5.1 - Planetary Evolution

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

To decide just how abundant planets are in the Galaxy, the most logical place to start is with planetary evolution theory. If we can specify conditions conducive to the birth and development of solar systems, we may then compare these requirements to the observed Galactic environment and form a reasonable opinion as to the likelihood and frequency of planet formation.

Unfortunately, the array of historical planetary evolution schemes^20,2033,2109 and the ongoing proliferation of both mundane¹²⁷⁸ and unusual^816,1264 models in modern times are beyond the scope of this book. We will not deal with them at length here, especially since excellent and comprehensive reviews are readily available elsewhere.^{20,600,816,1278,2025,2033}

While all conclusions regarding planetary formation even today must be viewed as tentative, it appears that accretion models suffice to account for most of the observed properties of bodies in our solar system. In one theory which is gaining wider acceptance, a large, slowly rotating cloud of interstellar gas and dust about a light-year in diameter begins to slowly shrink. As it draws itself together gravitationally over a period of perhaps ten million years,¹⁹⁴⁵ it becomes denser. Were it merely a glob of ordinary neutral gas, it would end up as a small, rapidly rotating ball of hydrogen. Most of its mass would be flung away unceremoniously -- and there would be no planets.¹⁵⁴⁹

But radiation generated during the contraction of the hydrogen ionizes the gas, converting it into a plasma -- an electrically-charged, highly conductive but tenuous fluid. The Swedish physicist Hannes Alfvén, of the Royal Institute of Technology in Stockholm, was the first to demonstrate a viable mechanism by which angular momentum could be readily transferred from the protostar (the contracting solar nebula) to the surrounding plasma medium. This was fortunate indeed, because until that time a major problem had been to figure out why the planets (with 0.2% of the solar system’s mass) should carry roughly 98% of the total angular momentum.

The magnetic coupling concept announced by Alfvén, and later wielded into a classical theory by world-famous astronomer Fred Hoyle, goes something like this: As the protostar collapses, its magnetic field lines of force are dragged closer together but are held firmly in place. Since the infalling clouds are ionized, the field lines are “glued” to the incoming particles. Thus the protostar’s magnetism is coupled directly to the solar nebula; when the protostar tries to speed up as it contracts, the external medium resists the attempt and absorbs the angular momentum itself. The final result is a small, still slowly turning protostar, surrounded by a rapidly rotating disk of matter.

(This theory helps to explain the observed sudden drop-off in stellar rotation later than spectral class F5 (see Chapter 4). Massive, hot stars earlier than F5 apparently are unable to “glue” the magnetic field lines as tightly as cooler suns can. As a result, the field lines wrap themselves uselessly around these bright stars and fail to effect a momentum transfer to the solar nebula. There is no accretion, no planets form, and the protostar retains much of its original rotation. Stars earlier than F5 are thus less likely to spawn worlds than later-class suns.)

The planets themselves form in the disk of matter surrounding the protostar. This tenuous material probably consists of 98% hydrogen and helium, 2% heavier elements -- much like the composition of Sol today. As the cloud becomes denser, gases and dust particles begin to adhere and condense to form tiny grains. Clumping of the grains in not unlikely, because such grains are believed to have a fluffy snowflake-like structure.²⁰³⁸ By the time the development of the protostar gets into full swing, these particles have become millimeter- or centimeter-sized -- small cosmic pebbles which naturally tend to gravitate toward the midplane of the nebula. The time required for this downfall is no longer than 10-100 years, and the nebular disk thus created probably measures on the order of 1 AU thick and 100 AU in diameter at this point.²⁰⁵¹

The disk material must accrete quickly into bodies large enough to avoid the pressure of the inrushing gases in the plane. Were the grains unable to pull themselves into boulder-sized chunks, most of the matter would be swept remorselessly into the yawning solar “vacuum cleaner” at the rotational center of the accretion disk.³³ A means has been proposed to solve this problem, called the “Goldreich-Ward instability mechanism.” According to this theory, a powerful gravitational instability can appear in the plane of the disk provided the cosmic pebbles are not moving too fast with respect to one another.²⁰³⁸

Calculations show that this instability should be sufficient to cause aggregation within the thin sheet of pebbles into hundred-ton bodies with the diameters of asteroids -- say, one to ten kilometers. Higher-order clustering might then ensue as these bodies begin collecting each other up by collision. This epoch of titanic surface impacts must be reflected in the cratering record we see on the Moon, Mercury, and elsewhere. In our solar system, such impacts were intense during the first 100-500 million years but rapidly tapered off to their present low level about four eons ago.^225,2063

Two general classes of planet are found forming in the accretion disk. These are jovians (Jupiter-like, gas giants, mostly hydrogen and helium) and terrestrials (Earth-like, rocky crust, dense metal core). The terrestrials tend to appear nearest to the protostar, in the hottest regions of the solar nebula. They are the result of simple mass accretion to build up small, rocky, dense bodies.

The jovians are formed far from the central regions. A small, heavy core serves as a seedling for the accumulation of vast quantities of material. The true jovians -- such as Saturn and Jupiter -- develop such massive central bodies that they cause the nebular gas to destabilize and condense into a thick, dense shell. This represents most of the final planetary mass. Jovians act much like miniature protostars, voraciously sweeping the nearby space clean of gas and dust.²⁰⁵¹ The subjovians -- represented by Uranus and Neptune in our system -- don’t have nearly so massive a core as the jovians. Thus, they can retain only those gases normally gravitationally concentrated near the planetary centrum. Subjovians do not grow as large as jovians.

This behavior can be explained in part by the process of differentiation of chemical elements in the condensing solar nebula. According to the detailed hydrodynamic model created by A. G. W. Cameron and his colleagues at the Harvard College Observatory, subjovians tend to form in the outermost regions of the nebula where the pressures are only about 10^-7 atm* and the temperatures under 100 K. Matter there consists largely of interstellar grains, mostly water-ice condensed upon a small rocky substrate.

Uranus and Neptune, then, consist mostly of ice with a little bit of rock. When sufficient mass has accreted, these bodies can gravitationally draw in some of the solar nebula for atmosphere. Hydrogen and helium will thus comprise perhaps 20% of the total mass of subjovian bodies.²⁰⁵¹ Comets are believed to have originated under similar conditions.²⁰³⁸

Jovians are found closer to the swollen protostar. Most likely they occur in a region where the pressure is about 10^-6 atm and temperatures are 100-200 K or more. At such high temperatures the ice evaporates, leaving only rocky materials to condense. However, due to the higher pressures there is more material around, and it turns out that accretion proceeds faster. This leads to the aforementioned instability and sudden, massive gas collection from the nebula.²⁰⁵¹

The amounts of gas gobbled by a jovian during this period is astounding. In fact, it appears that even now, 4.6 eons later, Jupiter and Saturn are still in the process of “swallowing” their great feast of hydrogen and helium. Both worlds emit roughly three times more energy than they receive from Sol.^2096,210 This heat is due to the slow collapse of the planets gravitationally.^{598,2032,2048,2057} (The shrinkage amounts to about 1 millimeter per year.²⁰³²)

The terrestrials form closest to the protosun, where pressures range from 10^-5 to 10^-4 atm and the temperature climbs from 200 K to well over 1400 K.¹⁵⁶⁴ It is a region of very high convection, so the matter is kept well-stirred. Only small cores with miniscule amounts of nebular gas can accrete. (The extent of this growth restriction is made more clear if we consider stripping the jovians down to their heavy elements. If we did this, we’d find both Jupiter and Saturn with 15-20 M_earth (Earth-masses) of heavies.^{2091,2096,2098} This is far more than Earth, the most massive terrestrial world in our system.) Total accretion time for terrestrials probably runs on the order of a thousand to a million years.^2043,2044

We see that the bulk composition of planets in any single-sun system should follow a quite regular, orderly progression (Figure 5.2). The innermost worlds will consist of the most refractory matter, with the planets at progressively greater distances from the primary consisting of the less refractory materials.²²

To sum up: We expect that planets lying within or close to the habitable zones of stars will be generally terrestrial in character. Far outside the habitable zone at great distance from the sun, jovians and subjovians put in an appearance. And no planets will be found closer to a star than perhaps one-quarter of the distance to the center of the habitable zone. No substance found in the solar nebula could condense in the extreme heat encountered there.

The fundamental correctness of the accretion model has been tentatively verified by Stephen H. Dole of the Rand Corporation.¹²⁵⁸ Dole set up a computer program to simulate the primitive solar system in the process of formation. Accretion nuclei with random orbits are shot into a nebula surrounding a theoretical protostar of 1 M_sun Nuclei aggregate dust in the nebula, assumed to be 2% of the total by mass, until a specified critical mass is reached beyond which gas can be accumulated as well. The growing planetesimals coalesce if their orbits cross or if they come too close. Nuclei continue to be injected until all dust has been swept from the system. The model is simplistic, to be sure,²⁰³⁷ and yet the results are most intriguing.

Despite the fact that Dole varied the initial conditions considerably, the final products always seemed remarkably similar (Figure 5.3). After each run, the end result was a solar system which looked much like our own. The total number of worlds formed varied from seven to thirteen, and the Titus-Bode “law”^1254,1304 of planetary orbital spacing (so well-known to beginning astronomy students) seemed to hold up approximately in all cases.²⁰⁵⁴ While every such system is quite unique, the surprising thing is that each shares many features of Sol’s system and yields results consonant with accretive evolutionary theories.

Dole’s program generated another unexpected result. It has long been suspected that the processes which give rise to binary and multiple star systems may actually preclude the formation of planets.^20,1300 In our Galaxy, the average separation of binary components is about 20 AU, corresponding roughly to the orbital distances of the jovian gas giants in our solar system. (Jupiter and Saturn have often been called “failed stars.”²⁰⁴⁸ In this view, we narrowly missed out on finding ourselves in the middle of a triple star system.)

By increasing the density of the initial protocloud an order of magnitude higher than before, Dole’s program generated larger and larger jovians (Figure 5.4). Eventually the threshold between planetology and astrophysics was crossed. In one high-density run, a class K6 orange dwarf star appears near Saturn’s present orbit, along with two superjovians and a faint red dwarf further sunward. No terrestrials are formed.

As Dole says, the general trend is clear. Jovians multiply at the expense of terrestrials. An increase of one critical parameter -- the nebular density -- may well result in the generation of binary and multiple star systems to the eventual exclusion of terrestrial worlds.¹²⁵⁸

Both theoretical and numerical accretion models of solar system formation suggest that planets are probably the rule rather than the exception, and that terrestrials should form near most single stars in the inner regions of the solar nebula. This augurs well for the abundance of habitable worlds and extraterrestrial life in the Galaxy.