Whereas there is almost no confusion about the vast numbers of proper stars, there is an apparent disagreement about planetary systems around them. This "debate" evaporates into a near uniformity of opinion once it is unraveled, however. Planet theorists and observational astronomers are arguing about whether clear evidence exists as yet for an extra-solar planetary system, leading some listeners, perhaps, to conclude that scientists think that planets are rare. Actually, astronomers are nearly universal in their belief that although planets are extremely difficult to detect with our current tools, they are commonplace, almost ubiquitous in the galactic disk. David Black, one of the most eminent planetary researchers, has stated that "Current planetary theories suggest that planets should be the rule rather that then exception" Black 1987. In fact he asserts that if, once our technology improves, we cannot find large numbers of other planetary systems, we will have to revise our whole theory of star formation.
Confidence in numerous planetary systems is based upon more than pure theory. Several lines of research have indicated the overwhelming likelihood of such systems. They include:
- Since, in terms of the mechanisms of formation, stars and planets differ from one another only in the amount of mass originally involved in their condensation, the formation of a second star orbiting about a primary is essentially no different than the formation of a big planet. Multiple stars are, therefore, planetary systems wherein at least one "planet" condensed from a lump of the cloud which was so large that it allowed nuclear fusion in the core, and the "planet" became self-luminous, a second star. We can see and count such "planetary systems" quite easily. About one half of our disk stars seem to be in such systems, and on that observation alone the pheonomenon of a larger mass with smaller masses allied to it must be common. Unless there is something unforseenly unique about stellar-sized objects which favors their formation while blocking that of slightly smaller planet-sized objects, planetary systems must be at least as common as double stars.
- Our own solar system provides several, not one, examples of such systems. Not only do we have our system at large, but also several mini-systems in the moons of the Jovian planets. Large rotating centers-of-mass seem to naturally acquire secondary bodies revolving about them. An intriguing added fact that the elemental composition of our solar system almost precisely matches the composition of the galactic disk leads to a further intuition as to the normalcy of our situation. Given similar basic materials and forces, what took place here should have taken place elsewhere in the galaxy as standard practice. Leading planetologists John Lewis and Ronald Prinn say: It is widely, but not universally, accepted that stars form from moderately dense nebulae comprising gases and dust with overall elemental abundances essentially identical to those in the sun and in other normal (Main Sequence) hydrogen-burning stars Lewis & Prinn 1984.
- Several physical measurements have indicated the probable existence of planets around specific nearby stars. These measurements include gravitational tugs or wobbles caused by the pull of large unseeable objects on the stars, or infrared indications of circumstellar dust disks (expected accompaniments of planet-formation), or the slow rotational movements of stars (as if they had transferred some of their rotary motion to other bodies which now revolve about them) Hobbs 1986 Hecht 1987 Gatewood 1987. Recent Doppler shift work by Campbell seems to confirm our positive expectations on the common occurence of planets around nearby stars Waltrop 1987.
The subsequent conclusions of almost all planetary theorists and astronomers are optimistic and eminently reasonable:
- Planets are a natural ordinary feature of the cosmos;
- Only our inadequate technology prevents us from directly settling the question. To this position, the current author would add the following corollary, which is the view of almost everyone interested in ETI;
- Probably all the sun-like stars in the galactic disk, as defined above, will have planetary systems. In the terms of the Drake Equation, the fraction of "suns" which are accompagnied by planets is very close to unity (fp = 1). There are perhaps 6 to 15 billion sun-like disk stars with associated planetary systems.