Most galaxies began during the first half of the Universe and contained a large number of massive stars that formed early in galactic histories. These galaxies have continuously been evolving through the eons as their stars synthesized elements (see page 20-7) and then dispersed them by the Supernova process. New, mostly Main Sequence stars, chemically enriched with elements of higher atomic number, have continued to form well into younger times up through the present. The smaller stars have lasted much longer and are probably the preferred sizes suited to planetary formation and survival. Even today stars are developing from the gases contained in remaining nebular material, so new planets could still be forming.
A telescope observation, reported in April, 1998, records the sighting (through the Keck II telescope on Mauna Kea, Hawaii) of what is interpreted to be another "solar" system around star HR 4796 (about 220 light years away). This image (at a lower resolution in which individual pixels stand out), taken in the IR, shows this central star (yellow white) surrounded by a lenticular (in an oblique view), flattened disk of gases and solid matter (glowing hot [reds] in the Infrared):
The diameter of the lens is about 200 A.U. No evidence of individual planets can be made out but the discoverers judge this feature, which has caused quite a stir of interest, to be an emerging planetary system in a "young adult" stage of development. It will certainly be a target for more detailed HST observations.
More recently, the Hubble Space Telescope has imaged star HR4796A, in our galaxy, which shows both a disk and irregular dust and gas clouds. This disk is interpreted to be in a more advanced (mature) stage of development than the protodisks shown above. Although not discernible, there may be planets already in the evolving gas/dust cloud which is made visible by the star's light.
This next Spitzer Space Telescope image shows HD6105, a collection of gas and dust that appears to be forming planets. Its central star has been blacked out in this image.
Another Hubble achievement is the imaging of a prominent disk around Beta Pictoris (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.
This next image of Beta Pictoris, made by a European Southern Observatory IR telescope, 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 its off-center star, is analogous to the Kuiper Belt of debris around our Sun. It appears to consist of icy dust. Time-motion studies indicates that there may already 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 the Fomalhaut disk, the first with the Spitzer Space Telescope, the second made with IRAS, and the third through the Hubble Space Telescope, appear here:
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 star light off myriads of small particles; the star, blocked out, is a Red Dwarf. On the right 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 of spectral bands used to create the image. This star-disk system may be as young as 250 million years since its protostar began to burn.
Theory indicates that, in the earlier stages of planetary formation, some number of broad rings should develop at various distances around the central star. One or more of these would appear as torus like glowing collections of dust and gases. At least two stars with this feature were imaged by HST and reported in January of 1999. Here is one of the star-ring systems:
Visible is a bright ring at some considerable distance out from the tiny parent star (white dot) and a more diffuse, darker mass extending beyond, both features occupying a flattened disc. In this instance, there are no rings close in (analogous to the regions occupied by the inner planets of the Solar System). The white circle is added by the astronomers to mark the boundary within which no visible planetary disk matter has been detected; the broad black cross (X) is an optical artifact. This star is about 350 light years from Earth.
A recent image, made from data detected at 1.3 mm by a French radio telescope, may have caught the formation of two large clots of matter likely to eventually contract into giant planets. These occur in the ring around the central star Vega, 25 light years away (in the Constellation Lyra). Here is an image based on observations made by D. Wilner and D. Aguilar of Harvard's Smithsonian Center for Astrophysics. (Note: this image has been enhanced artificially as an artist's rendition.)
C. Chen and M. Jura, Univ. of California-Berkeley have detected (monitoring Infrared radiation through the Keck telescope on Mauna Kea) a ring or disk of dust which seems to contain asteroid-like bodies around zeta Leporis, a star some 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 bodies around stars. Although imagery of the star, HR 1998, does not directly reveal these bodies, their presence is inferred from their average temperature of 350°K (77°C or 170°F). From the data they accumulated, Chen and Jura have produced a plot of the asteroidal ring around zeta Leporis, with a comparison of the Sun's asteroidal belt also displayed:
Chen and Jura propose this ring to be the precursor of eventual formation, by collision of asteroidal bodies, of rocky planets analogous to those of the Solar System. These bodies, form from smaller particles (dust) condensed from the gas-particle cloud associated with the forming star. Much of that dust can move inward towards the star by a process called Poynting-Robertson drag. This is caused by radiation from the parent star being absorbed and re-radiated differentially, leading to a Doppler effect (here, the energy of emission in the direction of dust motion is at shorter wavelengths [more energetic] and thus by retro-action slows the particles) that promotes drift of the dust towards the star.
There is evidence in our own Solar System that dust is commonplace and widespread. Astronomers can have their observations slightly diminished by the phenomenon called "Gegenschein" (German for "counter shine"). This has been attributed to dust in intersolar space beyond Earth. Here is a photo of this effect:
From the above, one necessary step in planetary formation is the development of a 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 or even ternary stars, thus increasing the likelihood of planetary systems being ubiquitous since more than half of galactic stars are of the binary mode). An inventory of detectable disks in nearby neighborhoods of the Milky Way found disks around 236 stars; the instrument used was the Spitzer Space Telescope whose Infrared sensors are adept at measuring hot gas and particles. As evident in the graph below, most of these disks are associated with younger stars but the distribution includes even older (>800 million years) stars:
The following is a generally accepted model (called "core accretion") for establishing a planetary system: A nebula is subject to gravitational irregularities and other perturbations that cause free-fall collapse to numerous clots around which surrounding gases and particulates usually adopt a disklike form. Over time, the disk tends to organize in spiral arms of gas and matter, which increasingly become disorganized by clotting (discussed below). Consider this generalized sequence:
Another example of these extrasolar planet-star formation gas/dust disks appears below; read the caption for details.
The influence of gravity, which builds up progressively as planetesimal clots enlarge, is the prime driving force promoting both planet and star formation. In some instances, shock waves from a Supernova can cause interstellar matter to initiate collapse and compress into protostars and debris orbiting them. Matter is also redistributed along magnetic field lines by magnetohydrodyamic processes. The main phases of planetary formation extend over about 10-20 x 106 years but it may require up to 108 years to progress from the early infall to the late T Tauri stage of a protostar's development. While a particular clot is organizing, the materials tend to redistribute such that Hydrogen and much of the lighter elements flow towards a growing center to accumulate in a gravitationally balanced sphere, the star. Under one set of conditions, instabilities lead to a double (binary) star pair. As protostars form, the rotating gases and dust particles collect in a spinning disk around each center and eventually organize by accretion into planets. The same process, with variants, works on single stars. The time frame for the above model suggests planets will appear within one hundred million years or less after the nebular gas and dust have begun to behave as a unit in space.
If our Solar System is the norm, inner planets should be rocky, with thin or absent atmosphere (lost from insufficient gravitational ability to retain the gases or by being swept away by the solar wind). Outer planets should have rocky cores and be less susceptible to loss of gases, so that their increased mass serves to gather in still more gases. However, the discovery that giant planets can lie quite close to their parent stars places this assumption of size distribution with distance into question.
Alan Boss of the Carnegie Institution of Washington has argued that the outer gas planets Uranus and Neptune have much less gas than would be expected from conventional planetary system models. He claims that large quantities of gas were driven away in the earlier history of these planets by UV radiation from nearby stars in the local cluster. It is reasonable to expect that stellar windw, UV radiation, and other "forces" from neighboring stars might affect planetary history but his hypothesis remains in dispute.
Two recent hypotheses are adding new twists to the above concepts. First, in addition to or in place of core accretion, another mechanism called "disc instability" may play an important, perhaps key role, in planetary inception. This is related to gravitational irregularities that can cause rather rapid accumulation of materials in the proto-planetary disc. Earlier-formed planets can contribute by setting up further instabilities. A second idea holds that planets can move inward or even outward in a form of migration or "wandering" so that their orbits change both in relative distance to their parent star(s) and in eccentricity.
But for the present, astronomers continue to build and refine their models on the much easier-to-make observations at the astronomically short distances within the Solar System. Like other stars, the Sun (whose diameter is 1,392,000 km [870,000 miles]) is an end-product of gravitationally-driven condensation and collapse of Hydrogen/Helium gases and associated matter (both solid and gaseous) consisting of other elements and compounds that once made up a diffuse (density ~ 1000 atoms/cc) nebula. Probably many stars were generated in the timeframe of a few hundred million years from this particular "cloud".
The protosun built up from centripetal, gravity-induced infall of nebular substances towards one of the concentration centers in the nebula. The bulk of the gases enters the resulting star itself along with much of the other materials, leaving an enveloping residue of matter enriched in Si, C, O (and H), N, Ca, Mg, Fe, Ti, Al, Na, K, and S (most organized into compounds, particularly silicates, that can be sampled by recovery of iron and stony meteorites - representing fragments of comets and broken protoplanets that are swept up onto Earth). This material, bound by gravity to the Sun but free to move inertially in encircling orbits, remained distributed in the space making up the Solar System. This system of particles rather rapidly organized into a disc-like shape whose present radius is about 100 A.U. (about 9300 million miles). The disc rotated slowly (counterclockwise relative to a viewpoint above the north celestial pole [which passes through Polaris, the North Star]), its motion influenced by external gravitational effects from nearby stars.
As this rotation got underway, and thereafter, the 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 than the Sun that underwent various degrees of condensation. This field also expels and guides this material into jets that carry matter out to great distances, as seen here in this Hubble Space Telescope view of a jet ejecting from another star in our galaxy:
Both jets and irregular nebular patches (e.g., the Horsehead and Eagle nebulas shown above) contain not only gases but significant amounts of dust. The dust is very small and consists of three types: 1) core-mantle elliptical particles, typically 0.3 to 0.5 microns in long dimension, with a silicate interior coated by icy forms of gases; 2) carbonaceous particles (~0.005 microns), and 3) open frothy clots called PAH dust (polycyclic aromatic hydrocarbons) (~0.002 microns). Shock waves and radiation can strip off the ice mantle leaving grains that are incorporated into coalescing bodies that form the prototypes which accrete into the planetesimals from which asteroids or planets then build up. Ultraviolet radiation can modify the organics into more complex molecular forms. In this way, organic molecules are introduced from space onto planetary surface and, if conditions are right, can eventually serve as viable ingredients for the inception of living things (see below).
The possible role of shock waves in planetary formation is now the subject of considerable study. Evidence for a shock wave that develops as material falls towards a nearby protostar against its remaining gas/dust cloud has been observed at L1157, in which the present cloud is about 20 times the Solar System diameter. As this cloud proceeds to infall into the newborn star, it organizes 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 such that some of it melted into tiny droplets which chill into chondrules. These spherical bodies then were caught up with remaining dust to produce the primitive small solid bodies (fluffy "rockballs") that populated much of the heliosphere surrounding the Sun. Today we can analyze samples of these accreted bodies as meteorites which are small pieces of larger bodies torn loose from asteroids and put in orbits that eventually reach Earth. (You can review some basic knowledge of meteorites by clicking to page 19-2.) Most infallen meteorites are ordinary chondrites that, in thin section, appear much like this sample from the Tieschitz meteorite:
The most primitive meteorites, called carbonaceous chondrites, are enriched in Carbon and contain water. Other meteorites are iron-rich (some with > 90% metallic Iron), and may have once been the interiors of planetary bodies since disrupted. The chondrules themselves generally show a very limited size range, suggesting that ones larger than these fell back into the Sun through gravitational pull whereas smaller ones were swept away into interstellar space through expulsion by shock waves and solar wind.
Magnetically-driven eddies within the gas/dust cloud helped to impart additional angular momentum to the larger condensed rotating objects beyond the spherical Sun (which possesses only 0.55% of this momentum even though it contains 99.87% of the total mass of the system). These objects now remain in orbits around the Sun in positions that remain stable because of the counterbalance between centrifugal forces related to angular momentum and inward-directed gravitational pull from the Sun.
What are the currently favored models for planet formation? Two general models (mainly for formation of large planets with thick gas atmospheres) - Accretion and Gas Collapse - are popular now, and both may have operated. These models are shown in these two panel sequences:
Both models start with a gas/particulate cloud denser than its surroundings. In many instances, this cloud is a remnant of a supernova explosion. This is the case for the Solar System nebula, as evidenced by the higher concentrations of heavier elements - the normal end-product of the destruction of a star in which these elements have been synthesized.
For the Accretion model, as the formative process operated, local instabilities in the nebula tied to the Sun caused the chondrule-laden rockballs within turbulent zones to cluster and further aggregate into objects ranging from meter-size up to planetesimal dimensions (tens to a few hundred kilometers, typified [perhaps coincidentally] by asteroid proportions).
From J. Silk, The Big Bang, 2nd Ed., © 1989. Reproduced by permission of W.H. Freeman Co., New York
During this growth stage, smaller planetesimals tended to break apart repeatedly from mutual collisions while larger ones survived by attracting most of the smaller ones gravitationally, growing by accretion as new matter impacted on their surfaces. Once started, "runaway" growth ensues so that many planetesimals combine into bodies that eventually enlarge into fullblown planets. The bulk of the matter beyond the Sun was swept into the planets and their satellites, although some remains in comets and cosmic dust. Mercury, and some Outer Planet satellites are preserved remnants of this later stage in planetary growth, as indicated by their heavily cratered surfaces that were never destroyed by subsequent processes such as erosion. In contrast, the Moon appears to have built up by re-aggregation of debris hurled into space as ejecta from a giant impact on Earth soon after our planet formed. Once collected into a sphere (which probably melted), the lunar surface continued to be bombarded with its own remnants as well as asteroids and other space debris. Its oldest craters are hundred of millions of years younger than the time at which the debris reassembled, melted and formed the lunar sphere; at least some of its larger basins are somewhat older.