This "crash course" on Cosmology culminates with the following synopsis of 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 start of life on Earth and, by extrapolation, the presence of other intellectual 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 the Planets", 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 the blue RealVideo link.
As this page unfolds, one dominant idea regarding how planets form around stars will be developed. This idea is summarized in the diagram below so as to serve 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:
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 besides Hydrogen that have rocky cores or they are dominantly rock with many elements making 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 is distinguished by the presence of millions of compounds of carbon - the organic molecules 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 a drawn sphere the size of the Earth to allow comparison:
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 (generally the term "exoplanet" is now in common use) was found in 1993 by Penn 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 the Milky 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 been detected in at least 88 other stellar (~solar) systems; as of October 2010 the latest count was about 400 individual planets associated with these and other systems -most being Giants and of low density. (Note: A few astronomers have disputed some of these observations.) There is a growing "feeling" (but not yet a consensus) that planets are the norm around many - perhaps most - stars, at least those of masses similar to the Sun. This is the basis of an argument with profound implications: Statistically, there seems to be enough star/planet occurrences in the Milky Way, and by inference other galaxies, for a reasonable likelihood of some having hosted the evolution of life, and with a 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 3 planets (a second triple planet system was recently discovered); 8 other stars elsewhere have at least 2 planets. The Upsilon Andromedae system, diagrammed below, consists of Giant (probably gas-ball) planets (much smaller ones presently are very hard to detect), all within orbits whose distances from its star are comparable to that of the four small terrestrial planets in our Solar System:
In August 2004, a star, mu Arae d, just 50 light years away, was shown to have three planets, two larger gaseous ones and a third having a size, mass (14 Earth masses), and orbital characteristics (rapid revolution around its parent) that indicates 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 masses respectively.
Much attention was given in late September 2010 to the announcement that at least one of the 6 planets orbiting the Red Dwarf Gleise 581, about 20 light years from Earth, had conditions that could favor inception of some form(s) of life. Read about Gleise 581g at Space.com.
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 low luminosities. Nearly all of the 200+ planets claimed to have been detected are deduced to exist from their interactions with their parent star, involving perceptible 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 their star (at distances less than Mercury's orbit; one is thought to be as close as 5 million miles from its star and rotates rapidly around its parent body). 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 and do not significantly distort the motions of their host star.
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. (One recent estimate concludes that as many as six billion giant planets exist within the Milky Way galaxy.) 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. (Some cosmologists are 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 endeavors near the 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 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 prime 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 in this way.
The second method, astrometry, also relies on star 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 in measuring shift results from increasd resolution achieved by positioning two telescopes physically some distance apart but keeping them 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 celestial 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) capable of observing wobbles is significantly improved by operating one or more telescopes in space, above the distorting atmosphere. Resolution is also improved by using a pair of telescopes mounted on a single boom but separated by many meters, combining the image signals using the interferometric method. SIM (Space Interferometry Mission) is a NASA probe slated to fly sometime after 2015. Its two telescopes will be 10 meters apart. 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. Here is the spacecraft: A study of potential for success of the SIM approach comes up with this estimate of planets expected to be detected:
Interferometry, discussed in the Section on Radar, is an important technique for enhancing resolution. Its principles will not be discussed on this page but the reader is referred to these two websites for some insight: Wikipedia and Absolute Astronomy.
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 is a generalized plot of the type of signal or signature that is produced by the transit method:
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:
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 satellites flying in formation and spaced to utilize the principle of interferometry to improve resolution and to detect 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 finding planets of various sizes and locations around their parent stars. Thus, there is a tendency to find planets located around stars smaller than the Sun, since these planets (usually large) have enough mass to induce noticeable wobble in the less massive parent stars. Likewise, very large planets bring about increased wobble; the wobble also increases for planets close to the parent. Thus, so far Red Dwarf stars (which are quite abundant) have been the types associated with a disproportionately higher number of planet discoveries, since these stars are prone to greater stellar-wobble.
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).
One of the most important missions since the Hubble Space Telescope is Kepler observatory (named after the German astronomer who formulated the laws of planetary motion). Using the transit method, Kepler will systematically search a small segment of the Milky Way in search of Earth-sized planets. Kepler was launched 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't have to contend with the reflected light of the moon and Earth.This overview illustration symbolizes the mission.
The spacecraft will occupy what is termed an "Earth-trailing orbit". Thus it will circle the Sun at a revolution cycle slightly greater than the Earth itself. This diagram amplifies this
Here is a sketch of the Kepler spacecraft with its main components and a photo of the spacecraft in its fabrication facility:
The Kepler telescope has a 95 megapixel (detector) camera that can integrate brightness measurements to single out variations as small as 0.01% (capable of detecting Earth mass-sized planets). These are mounted in 42 panels, shown below, that help the instrument function as a photometer. Using its 0.9 m mirror telescope, up to 100,000 stars nearby in the Milky Way can be conveniently monitored over time and individuals with multiple planets should reveal the relative number of stars that possess planetary systems. The expectation is that 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 Cygnus constellation. Kepler will seek out stars that reside between 10 and 3000 light years from Earth. These diagrams are appropriate:
The next paragraphs, italicized, have been taken off the Internet from several sites that discuss details of the Kepler mission.
A transit occurs when a planet crosses in front of its host star as viewed by an observer. These transits very slightly dim the brightness of a star which allow for the detection of extrasolar planets. This change in brightness is very difficult to detect for terrestrial-sized planets, such as Earth, because they only dim their host star by 100 parts per million (0.01%), over a passage time lasting only 2 to 16 hours. That's equivalent to measuring from several miles away the change in brightness caused by a flea crawling across a car's headlight. Adding to the difficulty 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 will have planets properly aligned so that they pass between the star and Kepler's camera. 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 same planet 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 one Earth year, Kepler will need to observe any transits discovered amongst the sample 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 about equal to the size of a human hand held at arm's length or about equal in area to 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 of sand 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 star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or 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-years to 3,000 light-years away. The first planets to be discovered in the coming months 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 of 300 or more Earth days after the first detection of a transit; this could be longer if planet orbits further out).
The first successful images from Kepler were obtained in April; here is a typical scene:
One of the first successful observations was of an occultation of star HAT-P-7 by a planet calculated to be a bit larger than Earth. The ground-based measurement, shown below, was notably noisier than the one by Kepler beneath it;
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 neighboring planetary systems, containing planets that might be capable of sustaining life. When compared to all the stars in the universe, even one discovery amongst the relatively small sample space of 100,000 stars will be significant enough for us to rethink our meaning and place in the universe. Proof that there are planets elsewhere whose size and characteristics are within the range of Earth, and therefore may be favorable for life will have profound philosophical and theological impact on mankind. (Of course, if none are discovered this too would be quite meaningful as it would favor the argument that "we" may indeed be alone - although this does not rule out possiblities elsewhere in the Universe.)
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 has already 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 diameter of 1.7 times the Earth. Here is a plot of data received that illustrates how transit data appear:
In September, 2004 a reasonable claim has been made by astronomers using the European Space Agency telescope in Chile of having seen the first actual planet. Look at this infrared image of their observation:
The star, 2M1207, just 50 light years away, is a brown dwarf (too small to initiate Hydrogen fusion). At a distance twice that of Neptune from the Sun is a reddish object five times the size of Jupiter but is cool (less than 2000░ C) and has a spectrum that includes heavier elements. This object is likely a planet but final confirmation 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 using the European Southern Observatory. They have obtained a picture of GC Lupi with 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 of Jupiter and has a surface temperature between 3 and 4 thousand degrees Celsius. Here is a telescopic view:
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 body circled near "11 o'clock" is judged to be a large planet about 8 times the mass of Jupiter:
The case for each of these being actual planets is still being debated. At least one may be a brown dwarf companion star. In November of 2008 two papers in the journal Science offer evidence supporting planets actually observed. The bright star Formalhaut, 25 l.y. from Earth, was imaged by HST: there is a dot about 119 A.U. from the star that appears to be a Giant planet; note the broad ring of luminous dust and debris (asteroidal?) around the star (masked out).
The second paper presents imagery showing three planets around star HR8799, 130 l.y. away. The planets are comparitively young (estimated age: 600 million years), as they are still quite hot (accounts for their redness): Read the caption for details.
Statistically speaking, the number of such planetary systems in the Universe 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. So far, however, when the number of stars that have been studied using any of the above detection techniques are used as a base line, only about 5% have yielded evidence of associated planets; this number is probably a minimum for determining the actual percentage that do have planets since smaller ones cannot yet been recognized as present.
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 200 billion stars they calculate to be the total population of the M.W, about 10% or 20 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 exoplanet systems probably show a wide range of individual types. Our Solar System is but one of many combinations of small-large, rocky-icy-gaseous planets. The results mentioned above are biased towards larger planets and give no real indication of the actual number in their systems. Some systems however may be almost like a binary star system in which the second or more planets approach brown dwarf mass but still too small to initiate nuclear burning.
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 forms of primitive life did form on it or associated satellites, those would have perished because of harsh conditions that 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.
A group lead by Sean Raymond of the University of Colorado has recently published results of a planet formation-distribution model that simulates (in the computer) conditions likely to produce planetary systems similar to those now being 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 orbits of 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 for life. The Colorado researchers conclude that at least one earthlike planet develops in about a third of the planetary systems that are produced by the model (which can generate different variations by changing key parameters).
In nearly all the planet discoveries, the orbital pathways (around ecliptic zones) of the planet(s) in a system coincide well with any residual planetary debris disks that were observed. This supports the model described below in which planetary formation takes place in the disk that develops around the parent star as it contracts 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 techniques can only detect larger planets. What still seems unusual is the indication of many big planets orbiting much closer to their parent (relative to the Jupiter/Saturn/Uranus/Neptune distances from the Sun). Two ideas have been postulated for this: 1) these planets were once placed much farther from their parent and have been drawn closer by the star's gravity (and thus are near their demise); and 2) the protoplanetary disks that now show these close-in stars may have been smaller relative to the Solar System dimensions, and hence the stars actually 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 recently reported study (July 2007) has pointed up a paradox: Giant planets that are more distant than a few Astronomical Units are rare in other planetary systems (in the study none were found around stars that should have such systems). Unlike our Solar System, giant planets so far detected are nearly all relatively close to their parent stars. The reason for this is presently uncertain.
On April 24, 2006 came the announcement that the first planet similar to Earth in size 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 diameter is 1.5 times that of Earth. It lies only about 11 million km (7 million miles) from its parent 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 to always 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). This range would allow liquid water to exist at the surface. While much more needs to be learned about Gliese 581c, it seems to have passed the "Goldilocks test" - conditions favoring life may indeed exist there. More about this planet is available at this Wikepedia web site.
Another Red Dwarf star, Gliese 436, has a single detected planet about the size of Neptune. Gliese 436b's surface temperature is ~310 ░ C. Its estimated density suggests water as the main constituent (hydrogen-rich atmosphere). Because of high interior pressures, some of the water is ice, but at temperatures approaching 100░ C, possible because its freezing temperature is elevated by the greater pressures.
While astronomers involved in planetary studies exercise caution about conclusions that specify the number of earthlike planets to be expected from various estimation procedures, 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 the 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 established 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, 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 to comprise the main body of the planet. 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 (Supernovae) large enough to synthesize Silicon, Oxygen, Iron and other heavier elements. A recent observation strongly supports this:
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 (particulate 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:
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:
Below are two views of nebular materials associated with the famed Eagle nebula (page 20-3): Top = full display of the M16 (Eagle) nebula (note the dark dust areas; the white dots are stars lying outside this nebula); 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):
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):
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):
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 materials 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.
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 young nebula in the Orion group); look like this in closer views:
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
Development of planetary cores, from which full larger planets then form, is a race against time. Examine this model:
The major threat to protoplanet formation is the stellar (solar) wind coming from the parent star. UV radiation is also able to break apart the smaller particles. Wind and UV radiation are capable of pulling apart small particles. These particles 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 again bump with others. By the stage in which some particle conglomerations have reached baseball size, they can resist stellar wind forces and survive to grow ever larger through collisions.
Thus, as the gas and dust cloud forms around a growing star, particles of solids begin to clump and some survive disrupting actions. However, much of the gas and dust may be pushed continuously away by the wind and radiation, so that the amount of material available to form planets generally diminishes over time. 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.