The formation of the Moon by collision between two separate but large planetary bodies likely was not a unique event in Solar System history. During the earlier stages of accretion more planets than now exist probably formed. Their number was reduced by a collision which could have caused both bodies to be disrupted into particles of varying size in a "collision cloud" which then reassembled into a single planet whose mass was approximately that of the two earlier bodies (some matter was lost to space). The asteroid belt between Mars and Jupiter may be an example of collisional debris that failed to reorganize into what would have been the fifth rocky planet.
As mentioned above, in our Solar System, the four inner planets (the Terrestrial Group) are largely rocky (silicates, oxides, and some free iron; three with atmospheres) and the outer four (Giant Group) are mostly gases with possible rock cores. These Giants developed large enough cores to attract and capture significant fractions of the nebular gases dispersed in the accretion disk.
Analysis of Argon, Nitrogen, and other gases in Jupiter indicates their amounts are such that this Giant must have formed under very cold conditions; if further work bears this out, Solar System scientists may adopt, as one plausible explanation, an origin of Jupiter (and perhaps the other Giants) at much greater initial distances from the Sun with these having since moved significantly closer through orbital contraction or decay. The Dwarf planet Pluto, the smallest and, at times, farthest out (its elliptical orbit periodically brings it within that of Neptune), appears to be made up of rock and ice and may be a captured satellite of Neptune.
Theoreticians differ as to the exact methods and sequence in which the planets accumulated after the condensation and planetesimal phases. Timing is a critical aspect of the formation history. One version - the equilibrium condensation model - considers condensation to happen early and quickly, in a few million years, with the observed sunward zoning of higher temperature minerals and greater densities in the rocky inner planets both being consequences of the increasing temperature profile inward across the solar nebula. Accretion was stretched out over 100 million years or so. The heterogeneous accretion model holds condensation and buildup of planetesimals to proceed simultaneously over a few tens of millions of years. Neither model adequately explains the fact that both high and low temperature minerals aggregate together in the inner planets to provide materials capable of generating the atmospheric gases released from these planets. The models also do not fully account for the strong preferential concentration of Iron and other siderophile ("iron-loving") elements in the inner, terrestrial planets. One solution is to add (by impact) low temperature material to the growing protoplanets carried in along eccentric orbits from asteroidal and Giant planet regions. This material is then homogenized during the total melting assumed for each inner planet early in its evolution. This melting is the consequence of heat deposited from accretionary impacts, from gravitational contraction, and from release during radioactive decay. As cooling ensues, materials are redistributed during the general differentiation that carries heavy metals and compounds towards the center and allows light materials to "float" upwards towards the surface.
Less is known about the long-term evolutionary history of planets and their eventual demise (destruction). Extrapolating from our Solar System with its two major types of planets - Rocky and Gaseous - and the variety of surfaces on them and their satellites, it is evident that a great range of sizes, compositions, and surficial states can be expected among the millions of planets that many believe exist in the Universe. In the Solar System its complement of planets have survived essentially intact (possible exception: the asteroid belt) since the Sun itself organized some 4.5 to 5 billion years ago). The Sun is expected to burn out its fuel in another 5 billion years, when it expands rapidly into a Red Giant. The outward surge of its gaseous envelope should consume many - maybe all - of the named planets as well as other solar material. This is probably the usual mechanism of most planetary destruction - consumption by Red Giant expansion or by Novae or Supernovae (see top of page 20-6). Another possibility: gravitational pull brings the planets into their parent stars. Generally, planetary systems around massive stars, if indeed these do form, will be short-lived as those stars themselves do not last billions of years (thus, such stars are not likely to harbor life since not enough time elapses to permit development by evolution [see below]). Smaller stars, such as G types, are much more favorable bodies for fostering life on any planets that may revolve about them, owing to their longer spans of existence.
There is a second form of dust around stars that has been produced after they were 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 "The Hidden Members of Planetary Systems". (Ardila points out that not all stars with this 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 planet belt that gives rise to Zodaical light at sunset; 2) dust of a range of sizes that exists further out (beyond Jupiter and the Kuiper belt for our system but for some stars the disk extends out to notably greater distances). The zodaical dust is produced by release from comets and grinding of asteroidal-sized bodies that collide and abrade over time. The larger particles tend to spiral into the parent star, 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 in planets within this circumstellar debris cloud, already formed or yet to form. The temperatures associated with dust belts and clouds varies, so that telescope sensors will pick up measurable EM radiation at different wavelengths. One of the best examples of a huge dust disk is found around the star Beta Pictoris, 63 light years from Earth. The disc extends out about 1100 A.U. (about a 460 billion kilometers in diameter). There is a suggestion of one or two planets within the disk. The Visible light HST image below shows the symmetrical disk (lower image is colored to indicate density differences); the black center is due to screening out the star itself using a coronagraph accessory on the telescope.
Recently, another plausible model for the origin of planets has been reported by Dr. Jeff Hester and colleagues at Arizona State University. The figure below is relevant:
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 of embryonic stars which include besides those of Earth-size, more massive stars that explode. The astronomers studying this "nursery" of stars point out that massive stars can produce the isotope of Iron Fe60 whose half life is about 1.5 million 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 enriched by radioactive Fe60 from one or more exploding (Supernovae) stars in its neighborhood. These stars were much more massive than the Sun. The nebulae like Trifid, e.g., Eagle and Orion looked at earlier in this Section, are enriched in HII (doubly ionized Hydrogen). As shock waves produce ionization of the Hydrogen, YSOs (Young Stellar Objects) will form at various sizes. In the ongoing process of star formation, EGGs (Evaporating Gaseous Globules) develop and evolve into associated propylids. The propylids later shed some of their 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, and Pringle in the Monthly Notices of the Royal Astronomical Society has put forth an argument that life must be rare. This is deduced from the observation of the ~200 planetary systems so far discovered. Nearly all of these apparently have only Giant gas ball planets that are much closer to their central star that Jupiter through Neptune in our Solar System. If this turns out to be the case then rocky planets are scarce - those closer to their stars are prone to having much of their gas envelopes blown away by stellar wind and high temperatures proximate to the star. The flaw (and saving grace, for those who want life discovered elsewhere) in the argument is that none of the current methods of planet detection are capable of finding smaller planets but are biased towards locating big gas balls.
While most planets are believed to have rocky cores, stellar wind and explosion processes tend to blow off gases from smaller inner planets. The essential conditions needed for organic molecules to develop are water (preferably in liquid form, but life in steam or ice is believed possible), an appropriate temperature range, 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 shall see on page 20-12a that the Drake equation provides a mathematical means of estimating the opportunity for organic molecules to form on planets in some fraction of the star systems. Likewise, the likelihood for life to occur on planets seems 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 formed on Earth by processes becoming ever better understood. After that we will return to the topic of planetary systems, now within the framework of life as we know it on Earth and suspect it on other planets outside the Solar System.