Models for the Origin of Planetary Systems

8/2/2019 Models for the Origin of Planetary Systems

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Models for the Origin of Planetary Systems

This "crash course" on Cosmology culminates with the following synopsis of themethods and results of astronomers' search for other planetary systems andconsideration of the origin of our own Solar System, plus a quick look at severalof the latest, provocative, and especially exciting theories (or brash speculations)on the start of life on Earth and, by extrapolation, the presence of otherintellectual beings in our Universe that might be "out there".

There are several JPL movies which you may wish to view before continuing on

this page. Access through the JPL Video Site, then the pathway Format-->Video-->Search to bring up the list that includes "Close up View of Plametary Birth",October 8, 2001; "Planet-forming Disks", January 11, 2002; "Beyond thePlanets", February 4, 2003; and "Pointing the Way to Exoplanets", December 11,2003 (this last is a full hour lecture). To start each one, once found, click on theblue RealVideo link.

As this page unfolds, one dominant idea regarding how planets form aroundstars will be developed. This idea is summarized in the diagram below so as toserve as a frame of reference for the prevailing views on planetary formation.The diagram simply shows a disc of gas and dust in a narrow plane around the

forming star:
http://www.jpl.nasa.gov/videos/index.cfmhttp://www.jpl.nasa.gov/videos/index.cfmhttp://www.jpl.nasa.gov/videos/index.cfm


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Two hallmarks in particular distinguish planets from the stars they orbit: First,they usually show marked differences in composition, being either gas balls(whose temperatures are well below fusion levels) with other elements besidesHydrogen that have rocky cores or they are dominantly rock with many elementsmaking up usually silicate minerals. (Some have satellites with frozen water or

other frozen gases in their outer shells.) And, at least one planet so far isdistinguished by the presence of millions of compounds of carbon - the organicmolecules found on Earth. 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 adrawn sphere the size of the Earth to allow comparison:

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

It is natural for humans to wonder if there is life elsewhere in the Milky Way, andby implication in other galaxies. The starting point in searching for life is to provethe existence of other planets and inventory their characteristics. In the lastdecade the hunt for planetary systems has intensified. The first extrasolar planet(generally the term "exoplanet" is now in common use) was found in 1993 byPenn State University astronomers. A more definitive sighting occurred in 1995 -an object orbiting the star 51 Pegasi, which lies 50 light years from Earth in theMilky Way. The closest exoplanet found so far is just 10.4 l.y. away from us,orbiting the star Epsilon Eridani. By June 2000, planetary bodies had beendetected in at least 88 other stellar (~solar) systems; as of October 2009 thelatest count was about 400 individual planets associated with these and othersystems -most being Giants and of low density. (Note: A few astronomers havedisputed some of these observations.) There is a growing "feeling" (but not yet aconsensus) that planets are the norm around many - perhaps most - stars, at


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least those of masses similar to the Sun. This is the basis of an argument withprofound implications: Statistically, there seems to be enough star/planetoccurrences in the Milky Way, and by inference other galaxies, for areasonable likelihood of some having hosted the evolution of life, and witha plausible probability of at least 10% intelligent life. Where is it? - this will be

discussed two pages later.

Analysis of one such system - Upsilon Andromedae -indicates it to have 3planets (a second triple planet system was recently discovered); 8 other starselsewhere have at least 2 planets. The Upsilon Andromedae system,diagrammed below, consists of Giant (probably gas-ball) planets (much smallerones presently are very hard to detect), all within orbits whose distances from itsstar are comparable to that of the four small terrestrial planets in our SolarSystem:

In August 2004, a star, mu Arae d, just 50 light years away, was shown to havethree planets, two larger gaseous ones and a third having a size, mass (14 Earthmasses), and orbital characteristics (rapid revolution around its parent) thatindicates it is likely the first rocky (inner Solar System-like) planet yet found.Then, at the end of August, announcement was made of star 55 Cancrie, 41 l.y.away and Cliese 436b, 33 l.y. away having a planet with 18 and 20 earth massesrespectively.

So far, only a very few possible planets have actually been seen (see below).Generally, planets are small relative to their parent stars and also have lowluminosities. Nearly all of the 200+ planets claimed to have been detected arededuced to exist from their interactions with their parent star, involvingperceptible movement of the star's position. Almost all discovered so far are large- Jupiter-sized or greater - and are gas balls. Several are extremely close to theirstar (at distances less than Mercury's orbit; one is thought to be as close as 5


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million miles from its star and rotates rapidly around its parent body). Manyplanets have much more elliptical orbits than those moving in the Solar System.It is hypothesized that most planetary systems will consist of multiple planets butthe smaller ones are presently still invisible to us and do not significantly distortthe motions of their host star.

A high point in the current search for planets occurred on June, 2002 whenseveral groups of astronomers announced jointly the discovery of up to 20 newplanets - including at least two of Jupiter size - in the Milky Way galaxy, whosestellar parents reside at distances ranging from 10.5 to 202 light years fromEarth. The closest in has been provisionally named Epsilon Eridani b (its star isthe one actually cited in the Star Trekseries as the location around which Mr.Spock's home planet, Vulcan, was orbiting). The rate of planet discovery seemsto be accelerating. With it is the growing belief by cosmologists that planets couldwell exist in the billions, i.e., they are the inevitable result of processes that takeplace 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 instellar evolution. (One recent estimate concludes that as many as six billion giantplanets exist within the Milky Way galaxy.) The proponents of SETI (Search forExtraterrestrial Life; seeking primarily radio signals that have non-randomcharacter [perhaps some form of mathematical organization]) have beengalvanized by these recent discoveries. (Some cosmologists are convinced that itis only a matter of time - probably during the 21st Century - until contactwithother intelligent beings is achieved.) We will return to the SETI endeavors nearthe bottom of this page.

Current search for extra-solar planets is restricted to the Milky Way and (in

principle) galaxies close enough to Earth for an individual star to be resolved tothe extent that changes in its motion can be measured. Gravitation attractionsfrom orbiting planetary bodies cause the central star to wobble. This is the basisfor the three prime methods currently being used to detect anomalies in a star'sbehavior 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-velocitytechnique. A component of a star'swobble 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 theapparent velocity of light. That, in turn, gives rise to small but measurableDoppler shifts in the frequencies of light radiation from specific excited elements,as expressed by lateral displacements of their spectral lines. From the wobblemagnitude and period, the approximate orbit and mass of its presumed cause -the orbiting planet - can be calculated. This method is sensitive to wobblevelocities as low as 2 meters per second. Jupiter, for example, causes a wobblevelocity of 12.5 m/sec imposed on light radial from the Sun. Generally, thismethod, applied to nearby stars, will detect mainly large planets close to their


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star but in March of 2000, two planets about Saturn-size (1/3rd the mass ofJupiter) were found in this way.

The second method, astrometry, also relies on star wobble but depends onmeasuring side to side displacements by direct observation through periodic

sightings. This determination of relative shifts can be done on photographicplates taken of part of the sky at different times, commonly using the sametelescope. But, considerable improvement in measuring shift results fromincreasd resolution achieved by positioning two telescopes physically somedistance apart but keeping them joined electronically. This permits application ofinterferometry such that the two telescopes act as though they were one largeone. Resolution as sharp as 20 millionths of an arc second (an arc second is1/3600ths of an arc degree on the sky hemisphere [0 at the sealevel horizon;90 at the zenith near Polaris in the celestial northern hemisphere]). The Keckinterferometric telescope in Hawaii will soon be operational. This should facilitatedetection 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)capable of observing wobbles is significantly improved by operating one or moretelescopes in space, above the distorting atmosphere. Resolution is alsoimproved by using a pair of telescopes mounted on a single boom but separatedby many meters, combining the image signals using the interferometric method.SIM (Space Interferometry Mission) is a NASA probe slated to fly sometime after2015. Its two telescopes will be 10 meters apart. This will lead to a millionth of anarcsecond resolution, capable of inferring the presence of planets just larger thanEarth-size or of large planets with far out orbits. Here is the spacecraft: A study of

potential for success of the SIM approach comes up with this estimate of planetsexpected to be detected:


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Interferometry, discussed in the Section on Radar, is an important technique forenhancing resolution. Its principles will not be discussed on this page but thereader is referred to these two websites for some insight: Wikipedia andAbsolute

Astronomy.

A third method is called transit photometry. When a planet's orbit takes it on apath where it passes across the face of the star under observation, the body willblock out a small amount of radiation (usually visible light) for the period oftransit. Sensitive detectors can note the slight diminution (up to about 2%). Todistinguish this from a "transient event" of other origin, the astronomer needs toestablish some regularity (reproducible at fixed intervals) of the drop in radiation,which will depend on the nature of the orbit (ellipticity; distance, etc.) Dependingon planet size and proximity, the drop in stellar luminosity will be a few percent orless (an accurate determination helps to establish the planet's actual size). Suchan 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 intensityunderwent a drop of 1.5%. In a June, 2002 meeting, two other groups usingtelescopes in Chile have reported 3 and 13 possible transit detections, but theseobservations have yet to be confirmed independently. This is a generalized plotof the type of signal or signature that is produced by the transit method:
http://en.wikipedia.org/wiki/Astronomical_interferometerhttp://www.absoluteastronomy.com/topics/Astronomical_interferometerhttp://www.absoluteastronomy.com/topics/Astronomical_interferometerhttp://www.absoluteastronomy.com/topics/Astronomical_interferometerhttp://en.wikipedia.org/wiki/Astronomical_interferometerhttp://www.absoluteastronomy.com/topics/Astronomical_interferometerhttp://www.absoluteastronomy.com/topics/Astronomical_interferometer


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

This is called the eclipsed or "winking" star method. In the left image (a photonegative), 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 cycleoccurs every 48 days. The eclipsing body could not be another star nor is it likelyto be a huge (star-size) planet. The interpretation is that there is a cloud ofasteroids and dust in a smeared-out clump orbiting KH 15D which block thestarlight when a clump passes across the parent star; speculation considers thatthere may already be one or more planets formed from this debris. There mayactually be two clumps (symmetrical pairing) at opposite positions in a single


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orbit; this has yet to be confirmed. A further anomaly: examination ofphotographic plates taken many years ago (although at limited intervals) doesnot 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 thistechnology is deemed ready, they hope to persuade NASA or some other agencyto use mirrors on several telescope-bearing satellites flying in formation andspaced to utilize the principle of interferometry to improve resolution and todetect small planetary objects.

Because (in part) of limitations at present in observational capabilities (mainly,telescopes are not yet powerful enough), there are inherent biases in findingplanets of various sizes and locations around their parent stars. Thus, there is atendency to find planets located around stars smaller than the Sun, since these

planets (usually large) have enough mass to induce noticeable wobble in the lessmassive parent stars. Likewise, very large planets bring about increased wobble;the wobble also increases for planets close to the parent. Thus, so far Red Dwarfstars (which are quite abundant) have been the types associated with adisproportionately higher number of planet discoveries, since these stars areprone to greater stellar-wobble.

The ultimate dream is to directly visualize individual planets. This may bepossible using several HST type spacecraft flying in formation ("clusters") withseparations 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 lowluminosity orbiting bodies detectable by resolution- and radiation-sensitiveinterferometry. Both NASA and ESA each have in the planning stage such amission (called The Terrestrial Planet Finder and Darwin, respectively).

One of the most important missions since the Hubble Space Telescope is Keplerobservatory (named after the German astronomer who formulated the laws ofplanetary motion). Using the transit method, Kepler will systematically search asmall segment of the Milky Way in search of Earth-sized planets. Kepler waslaunched on March 6, 2009 at 10:58 PM EST from Cape Canaveral on a three-stage Delta II rocket and eventually drift about 45 million miles away so it won'thave to contend with the reflected light of the moon and Earth.This overviewillustration symbolizes the mission.


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The spacecraft will occupy what is termed an "Earth-trailing orbit". Thus it willcircle the Sun at a revolution cycle slightly greater than the Earth itself. Thisdiagram amplifies this

Here is a sketch of the Kepler spacecraft with its main components and a photo

of the spacecraft in its fabrication facility:


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The Kepler telescope has a 95 megapixel (detector) camera that can integratebrightness measurements to single out variations as small as 0.01% (capable ofdetecting Earth mass-sized planets). These are mounted in 42 panels, shownbelow, that help the instrument function as a photometer. Using its 0.9 m mirrortelescope, up to 100,000 stars nearby in the Milky Way can be convenientlymonitored over time and individuals with multiple planets should reveal therelative number of stars that possess planetary systems. The expectation is that


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at least 1000 new planets will be discovered, including some that are Earth-sized.

The region of the Milky Way to be monitored is located near the Cygnusconstellation. Kepler will seek out stars that reside between 10 and 3000 lightyears from Earth. These diagrams are appropriate:


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The next paragraphs, italicized, have been taken off the Internet from severalsites that discuss details of the Kepler mission.

A transit occurs when a planet crosses in front of its host star as viewed by anobserver. These transits very slightly dim the brightness of a star which allow forthe detection of extrasolar planets. This change in brightness is very difficult to

detect for terrestrial-sized planets, such as Earth, because they only dim theirhost star by 100 parts per million (0.01%), over a passage time lasting only 2 to16 hours. That's equivalent to measuring from several miles away the change inbrightness caused by a flea crawling across a car's headlight. Adding to thedifficulty of the task is that dimming can be caused by events other than a transit,such as sunspots. Also, it is estimated that fewer than one star system in 100 willhave planets properly aligned so that they pass between the star and Kepler'scamera. In order for an extrasolar transit to be observed from our Solar System,the orbit must be viewed edge on. Any detectable change must be absolutely

periodic if it is caused by a planet. In addition, all transits produced by the sameplanet must be of the same change in brightness and last the same amount of

time, thus providing a highly repeatable signal and robust detection method.Because any planet in the habitable zone will require an orbit close to that of oneEarth year, Kepler will need to observe any transits discovered amongst thesample size of 100,000 stars for at least 3.5 years to determine if the transit is

periodic enough to be a planet.

Over a four-year period, Kepler will continuously view an amount of sky aboutequal to the size of a human hand held at arm's length or about equal in area to


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two "scoops" of the sky made with the Big Dipper constellation. In comparison,the Hubble Space Telescope can view only the amount of sky equal to a grain ofsand held at arms length, and then only for about a half-hour at a time.

Once detected, the planet's orbital size can be calculated from the period (how

long it takes the planet to orbit once around the star) and the mass of the starusing Kepler's Third Law of planetary motion. The size of the planet is found fromthe depth of the transit (how much the brightness of the star drops) and the sizeof the star. From the orbital size and the temperature of the star, the planet'scharacteristic temperature can be calculated. From this the question of whetheror not the planet is habitable (not necessarily inhabited) can be answered.

Most of the stars in Kepler's survey are relatively close, from tens of light-yearsto 3,000 light-years away. The first planets to be discovered in the comingmonths are likely to be more of the same gas giants that have been found so far.The earliest likely announcement that another Earth-sized planet has been found

may not come until December 2010 (assumes at least one repeat revolution of300 or more Earth days after the first detection of a transit; this could be longer ifplanet orbits further out).

The first successful images from Kepler were obtained in April; here is a typicalscene:

One of the first successful observations was of an occultation of star HAT-P-7 bya planet calculated to be a bit larger than Earth. The ground-based


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measurement, shown below, was notably noisier than the one by Kepler beneathit;

In sum, the Kepler Mission might not be able to directly determine whether or not

we are alone in the universe, but it will be able to tell us if we have neighboringplanetary systems, containing planets that might be capable of sustaining life.When compared to all the stars in the universe, even one discovery amongst therelatively small sample space of 100,000 stars will be significant enough for us torethink our meaning and place in the universe. Proof that there are planetselsewhere whose size and characteristics are within the range of Earth, andtherefore may be favorable for life will have profound philosophical andtheological impact on mankind. (Of course, if none are discovered this too wouldbe quite meaningful as it would favor the argument that "we" may indeed bealone - although this does not rule out possiblities elsewhere in the Universe.)

More about this mission is on the Internet at NASA's Kepler mission overviewand at a Wikipedia website.

Sponsored by the French and ESA, COROT was launched on December 27,2006. Its primary mission is to search for planets using the transit method. It hasalready found several in the Jupiter size range. A recent discovery is COROT-Exo-7b, which has a mass about 5 times that of Earth and an estimated diameterof 1.7 times the Earth. Here is a plot of data received that illustrates how transitdata appear:
http://kepler.nasa.gov/about/http://en.wikipedia.org/wiki/Kepler_Missionhttp://kepler.nasa.gov/about/http://en.wikipedia.org/wiki/Kepler_Mission


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In September, 2004 a reasonable claim has been made by astronomers usingthe European Space Agency telescope in Chile of having seen the first actualplanet. Look at this infrared image of their observation:

The star, 2M1207, just 50 light years away, is a brown dwarf (too small to initiateHydrogen fusion). At a distance twice that of Neptune from the Sun is a reddishobject five times the size of Jupiter but is cool (less than 2000 C) and has aspectrum that includes heavier elements. This object is likely a planet but finalconfirmation must await future observations of its changing positions as it orbits

(there is a small possibility it is another object beyond the dwarf star).

A stronger case was presented in March, 2005 by a group of astronomers usingthe European Southern Observatory. They have obtained a picture of GC Lupiwith a definite planetary body distanced about 1 Neptune orbit from the star (a;with an extended atmosphere). This planet (b) is about 2.5 times the mass ofJupiter and has a surface temperature between 3 and 4 thousand degreesCelsius. Here is a telescopic view:


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In September of 2008, astronomers released this image of a young star about

the mass of the Sun, located just 500 light years away. The luminous bodycircled near "11 o'clock" is judged to be a large planet about 8 times the mass ofJupiter:

The case for each of these being actual planets is still being debated. At leastone may be a brown dwarf companion star. In November of 2008 two papers inthe journal Science offer evidence supporting planets actually observed. The

bright star Formalhaut, 25 l.y. from Earth, was imaged by HST: there is a dotabout 119 A.U. from the star that appears to be a Giant planet; note the broadring of luminous dust and debris (asteroidal?) around the star (masked out).


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The second paper presents imagery showing three planets around star HR8799,130 l.y. away. The planets are comparitively young (estimated age: 600 millionyears), as they are still quite hot (accounts for their redness): Read the captionfor details.

Statistically speaking, the number of such planetary systems in the Universeshould extend into the millions within individual galaxies and the billions when thewhole Universe is considered. It would logically be likely therefore that non- orweakly-self-luminous bodies, i.e. planets, are the norm orbiting around a centralstar for at least some of the size classes on the Main Sequence of theHertzsprung-Russell diagram. As such stars proceed through their


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developmental stages (before they leave the Main Sequence), planets seem theinevitable outcome of the formational processes involved in star-making. So far,however, when the number of stars that have been studied using any of theabove detection techniques are used as a base line, only about 5% have yieldedevidence of associated planets; this number is probably a minimum for

determining the actual percentage that do have planets since smaller onescannot yet been recognized as present.

Two scientists, C. Lineweaver and D. Grether of the University of New SouthWales in Australia, have recently published a study that relies on reasonableprobabilities to estimate the number of planets just in the Milky Way. They arguethat, of the approximately 200 billion stars they calculate to be the totalpopulation of the M.W, about 10% or 20 billion consist of stars similar to our Sunand most likely to have favorable conditions for planet formation. Assuming that,of these, at least 10% will produce giant, Jupiter-like planets; thus their earliernumber 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 collectionprocess that leads to smaller companions. But, more importantly, the giantsserve as the principal attractors that gravitationally pull comets and asteroids intothem (remember the Shoemaker-Levy event discussed in Section 19) and thusfunction as "protectors" of the small planets by minimizing the impacts thesereceive. Now, in a more recent presentation at the 2003 International AstronomyUnion in Sydney, Australia they have raised their estimate to perhaps as muchas 30 billion giant planets and a similar number of earthllike planets. This bodeswell for future hunts as observational technology improves. Although this mayseem "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 rangesno greater than 10 solar masses.

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

The exoplanet systems probably show a wide range of individual types. OurSolar System is but one of many combinations of small-large, rocky-icy-gaseousplanets. The results mentioned above are biased towards larger planets and giveno real indication of the actual number in their systems. Some systems howevermay be almost like a binary star system in which the second or more planetsapproach brown dwarf mass but still too small to initiate nuclear burning.


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The expectation is that planets have formed over most of the time that stars havedeveloped in galaxies. One star pair (one a Pulsar; the other a White Dwarf) in aglobular cluster within the Milky Way some 5600 l.y. away (in the constellationScorpius), has been shown to be perhaps as old as 13 billion years. This isbased 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 almostcertainly the same age since prevailing theory holds planets to form roughly atthe same time as parent stars. It verified as a true planet, it must be aHydrogen/Helium gas ball similar to Jupiter. It is likely devoid of life owing to theturbulent history reconstructed for this star pairing and to the absence of life-forming elements. Even if some forms of primitive life did form on it or associatedsatellites, those would have perished because of harsh conditions that prevailedin its later years. But the chief implication of this observation (reported in July2003) is that planetary formation can be traced to the early days of the Universeand, as carbon accumulated from the many early supernova explosions, someplanets may have developed life of some type(s) since the first few billion years

of cosmic time or even earlier.

A group lead by Sean Raymond of the University of Colorado has recentlypublished results of a planet formation-distribution model that simulates (in thecomputer) conditions likely to produce planetary systems similar to those nowbeing found around stars. One typical end result is shown in this diagram:

The lower row shows the case where an earthlike planet forms beyond the orbitsof one or more giant planets. Also, smaller ice-crusted planets can form at even

greater distances. These, or precursors, may collide with the earthlike planet(s)to contribute water that produces oceans which make the planet(s) habitable forlife. The Colorado researchers conclude that at least one earthlike planetdevelops in about a third of the planetary systems that are produced by themodel (which can generate different variations by changing key parameters).

In nearly all the planet discoveries, the orbital pathways (around ecliptic zones) ofthe planet(s) in a system coincide well with any residual planetary debris disks


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that were observed. This supports the model described below in which planetaryformation takes place in the disk that develops around the parent star as itcontracts and begins its life as a Hydrogen fusion object.

Nearly all the planets found to date are much larger than Earth, being similar to

Jupiter as gas balls. This may be an observational bias: our present techniquescan only detect larger planets. What still seems unusual is the indication of manybig planets orbiting much closer to their parent (relative to theJupiter/Saturn/Uranus/Neptune distances from the Sun). Two ideas have beenpostulated for this: 1) these planets were once placed much farther from theirparent and have been drawn closer by the star's gravity (and thus are near theirdemise); and 2) the protoplanetary disks that now show these close-in stars mayhave been smaller relative to the Solar System dimensions, and hence the starsactually were formed nearer to the parent.

These planets are almost all in the "Giant" category, since they are large enough

to produce presently detectable effects on their parent stars. But a recentlyreported study (July 2007) has pointed up a paradox: Giant planets that are moredistant than a few Astronomical Units are rare in other planetary systems (in thestudy none were found around stars that should have such systems). Unlike ourSolar System, giant planets so far detected are nearly all relatively close to theirparent stars. The reason for this is presently uncertain.

On April 24, 2006 came the announcement that the first planet similar to Earth insize and in conditions for life had been discovered. Three planets, with masses 5,8, and 15 times that of Earth, have been found orbiting the star, a Red Dwarf,Gliese 581, located 20.5 l.y. from Earth. The 5 Earth mass planet, designated

Gliese 581c, shows convincing evidence of a rocky or icy sphere whose diameteris 1.5 times that of Earth. It lies only about 11 million km (7 million miles) from itsparent star (which has a much lower radiant energy output than the Sun).Because of this proximity, the planet may be locked onto the star such as toalways have one side facing it (like the Moon). The planet has an atmosphere(composition TBD) and a temperature range from 0 to 40 C (32 to 104 F). Thisrange would allow liquid water to exist at the surface. While much more needs tobe learned about Gliese 581c, it seems to have passed the "Goldilocks test" -conditions favoring life may indeed exist there. More about this planet is availableat this Wikepedia web site.

Another Red Dwarf star, Gliese 436, has a single detected planet about the sizeof Neptune. Gliese 436b's surface temperature is ~310 C. Its estimated densitysuggests water as the main constituent (hydrogen-rich atmosphere). Because ofhigh interior pressures, some of the water is ice, but at temperatures approaching100 C, possible because its freezing temperature is elevated by the greaterpressures.
http://en.wikipedia.org/wiki/Gliese_581_chttp://en.wikipedia.org/wiki/Gliese_581_c


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While astronomers involved in planetary studies exercise caution aboutconclusions that specify the number of earthlike planets to be expected fromvarious estimation procedures, they do propose that that number should be in themillions. Whether such planets also harbor life is much harder to pin downnumerically but the statistical approach suggests that a significant fraction of the

earthlike planets would possess the proper conditions. How much of that isintelligent life is still guesswork. The current sample of 1 (us!) is the onlyestablished data point. But if the reality is actually a much larger number, then,purely from statistical logic, we should expect that some of these intelligentcivilizations should be more advanced than ours. Why we have yet to "hear" fromthem remains uncertain (but now SETI improves the chances for this) unlessthere is some fundamental reason that makes space travel, even from nearbystars, very difficult.

Now, let's turn to consideration of the ways in which planets form. For planets ingeneral, both terrestrial and gas envelope types, dust maintained by the parent

star's gravity must be present in sufficient quantities to collect as cores or tocomprise the main body of the planet. There is plenty of dust in galaxies, mixedwith gases from which stars emerge. The source of the dust has been somewhatproblematic but a prime candidate is exploding stars (Supernovae) large enoughto synthesize Silicon, Oxygen, Iron and other heavier elements. A recentobservation strongly supports this:

This 300 year old Supernova is hard to see in Visible light because of asuperabundance of dust. When imaged in submillimeter light, the above patternstands out. The brightest areas are advancing gases with large quantities of dustthat give off light at these wavelengths.


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One essential requirement for a planetary system to develop is that it formsduring the organization of a central star (possible exception: a captured planet,probably rare). Also critical is the availability of dust and gas. The processesinvolved can be somewhat varied but are sensitive to a relatively narrow range ofconditions. The sequence of formative events probably begins during the T Tauri

stage of developing stars in which the conditions are favorable. These stars havenotable dust clouds (particulate nebulae) that can be monitored in the Infrared.Some evidence indicates the clouds will begin to reorganize their tiny particlesinto 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 cantake 10 million years or more to build up any planets that may result.

Another important factor, recently reported, is that stars which have a relativelyhigh content of Iron (from gases enriched by repeated mixing of Supernovaexplosions 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 thegravitation attraction that drives accretion through collisions and infall. Stars withthree times the Iron content of the Sun have an estimated chance of havingplanets 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 aprobability of about 8% for all stars of mass less than 10 times the Sun havingauxiliary planetary bodies). The results of this study are depicted in this graphicaldiagram:

The paradigm summarizing the processes involved in the formation of the Sunand its planets probably applies (with variations) to most other planetary systemsin general. The first realistic notion of how planets form was proposed by PierreLaplace in the 18th Century. In its modern version, both stars and their planetsare considered to evolve from individual clots or densifications within largernebular (cloudlike) concentrations dominantly of molecular Hydrogen mixed with


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some silicate dust particles that spread throughout the protogalaxies andpersisted even as these galaxies matured. In younger stars, much of theHydrogen and the heavier elements are derived from Nova/Supernovaexplosions that have dispersed them as interstellar matter that then may initiateclouds or mix with earlier clouds. Such nebulae are rather common throughout

the Universe, as is continually being confirmed by new observations with theHubble Space Telescope.

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

Below are two views of nebular materials associated with the famed Eaglenebula (page 20-3): Top = full display of the M16 (Eagle) nebula (note the darkdust areas; the white dots are stars lying outside this nebula); Bottom = details ofthe temperature variations in the dust making up the solid particulates in theEagle nebula as imaged by the European Space Agency's (ESA) Infrared SpaceObservatory (ISO) at two thermal infrared wavelengths, in which red is hot andblue is cooler (about - 100 C):


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As individual stars start to develop within these gas and dust clouds, in manyinstances that dust will organize into a protoplanetary disk (see third figurebelow). The NICMOS Infrared camera on the Hubble Space Telescope hasobserved a prime example of this stage, in which the glowing gases moving intothe 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 sideview in this image):


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

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 protoplanetarydisks called Propylids found in the neighborhood of a parent star. This may be amore advanced stage of materials concentration that results in a new star with anenvelope of gas-dust suited to accretion that produces planets. Three suchPropylids are evident in this image of the nebula associated with the Oriongalactic cluster.


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Other individuals, at least one possibly formed as recently as 100,000 years ago,

were found during the Orion study (see page 20-2 for a view of the entire youngnebula in the Orion group); look like this in closer views:

Propylids are vulnerable to being destroyed by UV radiation from massive, youngnearby 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
http://rst.gsfc.nasa.gov/Sect20/A2.htmlhttp://rst.gsfc.nasa.gov/Sect20/A2.html


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

The major threat to protoplanet formation is the stellar (solar) wind coming fromthe parent star. UV radiation is also able to break apart the smaller particles.Wind and UV radiation are capable of pulling apart small particles. Theseparticles in the early stages are tiny bits of dust (solid; solids with an ice coating;ice) which are charged electrostatically. These will collide from time to time and

may stick. If not disrupted by the stellar wind the now larger particles can againbump with others. By the stage in which some particle conglomerations havereached baseball size, they can resist stellar wind forces and survive to growever larger through collisions.

Thus, as the gas and dust cloud forms around a growing star, particles of solidsbegin to clump and some survive disrupting actions. However, much of the gasand dust may be pushed continuously away by the wind and radiation, so that


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the amount of material available to form planets generally diminishes over time.Evidence suggests that most Propylids are blown away before a planet growslarge enough to survive, implying that the planet formation process may be lessefficient and common than had been thought during the last decade. If theplanetary 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 timeframe of just 100,000 years or so; it is estimated that 90% of such clouds aredissipated before significant planetary cores can form. Planet accretion leading tosurvival is estimated to take up to 10,000,000 years.

Most galaxies began during the first half of the Universe and contained a largenumber of massive stars that formed early in galactic histories. These galaxieshave continuously been evolving through the eons as their stars synthesizedelements (see page 20-7) and then dispersed them by the Supernova process.New, mostly Main Sequence stars, chemically enriched with elements of higheratomic number, have continued to form well into younger times up through the

present. The smaller stars have lasted much longer and are probably thepreferred sizes suited to planetary formation and survival. Even today stars aredeveloping from the gases contained in remaining nebular material, so newplanets could still be forming.

A telescope observation, reported in April, 1998, records the sighting (throughthe 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 (ata lower resolution in which individual pixels stand out), taken in the IR, shows thiscentral star (yellow white) surrounded by a lenticular (in an oblique view),flattened disk of gases and solid matter (glowing hot [reds] in the Infrared):

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


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

This next Spitzer Space Telescope image shows HD6105, a collection of gasand dust that appears to be forming planets. Its central star has been blacked outin this image.

Another Hubble achievement is the imaging of a prominent disk around BetaPictoris (63 l.y. away) using the coronagraph to block out that star's surface-emanating light. There is a more obscure, but real, partial second disk developed

about 8 offset from the primary disk plane. At least one planet (roughly Jupiter-sized) was also indirectly found in this system.


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This next image of Beta Pictoris, made by a European Southern Observatory IRtelescope, has actually imaged a large planet inside the dust disk. The planet has

been given the provisional name Beta Pictoris b.

A very well-developed debris disk occurs around the young (200 million years)

star Fomalhaut, the 17th brightest star in the sky (southern celestial hemisphere),just 25 light years away. The disk, about 150 A.U (20 billion kilometers) from itsoff-center star, is analogous to the Kuiper Belt of debris around our Sun. Itappears to consist of icy dust. Time-motion studies indicates that there mayalready be one or more planets that are influencing the large objects in the disk,much like small moons perturb Saturn's rings. Three illustrations showing theFomalhaut disk, the first with the Spitzer Space Telescope, the second madewith IRAS, and the third through the Hubble Space Telescope, appear here:


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The HST has also imaged two other distinctive debris disks around a central star,

as shown in the next illustration. The disk on the left is caused by reflected starlight off myriads of small particles; the star, blocked out, is a Red Dwarf. On theright is another disk seen at nearly normal to a blocked out Red Dwarf 88 l.y.from Earth that here appears orange because of a different combination ofspectral bands used to create the image. This star-disk system may be as youngas 250 million years since its protostar began to burn.

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


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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 featuresoccupying 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 the boundary within whichno visible planetary disk matter has been detected; the broad black cross (X) isan 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 radiotelescope, 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 centralstar Vega, 25 light years away (in the Constellation Lyra). Here is an imagebased on observations made by D. Wilner and D. Aguilar of Harvard'sSmithsonian Center for Astrophysics. (Note: this image has been enhancedartificially as an artist's rendition.)


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C. Chen and M. Jura, Univ. of California-Berkeley have detected (monitoringInfrared radiation through the Keck telescope on Mauna Kea) a ring or disk ofdust which seems to contain asteroid-like bodies around zeta Leporis, a starsome 70 l.y. from Earth that is twice the mass of Sol and 15 times its luminosity.The ring, first detected by IRAS, is much closer (~5-12 A.U.) to its parent star

than the distances found for other recently observed or inferred planetary bodiesaround stars. Although imagery of the star, HR 1998, does not directly revealthese bodies, their presence is inferred from their average temperature of 350K(77C or 170F). From the data they accumulated, Chen and Jura have produceda plot of the asteroidal ring around zeta Leporis, with a comparison of the Sun'sasteroidal belt also displayed:

Chen and Jura propose this ring to be the precursor of eventual formation, bycollision of asteroidal bodies, of rocky planets analogous to those of the SolarSystem. These bodies, form from smaller particles (dust) condensed from the

gas-particle cloud associated with the forming star. Much of that dust can moveinward towards the star by a process called Poynting-Robertson drag. This iscaused by radiation from the parent star being absorbed and re-radiateddifferentially, leading to a Doppler effect (here, the energy of emission in thedirection of dust motion is at shorter wavelengths [more energetic] and thus byretro-action slows the particles) that promotes drift of the dust towards the star.

There is evidence in our own Solar System that dust is commonplace andwidespread. Astronomers can have their observations slightly diminished by thephenomenon called "Gegenschein" (German for "counter shine"). This has beenattributed to dust in intersolar space beyond Earth. Here is a photo of this effect:


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From the above, one necessary step in planetary formation is the development ofa protoplanetary disk around a suitable star (or a binary star pair; a significant

fraction of the planets found so far are [surprisingly] associated with binary oreven ternary stars, thus increasing the likelihood of planetary systems beingubiquitous since more than half of galactic stars are of the binary mode). Aninventory of detectable disks in nearby neighborhoods of the Milky Way founddisks around 236 stars; the instrument used was the Spitzer Space Telescopewhose Infrared sensors are adept at measuring hot gas and particles. As evidentin the graph below, most of these disks are associated with younger stars but thedistribution includes even older (>800 million years) stars:

The following is a generally accepted model (called "core accretion") forestablishing a planetary system: A nebula is subject to gravitational irregularitiesand other perturbations that cause free-fall collapse to numerous clots aroundwhich surrounding gases and particulates usually adopt a disklike form. Over


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time, the disk tends to organize in spiral arms of gas and matter, whichincreasingly become disorganized by clotting (discussed below). Consider thisgeneralized sequence:

Another example of these extrasolar planet-star formation gas/dust disksappears below; read the caption for details.

The influence of gravity, which builds up progressively as planetesimal clotsenlarge, is the prime driving force promoting both planet and star formation. Insome instances, shock waves from a Supernova can cause interstellar matter toinitiate collapse and compress into protostars and debris orbiting them. Matter isalso redistributed along magnetic field lines by magnetohydrodyamic processes.The main phases of planetary formation extend over about 10-20 x 106 years but


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it may require up to 108 years to progress from the early infall to the late T Tauristage of a protostar's development. While a particular clot is organizing, thematerials tend to redistribute such that Hydrogen and much of the lighterelements flow towards a growing center to accumulate in a gravitationallybalanced sphere, the star. Under one set of conditions, instabilities lead to a

double (binary) star pair. As protostars form, the rotating gases and dust particlescollect in a spinning disk around each center and eventually organize byaccretion into planets. The same process, with variants, works on single stars.The time frame for the above model suggests planets will appear within onehundred million years or less after the nebular gas and dust have begun tobehave as a unit in space.

If our Solar System is the norm, inner planets should be rocky, with thin or absentatmosphere (lost from insufficient gravitational ability to retain the gases or bybeing swept away by the solar wind). Outer planets should have rocky cores andbe 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 quiteclose to their parent stars places this assumption of size distribution with distanceinto question.

Alan Boss of the Carnegie Institution of Washington has argued that the outergas planets Uranus and Neptune have much less gas than would be expectedfrom conventional planetary system models. He claims that large quantities ofgas were driven away in the earlier history of these planets by UV radiation fromnearby stars in the local cluster. It is reasonable to expect that stellar windw, UVradiation, and other "forces" from neighboring stars might affect planetary historybut his hypothesis remains in dispute.

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

But for the present, astronomers continue to build and refine their models on themuch easier-to-make observations at the astronomically short distances withinthe 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 andcollapse of Hydrogen/Helium gases and associated matter (both solid andgaseous) consisting of other elements and compounds that once made up adiffuse (density ~ 1000 atoms/cc) nebula. Probably many stars were generated inthe timeframe of a few hundred million years from this particular "cloud".


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The protosun built up from centripetal, gravity-induced infall of nebularsubstances towards one of the concentration centers in the nebula. The bulk ofthe 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 - representingfragments of comets and broken protoplanets that are swept up onto Earth). Thismaterial, bound by gravity to the Sun but free to move inertially in encirclingorbits, remained distributed in the space making up the Solar System. Thissystem of particles rather rapidly organized into a disc-like shape whose presentradius is about 100 A.U. (about 9300 million miles). The disc rotated slowly(counterclockwise relative to a viewpoint above the north celestial pole [whichpasses through Polaris, the North Star]), its motion influenced by externalgravitational effects from nearby stars.

As this rotation got underway, and thereafter, the solar (stellar) 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 thanthe Sun that underwent various degrees of condensation. This field also expelsand guides this material into jets that carry matter out to great distances, as seenhere in this Hubble Space Telescope view of a jet ejecting from another star inour galaxy:

Both jets and irregular nebular patches (e.g., the Horsehead and Eagle nebulasshown above) contain not only gases but significant amounts of dust. The dust isvery small and consists of three types: 1) core-mantle elliptical particles, typically0.3 to 0.5 microns in long dimension, with a silicate interior coated by icy forms ofgases; 2) carbonaceous particles (~0.005 microns), and 3) open frothy clotscalled PAH dust (polycyclic aromatic hydrocarbons) (~0.002 microns). Shockwaves and radiation can strip off the ice mantle leaving grains that areincorporated into coalescing bodies that form the prototypes which accrete intothe planetesimals from which asteroids or planets then build up. Ultravioletradiation can modify the organics into more complex molecular forms. In thisway, organic molecules are introduced from space onto planetary surface and, ifconditions are right, can eventually serve as viable ingredients for the inceptionof living things (see below).

The possible role of shock waves in planetary formation is now the subject ofconsiderable study. Evidence for a shock wave that develops as material fallstowards a nearby protostar against its remaining gas/dust cloud has been


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observed at L1157, in which the present cloud is about 20 times the SolarSystem diameter. As this cloud proceeds to infall into the newborn star, itorganizes into a disk and produces shock waves that may clump dust together,as described in the next paragraph. Here is this cloud:

For the Solar System, shock waves and intense radiation acted on the dust suchthat some of it melted into tiny droplets which chill into chondrules. Thesespherical bodies then were caught up with remaining dust to produce theprimitive small solid bodies (fluffy "rockballs") that populated much of theheliosphere surrounding the Sun. Today we can analyze samples of theseaccreted bodies as meteorites which are small pieces of larger bodies torn loosefrom asteroids and put in orbits that eventually reach Earth. (You can reviewsome basic knowledge of meteorites by clicking to page 19-2.) Most infallenmeteorites are ordinary chondrites that, in thin section, appear much like this

sample from the Tieschitz meteorite:
http://rst.gsfc.nasa.gov/Sect19/Sect19_2.htmlhttp://rst.gsfc.nasa.gov/Sect19/Sect19_2.html


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The most primitive meteorites, called carbonaceous chondrites, are enriched inCarbon and contain water. Other meteorites are iron-rich (some with > 90%metallic Iron), and may have once been the interiors of planetary bodies sincedisrupted. 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 throughexpulsion by shock waves and solar wind.

Magnetically-driven eddies within the gas/dust cloud helped to impart additionalangular momentum to the larger condensed rotating objects beyond the sphericalSun (which possesses only 0.55% of this momentum even though it contains99.87% of the total mass of the system). These objects now remain in orbitsaround the Sun in positions that remain stable because of the counterbalancebetween centrifugal forces related to angular momentum and inward-directedgravitational pull from the Sun.

What are the currently favored models for planet formation? Two general models(mainly for formation of large planets with thick gas atmospheres) - Accretion andGas Collapse - are popular now, and both may have operated. These models areshown in these two panel sequences:


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Both models start with a gas/particulate cloud denser than its surroundings. Inmany instances, this cloud is a remnant of a supernova explosion. This is thecase for the Solar System nebula, as evidenced by the higher concentrations ofheavier elements - the normal end-product of the destruction of a star in whichthese elements have been synthesized.

For the Accretion model, as the formative process operated, local instabilities inthe nebula tied to the Sun caused the chondrule-laden rockballs within turbulentzones to cluster and further aggregate into objects ranging from meter-size up to

planetesimaldimensions (tens to a few hundred kilometers, typified [perhapscoincidentally] by asteroid proportions).


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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 repeatedlyfrom mutual collisions while larger ones survived by attracting most of the smallerones gravitationally, growing by accretion as new matter impacted on their

surfaces. Once started, "runaway" growth ensues so that many planetesimalscombine into bodies that eventually enlarge into fullblown planets. The bulk of thematter beyond the Sun was swept into the planets and their satellites, althoughsome remains in comets and cosmic dust. Mercury, and some Outer Planetsatellites are preserved remnants of this later stage in planetary growth, asindicated by their heavily cratered surfaces that were never destroyed bysubsequent processes such as erosion. In contrast, the Moon appears to havebuilt up by re-aggregation of debris hurled into space as ejecta from a giantimpact on Earth soon after our planet formed. Once collected into a sphere(which probably melted), the lunar surface continued to be bombarded with itsown 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 debrisreassembled, melted and formed the lunar sphere; at least some of its largerbasins are somewhat older.

The formation of the Moon by collision between two separate but large planetarybodies likely was not a unique event in Solar System history. During the earlierstages of accretion more planets than now exist probably formed. Their numberwas reduced by a collision which could have caused both bodies to be disrupted


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into particles of varying size in a "collision cloud" which then reassembled into asingle planet whose mass was approximately that of the two earlier bodies (somematter was lost to space). The asteroid belt between Mars and Jupiter may be anexample of collisional debris that failed to reorganize into what would have beenthe fifth rocky planet.

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

Analysis of Argon, Nitrogen, and other gases in Jupiter indicates their amountsare such that this Giant must have formed under very cold conditions; if furtherwork bears this out, Solar System scientists may adopt, as one plausibleexplanation, an origin of Jupiter (and perhaps the other Giants) at much greaterinitial distances from the Sun with these having since moved significantly closerthrough orbital contraction or decay. The Dwarf planet Pluto, the smallest and, attimes, 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 ofNeptune.

Theoreticians differ as to the exact methods and sequence in which the planetsaccumulated after the condensation and planetesimal phases. Timing is a criticalaspect of the formation history. One version - the equilibrium condensation model- considers condensation to happen early and quickly, in a few million years, withthe observed sunward zoning of higher temperature minerals and greaterdensities in the rocky inner planets both being consequences of the increasingtemperature profile inward across the solar nebula. Accretion was stretched outover 100 million years or so. The heterogeneous accretion model holds


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condensation and buildup of planetesimals to proceed simultaneously over a fewtens of millions of years. Neither model adequately explains the fact that bothhigh and low temperature minerals aggregate together in the inner planets toprovide materials capable of generating the atmospheric gases released fromthese 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 tothe growing protoplanets carried in along eccentric orbits from asteroidal andGiant planet regions. This material is then homogenized during the total meltingassumed for each inner planet early in its evolution. This melting is theconsequence of heat deposited from accretionary impacts, from gravitationalcontraction, and from release during radioactive decay. As cooling ensues,materials are redistributed during the general differentiation that carries heavymetals and compounds towards the center and allows light materials to "float"upwards towards the surface.

Less is known about the long-term evolutionary history of planets and theireventual demise (destruction). Extrapolating from our Solar System with its twomajor types of planets - Rocky and Gaseous - and the variety of surfaces onthem 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 manybelieve exist in the Universe. In the Solar System its complement of planets havesurvived essentially intact (possible exception: the asteroid belt) since the Sunitself organized some 4.5 to 5 billion years ago). The Sun is expected to burn outits fuel in another 5 billion years, when it expands rapidly into a Red Giant. Theoutward surge of its gaseous envelope should consume many - maybe all - of thenamed planets as well as other solar material. This is probably the usual

mechanism of most planetary destruction - consumption by Red Giant expansionor by Novae or Supernovae (see top ofpage 20-6). Another possibility:gravitational pull brings the planets into their parent stars. Generally, planetarysystems around massive stars, if indeed these do form, will be short-lived asthose stars themselves do not last billions of years (thus, such stars are not likelyto 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 forfostering life on any planets that may revolve about them, owing to their longerspans of existence.

There is a second form of dust around stars that has been produced aftertheywere formed and well along their trail of evolution. This is described in the April,2004 edition of Scientific American in an article by Davod Ardila entitled "TheHidden Members of Planetary Systems". (Ardila points out that not all stars withthis kind of dust necessarily have associated planets). The dust is of two types:1) micron-sized particles that are analogous to the dust in the solar inner planetbelt that gives rise to Zodaical light at sunset; 2) dust of a range of sizes thatexists further out (beyond Jupiter and the Kuiper belt for our system but for somestars the disk extends out to notably greater distances). The zodaical dust is


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produced by release from comets and grinding of asteroidal-sized bodies thatcollide and abrade over time. The larger particles tend to spiral into the parentstar, the smaller are pushed away from the star by radiation pressure. Over time,the amount of dust will diminish. But some of the dust may be incorporated inplanets within this circumstellar debris cloud, already formed or yet to form. The

temperatures associated with dust belts and clouds varies, so that telescopesensors will pick up measurable EM radiation at different wavelengths. One ofthe best examples of a huge dust disk is found around the star Beta Pictoris, 63light years from Earth. The disc extends out about 1100 A.U. (about a 460 billionkilometers in diameter). There is a suggestion of one or two planets within thedisk. The Visible light HST image below shows the symmetrical disk (lowerimage is colored to indicate density differences); the black center is due toscreening out the star itself using a coronagraph accessory on the telescope.

Recently, another plausible model for the origin of planets has been reported byDr. Jeff Hester and colleagues at Arizona State University. The figure below isrelevant:


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As with the other models, clouds of Hydrogen gas and silicate dust are needed.Shown here is the Trifid nebula. Within it are now being formed a range ofembryonic stars which include besides those of Earth-size, more massive starsthat explode. The astronomers studying this "nursery" of stars point out thatmassive stars can produce the isotope of Iron Fe60 whose half life is about 1.5million years. Its stable daughter product Ni60 has been found in meteorites,whose parent sources presumably formed along with the stars like the Sun. This

implies that the Sun was born out of a gas-debris cloud that had been enrichedby radioactive Fe60 from one or more exploding (Supernovae) stars in itsneighborhood. These stars were much more massive than the Sun. The nebulaelike Trifid, e.g., Eagle and Orion looked at earlier in this Section, are enriched inHII (doubly ionized Hydrogen). As shock waves produce ionization of theHydrogen, YSOs (Young Stellar Objects) will form at various sizes. In theongoing process of star formation, EGGs (Evaporating Gaseous Globules)develop and evolve into associated propylids. The propylids later shed some oftheir material leaving stars on the Main Sequence of sizes similar to the Sun.

Little has been said about life on planets on this page - this will now be reviewed

on the next page, 20-12. However, a recent article by Beer, King, Livio, andPringle in the Monthly Notices of the Royal Astronomical Society has put forth anargument that life must be rare. This is deduced from the observation of the ~200planetary systems so far discovered. Nearly all of these apparently have onlyGiant gas ball planets that are much closer to their central star that Jupiterthrough Neptune in our Solar System. If this turns out to be the case then rockyplanets are scarce - those closer to their stars are prone to having much of theirgas envelopes blown away by stellar wind and high temperatures proximate to


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the star. The flaw (and saving grace, for those who want life discoveredelsewhere) in the argument is that none of the current methods of planetdetection are capable of finding smaller planets but are biased towards locatingbig gas balls.

While most planets are believed to have rocky cores, stellar wind and explosionprocesses tend to blow off gases from smaller inner planets. The essentialconditions needed for organic molecules to develop are water (preferably in liquidform, but life in steam or ice is believed possible), an appropriate temperaturerange, some semblance of a favorable atmosphere (but anaerobic or Oxygen-free environments on Earth can contain life), and the appropriate ingredients (C,H, O, N, and P; an Si life system, instead of C, is theoretically possible). We shallsee on page 20-12a that the Drake equation provides a mathematical means ofestimating the opportunity for organic molecules to form on planets in somefraction of the star systems. Likewise, the likelihood for life to occur on planetsseems to follow the Goldilocks dictum (page 20-11a): "not too hot, not too cold,

just right".

Having postulated that planets are probably rather commonplace in the Universe,let us study on the next 3 pages the types of and conditions for life having formedon Earth by processes becoming ever better understood. After that we will returnto the topic of planetary systems, now within the framework of life as we know iton Earth and suspect it on other planets outside the Solar System.

Primary Author: Nicholas M. Short, Sr.

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Models for the Origin of Planetary Systems

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