-20 x 106 years but it may require up to 108 years to progress from the early infall to the late T Tauri stage of a protostar's development. While a particular clot is organizing, the materials tend to redistribute such that hydrogen and much of the lighter elements flow towards a growing center to accumulate in a gravitationally balanced sphere, the star. Under one set of conditions, instabilities lead to a double (binary) star pair. As protostars form, the rotating gases and dust particles collect in a spinning disk around each center and eventually organize by accretion into planets. The same process, with variants, works at single stars.

If our Solar System is the norm, inner planets should be rocky, with thin or absent atmosphere (lost from insufficient gravitational ability to retain the gases or by being swept away by the solar wind). Outer planets should have rocky cores and be less susceptible to loss of gases, so that their increased mass serves to gather in still more gases. However, the discovery that giant planets can lie quite close to their parent stars places this assumption of size distribution with distance into question.

Two recent hypotheses are adding new twists to the above concepts. First, in addition to or in place of core accretion, another mechanism called "disc instability" may play an important, perhaps key role, in planetary inception. This is related to gravitational irregularities that can cause rather rapid accumulation of materials in the proto-planetary disc. Earlier-formed planets can contribute to setting up further instabilities. A second idea holds that planets can move inward or even outward in a form of migration or "wandering" so that their orbits change both in relative distance to their parent star(s) and in eccentricity.

But for the present, astronomers continue to build and refine their models on the much easier-to-make observations at the astronomically short distances within the Solar System. Like other stars, the Sun (whose diameter is 1,392,000 km [870,000 miles]) is an end-product of gravitationally-driven condensation and collapse of hydrogen/helium gases and associated matter (both solid and gaseous) consisting of other elements and compounds that once made up a diffuse (density ~ 1000 atoms/cc) nebula. Probably many stars were generated in the timeframe of a few hundred million years from this particular "cloud".

The protosun built up from centripetal, gravity-induced infall of nebular substances towards one of the concentration centers in the nebula. The bulk of the gases enters the resulting star itself along with much of the other materials, leaving an enveloping residue of matter enriched in Si, C, O (and H), N, Ca, Mg, Fe, Ti, Al, Na, K, and S (most organized into compounds, particularly silicates, that can be sampled by recovery of iron and stony meteorites - representing fragments of comets and broken protoplanets that are swept up onto Earth). This material, bound by gravity to the Sun but free to move inertially in encircling orbits, remained distributed in the space making up the Solar System. This system of particles rather rapidly organized into a disc-like shape whose present radius is about 100 A.U. (Astronomical Unit, defined as the average distance [149.6 x 106 km, or about 93 million miles] between the centers of the Earth and Sun; solar light takes about 8 and a half minutes to travel that distance; Pluto, lies 39.5 A.U. from the Sun whose gravitational influence is exerted well beyond). The disc rotated slowly (counterclockwise relative to a viewpoint above the north celestial pole [which passes through Polaris, the North Star]), its motion influenced by external gravitational effects from nearby stars.

As this rotation got underway, and thereafter, the stellar (solar) magnetic field churned up the dust and gases (descriptively compared to the action of an "eggbeater" in a thin batter) causing them to collect into clots much smaller than the Sun that underwent various degrees of condensation. This field also expels and guides this material into jets that carry matter out to great distances, as seen here in this Hubble Space Telescope view of a jet ejecting from another star in our galaxy:

Hubble image of a jet ejecting from another star in our Solar System.

Both jets and irregular nebular patches (e.g., the Horsehead and Eagle nebulas shown above) contain not only gases but significant amounts of dust. The dust is very small and consists of three types: 1) core-mantle elliptical particles, typically 0.3 to 0.5 microns in long dimension, with a silicate interior coated by icy forms of gases; 2) carbonaceous particles (~0.005 microns), and 3) open frothy clots called PAH dust (polycyclic aromatic hydrocarbons) (~0.002 microns). Shock waves and radiation can strip off the ice mantle leaving grains that are incorporated into coalescing bodies that form the prototypes which accrete into the planetesimals from which asteroids or planets then build up. Ultraviolet radiation can modify the organics into more complex molecular forms. In this way, organic molecules are introduced from space onto planetary surface and, if conditions are right, can eventually serve as viable ingredients for the inception of living things (see below).

The possible role of shock waves in planetary formation is now the subject of considerable study. Evidence for a shock wave that develops as material falls towards a nearby protostar against its remaining gas/dust cloud has been observed at L1157, in which the present cloud is about 20 times the solar system diameter. As this cloud proceeds to infall into the newborn star as it organizes into a disk, it produces shock waves that may clump dust together, as described in the next paragraph. Here is this cloud:

Nebular dust envelope around a protostar in L1157.

For the Solar System, shock waves and intense radiation acted on the dust such that some of it melted into tiny droplets which chill into chondrules. These spherical bodies then were caught up with remaining dust to produce the primitive small solid bodies (fluffy "rockballs") that populated much of the heliosphere surrounding the Sun. We see samples of these bodies today as meteorites. Most infallen meteorites are ordinary chondrites that, in thin section, appear much like this sample from the Tieschitz meteorite:

 Photomicrograph of a thin section through the Tieschitz chondritic meteorite in which the round objects are crystalline chondrules.

The most primitive meteorites, called carbonaceous chondrites, are enriched in carbon and contain water. Other meteorites are iron-rich (some with > 90% metallic iron), and may have once been the interiors of planetary bodies since disrupted. The chondrules themselves generally show a very limited size range, suggesting that ones larger than these fell back into the Sun through gravitational pull whereas smaller ones were swept away into interstellar space through expulsion by shock waves and solar wind.

Magnetically-driven eddies within the gas/dust cloud helped to impart additional angular momentum to the larger condensed rotating objects beyond the spherical Sun (which possesses only 0.55% of this momentum even though it contains 99.87% of the total mass of the system). These objects now remain in orbits around the Sun in positions that remain stable because of the counterbalance between centrifugal forces related to angular momentum and inward-directed gravitational pull from the Sun.

Planets appear to form simultaneously with the star around which they associate in well-defined orbits. Two general models for planetary formation (mainly of large planets with thick gas atmospheres) - Accretion and Gas Collapse - are popular now, and both may have operated. These models are shown in these two panel sequences:

Two models for planetary formation; applies primarily to Giant gas planets.

For the Accretion model, as the formative process operated, local instabilities in the nebula tied to the Sun caused the chondrule-laden rockballs within turbulent zones to cluster and further aggregate into objects ranging from meter-size up to planetesimal dimensions (tens to a few hundred kilometers, typified [perhaps coincidentally] by asteroid proportions).

Planet formation diagram.

From J. Silk, The Big Bang, 2nd Ed., © 1989. Reproduced by permission of W.H. Freeman Co., New York

During this growth stage, smaller planetesimals tended to break apart repeatedly from mutual collisions while larger ones survived by attracting most of the smaller ones gravitationally, growing by accretion as new matter impacted on their surfaces. Once started, "runaway" growth ensues so that many planetesimals combine into bodies that eventually enlarge into fullblown planets. The bulk of the matter beyond the Sun was swept into the planets and their satellites, although some remains in comets and cosmic dust. Mercury, and some Outer Planet satellites are preserved remnants of this later stage in planetary growth, as indicated by their heavily cratered surfaces that were never destroyed by subsequent processes such as erosion. In contrast, the Moon appears to have built up by re-aggregation of debris hurled into space as ejecta from a giant impact on Earth soon after our planet formed; once collected into a sphere (which probably melted), the lunar surface continued to be bombarded with its own remnants as well as asteroids and other space debris. Its oldest craters are hundred of millions of years younger than the time at which the debris reassembled, melted and formed the lunar sphere; at least some of its larger basins are somewhat older.

As mentioned above, in our Solar System, the four inner planets (the Terrestrial Group) are largely rocky (silicates, oxides, and some free iron; three with atmospheres) and the outer four (Giant Group) are mostly gases with possible rock cores. These Giants developed large enough cores to attract and capture significant fractions of the nebular gases dispersed in the accretion disk.

Analysis of argon, nitrogen, and other gases in Jupiter indicates their amounts are such that this Giant must have formed under very cold conditions; if further work bears this out, Solar System scientists may adopt, as one plausible explanation, an origin of Jupiter (and perhaps the other Giants) at much greater initial distances from the Sun with these having since moved significantly closer through orbital contraction or decay. The ninth planet, Pluto, the smallest and, at times, farthest out (its elliptical orbit periodically brings it within that of Neptune), appears to be made up of rock and ice and may be a captured satellite of Neptune.

Theoreticians differ as to the exact methods and sequence in which the planets accumulated after the condensation and planetesimal phases. Timing is a critical aspect of the formation history. One version - the equilibrium condensation model - considers condensation to happen early and quickly, in a few million years, with the observed sunward zoning of higher temperature minerals and greater densities in the rocky inner planets both being consequences of the increasing temperature profile inward across the solar nebula. Accretion was stretched out over 100 million years or so. The heterogeneous accretion model holds condensation and buildup of planetesimals to proceed simultaneously over a few tens of millions of years. Neither model adequately explains the fact that both high and low temperature minerals aggregate together in the inner planets to provide materials capable of generating the atmospheric gases released from these planets. The models also do not fully account for the strong preferential concentration of iron and other siderophile ("iron-loving") elements in the inner, terrestrial planets. One solution is to add (by impact) low temperature material to the growing protoplanets carried in along eccentric orbits from asteroidal and giant planet regions. This material is then homogenized during the total melting assumed for each inner planet early in its evolution (this melting is the consequence of heat deposited from accretionary impacts, from gravitational contraction, and from release during radioactive decay). As cooling ensues, materials are redistributed during the general differentiation that carries heavy metals and compounds towards the center and allows light materials to "float" upwards towards the surface.

Much less is known about the evolutionary history of planets and their eventual demise (destruction). Extrapolating from our Solar System with its two major types of planets - Rocky and Gaseous - and the variety of surfaces on them and their satellites, it is evident that a great range of sizes, compositions, and surficial states can be expected among the millions of planets that many believe exist in the Universe. In the Solar System its complement of planets have survived essentially intact (possible exception: the asteroid belt) since the Sun itself organized some 4.5 to 5 billion years ago). The Sun is expected to burn out its fuel in another 5 billion years, the expanding rapidly into a Red Giant. The outward surge of its gaseous envelope should consume many - maybe all - of the named planets as well as other solar material. This is probably the usual mechanism of most planetary destruction - consumption by Red Giant expansion or by novae or supernovae (see top of page 20-6). Another possibility: gravitational pull brings the planets into their parent stars. Generally, planetary systems around massive stars, if indeed these do form, will be short-lived as those stars themselves do not last billions of years (thus, such stars are not likely to harbor life since not enough time elapses to permit development by evolution [see below]). Smaller stars, such as G types, are much more favorable bodies for fostering life on any planets that may revolve about them, owing to their longer spans of existence.

Life on Planetary Bodies

From an anthropocentric outlook, the importance in understanding planetary formation mechanisms and history is the assumption (not yet a clearcut fact) that planets possessing certain appropriate conditions are the harbors of life. Life, it is believed by Earth dwellers who can think, may well be the most complex and advanced feature in the Universe, based on the presumption that it has evolved into a state resulting in lifeforms that perceive beyond sensing, analyze through reason, and evaluate most other aspects of known existence. Life, under this viewpoint, is the quintessential achievement in the evolution of the Universe to date. Whether life on Earth stands at the pinnacle, or somewhere below, has yet to be established - statistically, it is most likely that somewhere in the Universe even more highly developed living creatures, with superior intellects, exist today or have in the past. (The ideas just enunciated are closely associated with the modern doctrine called humanism.

Thus, the capstone of this Section on Cosmology must surely be a consideration of the most provocative and fascinating Quest in the history of life - the attempts to determine whether life - and specifically intelligent life - exists elsewhere in the Universe. Philosophically, many on Earth hope that we are unique - thinking beings that are the pinnacle and teleological goal of a Creator's act. Scientifically, most cosmologists, biologists, etc. are coming around to the firm conviction that life does indeed exist elsewhere - throughout the Universe. This is a logical conclusion, since a huge Universe with just one tiny inhabited body on which conscious creatures exist strikes most scientists, and a growing number of philosophers, as extremely unlikely, and, from a practical sense, even a foolish, wasteful action by any Creator (this viewpoint is touched upon again later on this page).

One of the prime motivations that has stemmed from space exploration continues to the the Search for ExtraTerrestrial Intelligence (SETI) - which is a much higher goal than simply seeking evidence that lower levels of life exist elsewhere. This has so far been something of an ad hoc effort by a small number of dedicated astronomers who have had limited support from private sources (the movie "Contact" captures the essence of this effort). Now, NASA and other large organizations have become involved and a more concerted and systematic hunt for advanced life forms is being funded.

The ways in which SETI now seeks evidence for intelligent life are diverse and innovative. This is a very daunting and intriguing subject that could warrant considerable coverage space on this page but reluctantly must be confined to a synopsis of a few key ideas. However, we choose to guide you to several sites on the Internet for many of the omitted details. The starting point is the SETI site itself. The SETI Institute is currently being directed by Dr. Frank Drake, the originator of what is now known as Drake's Equation. Here are three Internet sites that discuss in some depth that equation: (1), (2), and (3).

For the record, we now state the Drake Equation (in its "dimensional analysis" format) and add some comments on values used in its terms (you can choose your own set of value in (2) above to see how changes affect the outcome):

N = R fp ne fl fi fc L


N = the number of communicating civilizations in the Universe.

R = the rate of formation of stars of types around which planets can form

ne = the number of Earth-like bodies in the planetary system

fl = the fraction of planets with life

fi = the fraction with intelligent life

fc = the fraction that has developed interstellar communication systems

L = the lifetime (span) of civilizations (up to extinction)

Of course, none of these parameters has yet been fixed with reasonably certainty, so that many values (and ranges thereof) have been proposed. One common set (but still provisional) has R = 10; fp = 0.5; ne = 0.2; fl = 0.2; fi = 0.2; fc = 0.2, and L = 50000 (source: Article "Why ET hasn't Called", by M. Shermer; Scientific American, August 2002). For our galaxy, this set of values give N = 400 civilizations.

Each of these term inputs is easily challenged. For fp, any number chosen would depend on such variables as the total number of stars in a galaxy and the type of star suited to planetary formation (usually limited to G and K types) and its percentage of the total population. Recent estimates center around 10% (factor = 0.1). At the 2001 Annual Meeting of the American Astronomical Society, Dr. J. Bally of the Univ. of Colorado presented an argument which concludes that only about 5% of the stars in the Universe are capable of producing (surviving) planetary systems. Massive stars will blow away the gas and dust needed for planets to form. Binary or ternary star systems, which are the most common arrangement, also are unfavorable for planetary growth, especially since one of the pair or trio is likely to be a Giant, and matter is collected as jets owing to attraction. He concludes that planets need to form soon after a star is born in order to have a reasonable chance for survival.

Plausible arguments can be made for values/ranges of each of the others. For example, fl will be sensitive to such related variables as the presence and importance of water, the nature of the evolved atmosphere, and the time needed for higher order life forms to evolve (relative to planet age; rate can vary as these others assume values other than that of our Earth). The possibility of non-carbon-based life forms must be factored in. In Shermer's article, he calls attention to the great uncertainty of L, the time over which any intelligences will persist before extinction. Pessimists feel that this number can be small. For Earth, civilized life is only about 5000 years old (based on the time at which agriculture and earliest urbanization become practiced). With the advent of the atomic bomb, these doomsayers consider that total mass destruction may be likely. On the other hand, one can conceptualize human society as overcoming its self-destruction tendencies and lasting (into the future) for millions of years (the upper limit then may relate either to a catastrophic impact or the stage in which the Sun burns out and expands to envelop the inner planets).

Still another factor that is not stated in the Drake Equation (or commonly discussed online) is the nature and strength of the communication signal. Present-day radio waves are probably too weak to have much effect in all but the nearest part of our galaxy. Higher energy waves (such as gamma radiation) would be more powerful but we on Earth have not yet devised suitable transmitters of these rays. And any signal sent must move away in many directions from it planetary (spherical) source; if only directional beams are emitted, the number of planetary receivers (made by other intelligent recipients) in the right position to pick the signals will be greatly reduced. But, directional beams (those using laser light are especially promising) have one advantage - they remain concentrated over a small angular volume. As we on Earth continue to find many more planets (most would be in our Galaxy at this stage of our detection capabilities), we can systematically send signals to these with a good chance to intercept them in the narrow field of view encompassing our signals. Likewise, any intelligent and technically capable life on any one(s) of these may have by now spotted our solar system and have, or will, send signals to us. What the "message" should be, owing to uncertainties of language recognition and translation, is probably dictated by the need to transmit something of a universal nature: sending intermittent signals (sound or light) that consist of a series of prime numbers is a favorite suggestion. Since mathematics has a unifying generality to it - all intelligent beings should find some of the same fundamental theorems and expressions - the message we receive (or send) is most likely to be in that format (spoken/written anguage is ruled out because each one on Earth has a unique set of meanings attached to words that therefore precludes universality).

Start with hypothetical observers at two points A and B not then in contact in early spacetime. Over expansion time, their light cones would eventually intersect, allowing each to see (at time t1) other parts of the Universe in common but not yet one another. At a later time, beyond t2 ("now") in the future, the horizons of A and B (boundaries of the two light cones) will finally intersect, allowing each to peer back into the past history of the other.

All in all, we are still a "far cry" removed from having any reasonable estimate of probabilities of other civilizations actually extant. Shermer develops arguments that produce numbers as low as 2 to 3 for our galaxy (if really strong signals can reach other from other galaxies, this number would then greatly rise). T.R. McDonough of the Planetary Society arrives at 4000; Carl Sagan during an optimistic moment, came up with 1,000,000. Today, no one is arriving at zero, so those seeking ET can remain hopeful, even optimistic.

In October 2001, the Scientific American magazine carried a review article (pages 61-67) by G. Gonzalez, D. Brownlee, and P. D. Ward bearing the provocative title of "Refuges for Life in a Hostile Universe". We will not paraphrase its many intriguing statements and conclusions but urge you to track the article down and read through it. Its bottom line is that there appears to be only a narrow range of conditions around stars and within galaxies that would likely harbor life (organic matter; not necessarily intelligent). They define CHZ and GHZ as Circumstellar and Glalactic Habitable Zones respectively. In systems like our Sun's this is confined to a narrow inner zone. In spiral galaxies the GHZ is beyond the inner bulge, halo, and thick disk that consist mostly of old stars; the more favorable region is an annulus about midway from the center to the edge defined by the spiral arms, in that part known as the thin disk.

The condition most pertinent to possibilities for life is the metallicity of the star groups. Metallicity defines the proportion or percentage of chemical elements with atomic numbers above 2 in the total mix of elemental and molecular gases and dust available for star formation. Smaller stars with metallicities between 60 and 200% of the Sun's value are favored. These stars comprise about 20% of the total in a galaxy. Other factors include the frequency of supernovae (which over time build up the supply of the "metal" atoms), excessive radiation (continuous or in bursts), and the distribution and numbers of objects capable of destroying protoplanets by impact. Giant planets seem less likely to foster conditions that would aid life's establishment. They conclude that 1) most stars don't have planets and 2) complex life is rare even on those stars with planets. While they don't propose that Earth is unique, they do caution that the statistical distribution from their GHZ and CHZ models support the notion that it may prove very hard to find evidence of life of any kind either in the Milky Way or (even more so) in more distant galaxies.

The expectation that some life exists elsewhere in the Universe will depend on the nature of life itself. Life can be defined by properties that are both chemical and functional. Judging on what we know conclusively from the one sample available to earthlings - namely, life on Earth itself as the only confirmed example in the Solar System - the essential chemical incredients are carbon, the crucial element in organic molecules (of which proteins are the fundamental component) of great complexity and variety that are the basis of life, together with hydrogen, oxygen (and water), nitrogen, and to a lesser degree phosphorus and sulphur and other elements as important traces. The amazing thing about this assemblage of critical elements is that they all at times in the past resided in stars and much of the hydrogen itself can be traced back to the first minute of the Big Bang. You and I, as humans, are truly star people - our heritage is cosmic in that our ingredients are either primordial- or stellar-derived.

The principal functional manifestations of life are: metabolism; reproducibility; cellular-organization; growth cycle and dependence on nutrition; respiration (in some types); (usually) movement of some kind; propensity to evolutionary modification, and, for some types, utilization of photosynthesis. Intelligent life, furthermore, is marked by consciousness, reasoning, abstraction, reliance on memory, communication, and awareness of time and other essentials of existence; free will and "soul" are properties of a more metaphysical nature and harder to prove as realities.

If Earth defines the standard state, from which there can be no major deviations if life is to form and survive, then life-supporting planets are constrained to vary in their physical and chemical properties only within a narrow range. A planet must have accessible and reactive carbon capable of polymerizing (some have postulated an alternative element, silicon, as the keystone in complex life-sustaining molecules but no such compounds have been successfully synthesized and made to function like carbon life). It appears (but is not totally certain) that water is also essential. If so, in an environ like Earth, this limits the range of temperatures at which life originates to the freezing and vaporization (boiling) point values of 0 to 100 °C (however, life, once formed, has been found to exist at temperatures that are higher and lower, but generally in the presence of liquid water; witness, the "smokers", which host symbiotically specialized life, that eject superheated water and steam on active divergent ridges on the ocean floor). Atmospheres of oxygen and nitrogen favor many forms of life; planets that are either airless or contain hostile chemicals such as methane and sulphur compounds tend to suppress life.

If such conditions, and their ranges, are indeed limiting factors, then only a few, if any, planets in a given planetary system are properly suited to the origin, development, and persistence of living creatures. Thus, while billions of planets are probably existent universally today, only a small fraction are suited to supporting life. These will be confined to those at a distance from their star where temperatures are in appropriate ranges to allow water in a suitable state (not chemically bound or heated so that all has evaporated and escaped to space). They will have proper sizes to maintain fostering atmospheres. Their chemistry will allow production of molecules (most likely carbon-based) that can, over time, evolve into organic ones of sufficient complexity to merit the status of "living".

Water has been detected in the Solar System, mainly on icy satellites, on Mars, and in comets. A search for this compound beyond the Solar System has finally met with success. Astronomers have now detected water around CW Leonus, a carbon-rich star in its waning life, some 500 l.y. from Earth. This water is believed to now be vapor derived from billions of comets about the star, as it rapidly releases its heat during an explosive phase.

The best review of the Origin of Life found so far by the writer is the 1999 book by Paul Davies, The Fifth Miracle: The Search for the Origin and Meaning of Life, Simon & Schuster.

Whether the evolutionary mechanisms we have found to operate on Earth - change into diverse and usually more complex forms in response to changing conditions, aided by genetic processes and natural selection - lead to intelligent life elsewhere can only so far be the subject of speculation (devoid of any direct evidence). But, again, considering the large number of favorable planets - almost certainly in the millions - spread throughout the billions of galaxies, it would not be surprising, and is to be expected, that organisms with consciousness and other aspects of intelligence will someday be communicated with, thus supporting the thesis of Universal life. It is provocative to conjecture whether these "alien" thinkers have some insight into the concept of an Intelligent Designer (Creator or God) and whether they believe, as most here still do, in the special gift of that God of the "soul" destined to persist in some form of immortality.

Notice that we have progressed well down this page without mentioning the favorite topic among those - scientists and laypersons - who speculate on the possibilities and ramifications of intelligent "alien" life in our galaxy, and by reasonable inference, in most other galaxies: the reality of whether we have been visited by these creatures in their spaceships (UFO's) at times in the past, and a corollary, whether we on Earth will ever have the means to visit other planets by some means of space travel. In light of present knowledge, any extraterrestrials will almost certainly not originate in any other solar sytem planet but would reach us from beyond - well into outer space. We, in turn, must gain experience in space travel by first journeying to one or more of our neighboring planets. Probably in the lifetime of some who read these words this will happen. But extending this travel to other stars - interstellar travel within the Milky Way - may not happen in this timeframe, although 100, 500, 1000, a million years hence, the advances in science and technology should bring this about. But, complications and limitations must be overcome.

By far the biggest problem in interstellar travel is distance. A simple example illustrates the difficulty. The nearest star is Proxima Centauri, 4.2 light years away (actually, Alpha Centauri, 0.1 l.y. farther away, is a better choice owing to its size), or about 42 trillion kilometers (26 billion miles) from Earth, would be a reasonable first target. To help you visualize these distances, look at this diagram (the distances are in Astronomical Units [1 A.U. is the distance between Earth and Sun, or 149 million kilometers {~93 million miles}]).

The Solar System, to its outer edges (note Kuiper Belt and the Oort Cloud region), and beyond to the Centauri star group; distances given in power of ten scale using Astronomical Unites.

If a manned spacecraft were to leave the solar system at the same velocity that Pioneer 10 had when it actually did escape the system, namely, 60000 km/hr (37000 mph), it would take approximately 80000 years to reach one of these two stars. (And, the same time to return to Earth, unless a one-way trip is the choice, then at least double that total.) This obviously would be impractical under the psychology of today's thinking. The solution should be obvious: Earthlings must build a spacecraft that contains all those materials and provisions needed to sustain life for eons. Chief among these would be foodstuffs, water, oxygen and other essentials. Then, even if human life spans can be extended, the strategy would still have to be: to continually recreate people on board through breeding, so that those who arrive at Alpha Centauri (hopefully, we will discover planets there; none have been detected as yet) will be many generations down the time path of continuing life of the future. Trips to other more distant stars may be more enticing if these show evidence of planets that can sustain life. Surely, from the billions of humans on Earth when this time of launch comes there may be volunteers who are agreeable to setting forth on this voyage; if and when we develop the appropriate technology, the travelers may need to consent to being placed in some kind of suspended animation (a body freeze technique is commonly proposed to put living creatures into hibernation). Such a trip is likely to fulfill at least one of four motivations: 1) either the innate need for man to explore; (2) and/or a desire to establish contact and exchange knowledge with other civilizations; (3) and/or a compelling need to survive if Earth should threatened to become uninhabitable); (4) and/or a decision to colonize another (uninhabited) planet with suitable living conditions for organisms, or if intelligent beings are found to settle with them.

However, most readers of this Tutorial would judge the long duration travel scenario (with or without hibernation) to be rather undesirable even if altruistic. Is there any alternative. Yes, if we can find new means of propulsion through space at much greater speeds than presently achieved. For instance, let the inertial velocity reach 0.2 the speed of light 'c'. If that occurs soon after launch, the spacecraft should reach Alpha Centauri in 5 x 4.3 l.y., or 21 years (earthtime), or 42+ years roundtrip. Theoretically, this transit time can be greatly shortened if the spacecraft attains a velocity near light speed. However, relativistic effects will come into play (see Preface to this Section), including differential aging between space travelers and those remaining on Earth. Thus, time dilation would make the high speed trip appear to those on Earth to have taken longer than the 4.2 years plus adjustments for being somewhat under light speed.

There are many other factors required for success and safety to take into consideration. Perhaps at the top of the list is to find propulsion systems that can reach these high speeds (significant fractions of the speed of light); to get to such speeds requires huge expenditures of energy. Presently, none is known, but some sound proposals for possibilities already have surfaced: ion engines, anti-matter engines, controlled nuclear processes, gravitational "slings", laser beams pushing on light sails, and various other innovative but speculative mechanisms for powerful propulsion systems. Quantum-minded thinkers can conjure up schemes that depend on "wormholes" and "quantum tunneling", "timewarps", and the like. A long shot depends on the proof of existence of the hypothesized "tachyons", particles that travel at faster than the speed of light; if real and accessible, some technique would be needed to harness them as aids to propulsion. Whatever propulsion systems is eventually proven feasible, one requisite is that it be one that travels with the spacecraft (instead of a "one-shot" push at the outset) so that there would always be a means for course corrections, maneuvering, handling the unforseen, possible landing, and eventual return to Earth if that is a mission requirement. If we base our predictions for space travel eventuality on the huge advances in science and technology over the last two centuries - now seeming to occur as though growing exponentially - it is reasonable to expect that the possibility of interstellar travel will turn into reality in the not too distant future (say, in this millenium). If that indeed happens, then the counterpoint argument is that aliens "out there" may be ahead of us and have in fact visited Earth - if we are judged to be interesting enough.

These last paragraphs have no doubt been fascinating for some. There are a large number of books devoted to this mind-boggling subject. These are referenced on the Internet; just use your Search Engine to look for topics such as "Space travel", "Interstellar travel", or similar keywords. The writer found many that discuss the potentialities and the difficulties underlying outer space exploration; here are two of these URLs: (1); (3)

To sum up the hard realities of space travel: 1) present and foreseeable technologies still far short of making such trips plausible, safe and worthwhile 2) in time (probably many centuries), humans may learn enough to engage in such endeavors; 3) almost certainly, a manned trip will be preceded by unmanned spacecraft to prove the workability of the technology; 4) if mankind survives itself (or evolves into some form of superintelligence that can control the various threats to self-destruction (wars; environmental nihilism), trips to other stars will someday be inevitable; and 5) in the meantime, we should continue to inventory suitable planets and search for intelligent life elsewhere (SETI), so as to develop the incentive for undertaking travel to candidate planets; we should also hunt for any evidence that Earth has previously been visited (the Fermi Paradox: If aliens have already come to Earth, as might be expected since statistically there could well be many more advanced civilizations than those on Earth today if intelligent life is widespread in the Universe, then why haven't we found any valid signs of their visit?)

After the above comments on space travel were written, an excellent article for the layperson on its feasibility, simply titled Star Trek, by W.S. Weed, appeared in the August 2003 issue of Discovery Magazine. It is well worth reading to realize both potential and problems in visiting even the nearby stars. We will list here the 5 propulsion systems reviewed: Atomic Rockets; Nuclear Fusion: Antimatter; Laser Sail; and Fusion Ramjet. At present, technology is still way short of achieving a functional system but the concepts all appear viable. Within several decades one or more of these systems, and probably others yet conceived, may become practical working modes that can propel space travelers at speeds from 0.1 to perhaps 0.8 that of light. Most of the systems discussed would require that humans onboard would have to spend from 40 to 100 years to reach stars close-by. The various problems they would face: food/water; air; deletorious gravitational effects; radiation threats, and, not the least, psychological adjustments are all solvable but require considerable refinement from present-day capabilities. Of course, the age problem (not helped if the speed of light is not approached) can be handled by onboard breeding and birth, i.e, resorting to multiple generations.

Perhaps this is the point to introduce a note of caution. Forecasting into the future it is largely unclear as to how long, and even whether, the human race - or subsequent evolutionary offshoots (improvement?) - will continue to inhabit Earth. Based on some current trends, we ourselves may do intelligent life in, or a super asteroid, might extinguish us. That is one motivation for continuing to seek means to escape the confines of our Solar System and seek out new planetary systems around other stars. There is another compelling reason, suggested by this illustration:

The future of the Solar System

Thus, the Earth and its siblings are doomed ultimately by the exhaustion of fuel in the Sun which will lead to a catastrophic (if life still exists then) event associated with the Red Giant phase (mentioned on page 20-5a). Even before the next 5 billion years, severe warming may wipe out life if countermeasures haven't been found in the next billion years. Or, the ultimate Doomsday scenario: Man, or some successive genus or new type of intelligence, destroys "himself" through acts of his own (common example - total nuclear holocaust). Yet, somehow I fail to feel much personal concern at this time, as I happily believe that humans will mature enough in the future to assure their control over their survival.

In closing Section 20, contemplation of Cosmology, and its astronomical substructure, is a truly humbling experience for one's brain. The wonder is: that there exists on one tiny point in a humongous Universe something called the "conscious" and that the human mind (yours, for instance) can conceive of, and begin to understand, the truths of this Universe's attributes and history that are continually being discovered and refined. Finally, it is both astonishing and reassuring to realize that all beings - be they people or animals/plants or inanimate matter - are remarkably cosmological in nature: All the atoms in our bodies were once contained in stars or interstellar space; our parts in a sense are variously billions of years old and their atomic constituents, even after multiple dispersions and reassemblies, will last at least as long as the present Universe - estimated to continue for perhaps 50 b.y or more. In one way, then, our essences will have achieved some kind of plausible Immortality in view of the many incarnations that preceded our current atomic arrangement and are yet to happen. But, from the humble side, perhaps "We are not alone" and certainly we are not at the center of the known Universe; our importance is sui generis and our rank among the Universe's populations is probably just average.

This Section on Cosmology has doubtless been heavy going for most readers. Some may be inspired to wish to learn more. Refer to the Preface link accessed from page 20-1 for the references to books consulted by the writer in preparing this overview:

For those who would like to learn much more about astronomy through illustrations, we want to steer you to a Web site that allows you to seek more pictures and textual information about most of the topics that have been covered or touched upon so far in this Appendix. NASA Goddard astronomers have put together a Web site that features many previous Astronomy Picture of the Day (APOD). In the Search box, you simply type in a topic, e.g., young stars; supernova; black holes; spiral galaxies, planet, etc. and if the category is there a running text with links to subtopics and pictures will be delivered. This can be an adventure.

And, finally, here is a quiz of sorts - actually a Web Page that poses many questions (FACs) coming from those who have accessed the site. Many of these you should be able to answer now that you have gone through this Section. Others touch upon topics that may not have been considered before. Still others provide concise reviews of ideas we've presented but perhaps will give you a different slant or will offer supplemental information. Anyway, log on to "Ask the Space Scientist" and choose the group of topics that obviously fall within The Cosmos pervue. But, you may also want to check out the Planetology group or even some of the other subjects.

Some Additional Comments

For the curious, these paperback books by Paul Davies offer valuable insights into both scientific and metaphysical aspects of Cosmology: God and the New Physics and The Mind of God [especially Ch. 2]., Touchstone Books, Simon & Schuster, Inc.; this author considers the question of life elsewhere in the Universe in Are We Alone: Philosophical Implications of the Discovery of Extraterrestrial Life, Basic Books, 1995. Dr. Davies has followed up these books with an extremely insightful and provocative article in the September 2003 issue of The Atlantic Monthly entitled "E.T. and God" - highly recommended for its synoptic overview.

A book that relates cosmological discoveries to teachings associated with the Christian God is Beyond the Cosmos by Hugh Ross, Oxford Press, 1996. A balanced review that considers how religious beliefs and observations of the physical Universe are not necessarily incompatible is When Science meets Religion by Ian Barbour, Harper, 1999. A general overview of arguments contrary to the exclusively natural and spontaneous Universe such as described in this Section is A Case against Accident and Self-Organization, Rowman & Littlefield.

In the March, 2002 issue of the magazine First Things, which deals with topics in philosophy, theology, and the social order, an outstanding review of the possibilities of life elsewhere in the Universe and its implications for humankind, written by Fred Heeren, and entitled Home Alone in the Universe?, provides comprehensive and provocative insights into the question of how discovery of intelligent life beyond the Solar System would affect and change the outlook of Earth's inhabitants towards their place and role in the Universe. Highly recommended! And, as of August, 2002, the full text remains online at this URL.

Two Internet Sites that address the possibilities of other worlds, extraterrestrial life, and the implications that such life, and cosmology in general, would have on the religions of our world has been prepared by Florida Today and Stanford Encyclopedia (the latter rather heavy-going).

An interesting article dealing with the Anthropic Principle applied to cosmological ideas by Victor J. Stenger is found on the Internet. In this essay, Stenger argues against a recent resurgence in reconciliation between Science and Religion as still fraught with false premises. He concludes that a purely natural Universe is a very real possibility but while discounting the idea of Intelligent Design, he does not rule it out based on some overriding proof of its falsity but includes it in the list of possibilities. We strongly urge you to read this article and ponder its consequences. Then, you might wish to consider his article on The other side of time.

And last (but hopefully not least) you have the option of clicking here to read two letters written by the writer (NMS) to his local newspaper (The Press-Enterprise) as he joined a running debate on the editorial pages between one faction - the conservative Creationists - and another - the progressive Scientists - concerning the role that a God-Creator may have had in producing the Universe we have just been studying. The ideas in these two letters summarize my still developing viewpoint on the philosophical/scientific interweavings of the notions that God does/does not (+/-) exist. Read these letters if you are curious.

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