Other Worlds Than Ours

If the life of our solar system,” writes DONALD H. MENZEL, Director of the Harvard Observatory, “is to be limited by chance collision with a wandering star, the earth is likely to live to a ripe old age.” A leading astrophysicist, Dr. Menzel is the author of several books in the field of physics, radio propagation, and astronomy, and his book debunking flying saucers attracted wide attention. In the following paper, remarkable for the clarity of its elucidation, Dr. Menzel proposes a new theory about the origin of our solar system.

MANKIND is becoming increasingly aware of the fact that the earth is probably not unique. In our own solar system one planet, Venus, resembles the earth in many ways. The two bodies are nearly the same size. Both have extensive atmospheres, and Venus is even more troubled with clouds than we are. In such details as the composition of the two atmospheres, many differences occur. On Venus, for example, we find large quantities of carbon dioxide but no evidence of free oxygen. We may therefore question whether life exists on Venus. However, even with this handicap, if man were to visit Venus with the appropriate equipment, he would probably be able to survive. Certain types of vegetation might easily adapt themselves to the peculiarities of the Venusian climate.

Our sun is only one of approximately one hundred billion stars that make up our galaxy. Also there are probably more than one million galaxies within reach of our greatest telescopes. Hence, if each star had ten planets around it, the total number of planets would be about a billion billion. Planets would then be as numerous as the grains of sand in a pile as large as the Empire State Building. A plurality of worlds, indeed! Plenty of room for living creatures to originate and to evolve.

To view the earth, the sun, the planets, and the stars in true perspective, let us erect, in our imaginations, a huge building over an area the size of the United States and Canada. As we enter this tremendous structure, our first impression is that someone has deceived us, that the building is entirely empty. In order to see any stars at all, we need a super microscope; for on the scale on which our universe is constructed, no ordinary microscope would be able to render the stars visible. The largest are scarcely one millionth of an inch across, and the smaller ones are about the size of atoms.

When our eyes become adapted to this microscopic vision, so that we can see the individual stars, we note immediately some semblance of order and arrangement even in the larger portion of the universe. Stars are by no means scattered uniformly throughout the enormous building. They form groups or clusters several hundred feet in diameter, something like giant swarms of gnats. The stellar population is concentrated into these regions, with vast realms of nothingness in between.

We note that some of these groupings of stars are irregular in shape. Others are round and flat, like a pie plate. Each group contains hundreds of millions — sometimes even as many as a hundred billion — stars, with the model stars spaced a few tenths of an inch apart, on the average. Millions of such groups exist. In many of them the stars are arranged like the coils of a watch spring, giving a sort of pinwheel appearance to the aggregation. These are the great spiral nebulae.

Our first task is to discover which of these galaxies represents our own, the one we call the Milky Way. The belt of faint light that girdles the sky, dividing it into two hemispheres much as the equator does the earth, owes its luminosity to the glow of billions of distant suns individually too faint to be seen by the naked eye but collectively rich enough in light to delineate the rim of our flattened stellar system.

Our own Milky Way, like many other galaxies, has spiral arms. In our model, it is a disk-shaped aggregation of stars some 100 feet in diameter and 10 feet in thickness. It contains some hundred billion stars, and the task of searching out our sun from among the vast horde is indeed herculean. Talk about looking for a needle in a haystack! A hundred billion one-cent pieces, spread over a football field, would make a pile nearly 50 feet high. Merely to count all the stars within the Milky Way at the rate of one per second would take about a thousand years. When, at last, we find our solar system, we discover it to be a very small speck indeed. The largest orbit of all, that of the planet Pluto, is invisible to the eye, and our earth is even smaller than an atom.

This model of the universe will have served its purpose if we have gained some sense of proportion regarding the parts from which the universe is built. In particular we should carry away the impression of how empty the universe is, in spite of the enormous number of stars within it. We should have noted the tendency of stars to form groups of various orders. And finally we should have gained some idea of the relative insignificance of the sun, of the solar system, and especially of our earth.

Let us now turn from the universe of stars and interstellar depths to our own part of the world, the sun and family. To get a proper perspective, we shall let our model expand, stopping when the sun has reached the size of a basketball, and again survey the solar system. The scale of the newmodel is some thousand million times greater than that of the old.

The earth now appears about as large as a grain of wheat and revolves, at a distance of 100 feet, in a circle about the miniature sun. Within the earth’s orbit lie Venus and Mercury, a second grain of wheat and a mustard seed, spaced respectively at about 70 and 40 feet from the model sun. Mars, a trifle larger than Mercury, stands at 160 feet. We shall represent Jupiter, the largest of all the planets, by a sphere the size of a walnut, and place it 500 feet away from the sun. Saturn, as big as a cherry and circled by its famous ring system, lies at 1000 feet. Uranus and Neptune, both as large as good-sized peas, fall at 2000 and 3000 feet respectively. Finally Pluto, whose size is still somewhat uncertain — though it probably is even smaller than Mars — fits into the picture just under a mile away. And we must not forget the asteroids, two thousand or more minor planets, mere specks of dust King between the orbits of Mars and Jupiter.

What effect has this expansion of our model had upon the stars? The answer is, a tremendous one. The nearest star, another basketball similar to the one that represents our sun, has receded to a distance of 4000 miles. Within the great building that contained the entire universe we now find but a single star other than the sun! Our galaxy has swelled until it fills the entire orbit of Venus!

Again we are impressed by the emptiness of interstellar space. At the same time we note, perhaps even drawing a sigh of relief, that if the life of our solar system is to be limited by chance collision with a wandering star, the earth is likely to live to a ripe old age.


WE HAVE no direct proof that any planets exist beyond our solar system. Although we have deduced the presence of a few dark bodies that revolve around certain bright stars, because they eclipse a portion of the light at periodic intervals, we cannot decide whether these bodies are planets or simply small, relatively cool stars. A planet must be at least cold enough not to shine by its own light.

The question whether other worlds exist is one of the most fascinating topics in astronomy as well as one of the most difficult to discuss. It is dangerous to give free play to our imaginations; and yet, if we are to come up with any sort of answer at all, we have to resort to indirect reasoning.

To approach the problem about the likelihood of other planets, we may inquire into the manner of their birth. If it is as natural for a sun to have planets as it is for a cat to have kittens, the majority of stars may possess planetary families. If, on the other hand, planets come into being only as a result of some unusual cosmic event, like the collision of two stars, planets may be extremely rare. Under such circumstances, our solar system might then be unique in the entire universe.

Someone recently remarked that theories of the origin of the solar system are practically as numerous as astronomers. The fact that some astronomers hold no views at all on this subject is more than offset by the fact that others have proposed a number of theories.

We can divide theories of creation into two broad classes: the unitary theories and the binary theories. In the unitary theory the planets appear as the result of purely natural processes during the life of a single star. The planets, after their birth, repeat the process on a smaller scale, to form the satellites. Everything necessary for the entire solar system was contained in the original matter that eventually formed the sun and planets.

On the binary hypothesis, however, we start off with a sun that has no planetary retinue. The star acquires its family by capture, adoption, or catastrophe. Those processes require the action of either another star or a large volume containing meteoric material and cosmic dust. The great majority of modern theories are of the binary variety. These were forced upon us because the unitary hypothesis ran into seemingly insurmountable difficulties.

Two names associated with the development of unitary hypotheses are Kant (1755), a German philosopher, and Laplace (1796), a French mathematical astronomer. Both of these scientists supposed that, once upon a time, matter uniformly filled all space. The primordial material was a nebula; hence scientists called such a theory the “nebular hypothesis.” “A fire mist that began to contract as it grew cold,” according to Kant. “An enormous mass of cold gas that grew warm as it contracted,“ according to Laplace. Of these divergent views, the latter is probably more acceptable. In any event, gravitation was the controlling force. Slight condensations in the cosmic cloud began to grow and form the stars. Kant thought that smaller condensations might produce the planets.

Laplace suggested a different origin for the planets and satellites. The central condensation that eventually became the sun, also began to rotate upon its axis. It spun faster as contraction went on, by virtue of a well-known principle of mechanics called by the technical phrase “the law of conservation of angular momentum.”

Angular momentum is a phenomenon well known to ballerinas or fancy skaters. For example, a skater or dancer wishing to execute a whirl starts spinning slowly upon one foot with both arms and one leg extended. An ice skater often goes into a crouching position at the beginning of the maneuver. Then the person draws the arms and leg closer to the body, trying to achieve pencil slimness as far as possible. The nearer the arms and legs reach the axis of rotation the faster the spin becomes.

Anyone possessing a rotatable desk chair or piano stool can make an even more spectacular demonstration. Holding two fairly heavy books at arm’s length, start yourself spinning. Pull the books in toward your body and the speed or rotation increases noticeably. In fact, you can pull the books in only with considerable effort. They tend to fly off unless you hold them tightly.

Laplace visualized that the contracting sun, increasing its rate of spin, eventually became so unstable that it threw out a ring of gas. Temporarily relieved, the sun started contracting again until a second instability occurred, leading to the formation of a second ring. And so on, successively, one ring for each planet. The idea was that each ring of gas would then condense to form a planet. In this way the outer planets were formed first. Mercury, the innermost planet, is the baby of the solar system.

Laplace further postulated a repetition of the process, this time on a planetary scale, leading to the formation of satellite systems. Saturn possessed rings, he reasoned, because one of the ejected rings failed to condense into a satellite.

The idea had many attractive features. It certainly seemed to account for most of the regularities of the solar system — the flatness of the orbits, the fact that the planets all go around the sun the same way and that most of them rotate in the same direction that they revolve. It explained many features of the satellite systems.


THE first doubt we had of this theory came in 1898 when William H. Pickering discovered Phoebe, a satellite of Saturn, the first known object that revolved counter to the general motion so prevalent in the solar system. The discovery of Phoebe was not as fatal to the nebular hypothesis as some persons imagined, because an asteroid, captured by a planet with an assist from the sun’s gravitation, can become a satellite that revolves in retrograde fashion. But numerous other difficulties of even more basic nature began to show up.

To study the past, imagine that we reverse the evolutionary process. Let the sun gradually expand until it fills the orbit of Mercury and finally engulfs the planet itself. We can thus reconstruct the situation that prevailed when the sun was supposed to have been so unstable that it tossed out Mercury to relieve the situation.

Today, in our modern era, the sun certainly shows no tendency toward rotational instability. It turns slowly on its axis once every twenty-five days. The sun, as far as our measures show, is a perfect sphere. Any instability caused by too rapid a spin would make the sun look more like a doorknob than a sphere. As we go backward in time, making the sun expand, we note that it rotates even more slowly than it does today. This expansion is equivalent to the pushing out of the heavy books. Thus, by the time that the sun has expanded to fill the orbit of Mercury, the time required for a single rotation would be about five hundred years instead of twenty-five days. And now, as we add Mercury — the planet supposed to be responsible for the instability — the sun speeds up by just about one hour! A difference of one hour in five hundred years scarcely indicates a tendency for the sun to break up because it is rotating too fast.

We continue this process of turning time backward, having the sun pick up Venus, the earth, and the rest of the planets in turn. It gets the biggest jump when it swallows Jupiter, the most massive planet of all the planets, but even then the sun never exhibits any trace of instability.

What this analysis demonstrates is simply that the planets themselves possess less than one per cent of the total mass of the solar system, and yet they have 98 per cent of the angular momentum. Such an unequal distribution cannot come about naturally as the result of gravitational forces.

The force of gravitation is remarkable in many ways. One is this property we have just employed, of permitting us to reverse time, to explore the past. As long as no frictional forces act in addition, the universe will run just as well backward as forward. From the motions alone we have no way of telling which is past and which is future.

We meet one vital consequence of gravitation: a single body, acting by itself, cannot possibly capture another body. Various persons have surmised, for example, that the earth may have acquired the moon by capture, the moon coming in from a distance, spiraling slowly around the earth like a plane about to land, until it settled down in its present orbit. According to the law of gravitation, if a body moves in closer and closer to the sun or a planet, finally reaching the point of nearest approach, the body must recede in an orbit that is the mirror image of the one coming in. Spiral paths are ruled out. If the body came from infinity it must recede to infinity. The orbit must be a hyperbola.

To make capture possible, we should have to invoke the aid of a third body or introduce some form of friction. A large meteorite from outer space, hitting the earth a glancing blow, might be so slowed down by atmospheric friction that the earth could capture it and thus acquire a satellite. But the captured meteor, after receding to a distance, would return again and suffer another slowing down in the earth’s atmosphere. After a few revolutions, the meteor would be so retarded that it would fall to earth and cease to be a satellite.

Chamberlain, a geologist, and Moulton, an astronomer, pointed out some of these difficulties with the early theories and proposed, about 1900, what they called the “planetesimal hypothesis.” This was the first and perhaps the most important of all the binary theories of planetary origin, specifically devised to avoid the difficulty with angular momentum.

These binary hypotheses possess one feature in common. They all start with the sun (or perhaps with a mass of gas that later condensed into the sun). And they introduce a second system coming from outer space: a star, a group of meteors, or a cloud of dust and gas.

Somehow or other, the second system starts revolving about the sun. The angular momentum, therefore, is inherent in the body coming in from space. Hence we do not have to examine the problem from the standpoint of angular momentum, but rather from the standpoint of how probable a given type of encounter may be and whether it will result in the formation of a planetary system like ours.

We have already noted that if our sun encountered a second sun, no capture would result. Double stars or other binary systems cannot arise in this fashion. The transient visitor swings in toward the sun in a long graceful arc, and then swings away just as gracefully. If the original sun came from the depths of space, it returns to the depths. Gravitational forces alone, acting on two bodies such as the sun and a star, cannot produce a capture. The sun might capture a comet or a great meteor, however, if the object happened to penetrate deeply enough into the sun’s atmosphere to lose energy by friction.


ACCORDING to the planetesimal hypothesis, the solar system originated in the chance encounter of our sun with some intruding star. As the intruder swung in close, enormous tides swelled up on the solar surface, enhancing the natural explosiveness of the sun’s atmosphere until suddenly vast clouds of gas streamed into space to form a seething whirlpool of hot matter, rotating vigorously as the result of the gravitational pull of the other star. The intruding star disappeared into the distance, leaving the sun enveloped in a hot, spinning cloud.

The mass of gas eventually cooled, congealing into small solid lumps, the planetesimals. A few of the larger lumps acted as centers of condensation, sweeping up the smaller planetesimals and gradually clearing out the debris caused by the near collision with a hit-run star. In a sense, the final picture differed little from Kant’s, except that the planetesimal cloud developed as the result of a collision instead of natural evolution.

The planetesimal hypothesis proved to be especially popular with the geologists, because it supposed that the mantle or outer covering of the earth grew by the accretion of cold planetesimals rather than by the cooling of a large, gaseous mass. Certain characteristics of the earth, such as its dense central core and the chemical nature of the atmosphere, seemed to accord better with the concept of planetesimals. The proponents of the hypothesis pointed toward the scarred surface of the moon as further evidence for the impacts of numerous small bodies upon the surface of the larger mass.

A variant of the planetesimal hypothesis was the so-called “tidal” hypothesis, suggested by Sir James Jeans about 1917 and later developed by Sir Harold Jeffreys. This theory also relied on the gravitational effect of the passing star. The sun, however, instead of squirting matter wildly in all directions, ejected a single long snake-like filament that stretched out between the two bodies. This filament cooled and eventually broke up into planets to form the solar system.

Still other variants have been proposed by scientists over the past fifty years. Despite the apparent diversity of opinion about details, we can agree on the basic picture. At some late stage we find a central sun surrounded by a rotating cloud of planetesimals that rain down upon centers of condensation to build up the planets. But we are unable to explain how the spinning cloud came into existence. More precisely, we cannot decide which, if any, of a number of alternative theories is correct.

Solar systems must be extremely rare under any theory which depends upon chance encounters between stars. I find the odds hard to figure. But the chances of a star’s having planetary attendants as the result of collision, with the entire age of the universe available for the collision to happen, is not greater than one in a hundred million and probably not more than one in a billion. In other words, in our entire galaxy, with its hundred billion stars, we may expect to find a scant few hundred stars with families of planets.

Cosmic insurance rates ought to be extremely low. We are forced to admit that, if either one of these collision theories is right, then our earth is a freak. And we who inhabit it are an even greater freak.

If we want to increase the probability of planetary systems in the universe, we must find some way of reinstating the unitary system. Every star must have — built into it from the beginning — the potentiality for the production of planets.

Many persons have tried to discover this natural way. They have pictured the sun as shooting out clouds of gas into space. If these clouds act under gravitation alone, they will either fall back into the sun or recede to the depths of space, never to return. In 1930 a Dutch meteorologist, Berlage, invoked the aid of solar magnetism to deflect the charged particles into paths revolving around the sun. His theory thus resembles that of Alfvén, who employed electromagnetic forces in connection with a binary theory, with the matter falling in toward the sun. Neither of these theories appears to provide an adequate amount of matter to form the planets. And the slowly rotating sun could not impart enough angular momentum for the planetary system.


PERHAPS we can get a clue from our sun itself. Motion pictures of the sun’s atmosphere, taken with special telescopes, portray the sun’s activity in graphic fashion. These records, photographed in the light of glowing hydrogen, show the sun in action. Astronomers confess that the activity so depicted presents problems that are difficult for us to understand.

The sun, in apparent defiance of gravitation, shoots enormous clouds of gas into space. Most of this material cascades back to the sun’s surface in threads or clouds many times larger than our earth. But some evidently reaches the earth’s atmosphere, where it may cause the aurora borealis, make compass needles wobble, or disturb commercial radio communications.

Two features stand out. These clouds of glowing hydrogen really do move against gravity and at enormous velocities, sometimes as high as several hundred miles a second. Also, we are surprised to find that they do not dissipate in space as we expected them to. In fact, although the matter contained in one of these filaments would scarcely build a mountain, let alone a planet, the sun may have once been much more active than at present and may have ejected far greater amounts of material, perhaps enough to form the planets.

These motion pictures suggest that the forces tending to overcome gravitation are due to electrical currents and their associated magnetic fields. The possibility of the existence of such forces is not new. We have known for nearly fifty years that intense electric currents exist on the solar surface. They are needed to produce the known powerful magnetic fields associated with sunspots.

We have recently been studying the flow of electricity through large filaments of gas. The predictions agree closely with our observations. When a fuse blows or when a short circuit occurs in a power line, the electric discharge flares out in the form of an expanding loop or arch. Many of the gaseous filaments ejected from the sun act in similar fashion. Also an intense electric current along the filament tends to restrict the escape of gases from the filament. In other words, an electric current can overcome the natural explosive character of an ordinary gaseous filament, perhaps long enough to permit condensation.

Theory indicates that solar electric currents must be slowly dying. Billions of years ago, when the sun was new, the electric currents may have been very much more powerful than they are today. The details are vague, but one may reasonably argue that an electric current of enormous intensity may have flowed around the equator of the primitive sun. The forces of such a current would cause the equator to bulge. We can even see how such a current might tear itself loose from the sun, carrying with it a doughnut ring of matter, so that the sun may momentarily have resembled the planet Saturn. Even today we see the sun ejecting great rings of glowing gas into space; so the idea is by no means highly speculative.

Such a ring could exert a braking action, slowing down the sun’s rotation and, at the same time, speeding up so as to acquire the necessary angular momentum. The ring would rapidly disintegrate and form planetesimals, and perhaps provide some larger bodies to act as centers of condensation. From here on, the course of evolution would follow that of the other theories. But every star with a powerful magnetic field and associated electric currents becomes a potential parent of a planetary family.

The idea of an electromagnetic brake appears to be new. Further theoretical studies are necessary to find out just how effective the braking will be, but the preliminary analysis is encouraging.

To see how a relatively light ring exerts such a force upon the much heavier sun, consider again the rotating chair. I previously emphasized that the books held in the hand have to be heavy. If you should merely toss out a lead pencil, say, instead of a heavy book, no appreciable slowing down would occur. Thus, if the sun had tossed out the planets, its speed of rotation would not have appreciably diminished. However, suppose that you hold out the lead pencil on a thin weightless stick 20 feet long, say, instead of tossing it away. Now the pencil, light as it is, exerts a considerable braking power on the revolving chair. In the same way, as long as the ejected ring current remains attached to the sun by invisible lines of magnetic force, the braking action continues and the sun slows down, transferring its angular momentum to the ring.

As we look into space we see some possible substantiation of this hypothesis. Many stars possess atmospheres far more distended than they should be on a purely gravitational theory. I have long believed that electromagnetic forces were the only ones that could counteract gravitation. We have seen stars explode and, although many alternative theories of novae exist, I prefer to believe that the explosions result from electric currents.

In brief, if you wish to sum up my speculative theory in a sentence, the solar system resulted when the sun “blew a fuse.”

I want to emphasize that my electromagnetic hypothesis is new, and that it does not possess the seal of approval of my astronomical colleagues. I point out, however, that it is not altogether revolutionary. It borrows significant features from most of the major hypotheses discussed previously.

First of all, it is a unitary hypothesis, like that of Kant. The sun ejects a ring or rings of matter, as in Laplace’s theory, though the motive power consists of electric currents instead of the centrifugal force of rotation. The theory relies on the naturally explosive character of the solar atmosphere, as in the planetesimal hypothesis of Chamberlain and Moulton. However, the explosion occurs naturally instead of being induced by a passing star.

The ejected ring possesses the significant features of the filament of Jeans and Jeffreys. The electric current seals the gases in, giving them time to condense rather than dissipate in a violent explosion. Electromagnetic forces play a part, as in the theories of Berlage and Alfvén. A nova-like or perhaps a true nova outburst occurs, as postulated by Hoyle.

As with most other theories, we end up with a sun, a swarm of planetesimals, and centers of condensation. From here on, the evolution must roughly parallel that generally assumed in other theories. One exception may be the regular satellite systems, especially those of the larger planets. These may also arise from a repetition of the electric explosion, on a smaller scale.

Our own moon presents some difficulties, principally because of its large size relative to the earth. We call it a satellite, but hypothetical inhabitants of Venus or Mars would most surely refer to the earth-moon system as a double planet.

At present the moon is about a quarter of a million miles distant from the earth and is slowly — very slowly — moving still farther away. This recession results from the tides, which the moon continuously drags over the earth’s surface in such a way as to slow down the earth’s rotation. The day is getting longer. In 1954 the day is about one thousandth of a second longer than it was in 1854.

Using our earlier device of turning time backward, we note that the earth speeds up and that the moon comes closer and closer, as we approach the distant past. A few billion years ago, when the moon was almost in contact with the earth, the day was only four hours in length. And the moon swung around the earth in about the same period of time.

The great British mathematician, Sir George Darwin, suggested that the earth split into two parts and that the smaller fragment formed the moon. We cannot be sure — but the idea is attractive.

If we admit miracles into the world of science, then our universe is full of them. Every step in the development of the cosmos from star to man borders on the miraculous. For the world must have had built into it, from the very start, the potentiality not only for the building of planets but also for the building of men.

And if my speculative theory of planetary origin should prove to be correct, then we may expect to find a plurality of planets in our universe. And what has happened on earth is surely not unique. Life, even human or superhuman life, may exist in millions of places in the universe.

(In his article,The Bounds of Space and Time,” which will appear in the December Atlantic, Dr. Menzel discusses the current theories about the end of the world and then draws his own picture of the future.)