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The Planet That Wasn't

By Isaac Asimov


I was once asked whether it was at all possible that the ancient Greeks had known about the rings of Saturn. The reason such a question is raised at all comes about as follows--


Saturn is the name of an agricultural deity of the ancient Romans. When the Romans had reached the paint where they wanted to match the Greeks in cultural eminence, they decided to equate their own uninteresting deities with the fascinating ones of the imaginative Greeks. They made Saturn correspond with Kronos, the father of Zeus and of the other Olympian gods and goddesses.

The most famous mythical story of Kronos (Saturn) tells of his castration of his father Ouranos (Uranus), whom he then replaced as ruler of the Universe. Very naturally, Kronos feared that his own children might learn by his example and decided to take action to prevent that. Since he was unaware of birth-control methods and was incapable of practicing restraint, he fathered six children (three sons and three daughters) upon his wife, Rhea. Taking action after the fact, he swallowed each child immediately after if was born.

When the sixth, Zeus, was born, Rhea (tired of bearing children for nothing) wrapped a stone in swaddling clothes and let the dim-witted lord of the Universe swallow that. Zeus was raised in secret and when he grew up he managed, by guile, to have Kronos vomit up his swallowed brothers and sisters (still alive!). Zeus and his siblings then went to war against Kronos and his siblings (the Titans). After a great ten-year struggle, Zeus defeated Kronos and took over the lordship of the Universe.

Now, then, let's return to the planet which the Greeks had named Kronos, because it moved more slowly against the background of the stars than any other planet and therefore behaved as though it were an older god. Of course, the Romans called it Saturn, and so do we.

Around Saturn are its beautiful rings that we all know about. These rings are in Saturn's equatorial plane, which is tipped to the plane of its orbit by 26.7 degrees. Because of this tipping, we can see the rings at a slant. The degree of tip is constant with respect to the stars, but not with respect to ourselves. It appears tipped to us in varying amounts depending an where Saturn is in its orbit. At one point in its orbit, Saturn will display its rings tipped downward, so that we see them from above. At the opposite point they are tipped upward, so that we see them from below.

As Saturn revolves in its orbit, the amount of tipping varies smoothly from down to up and back again. Halfway between the down and the up, and then halfway between the up and the down, at two opposite points in Saturn's orbit, the rings are presented to us edge-on. They are so thin that at this time they can't be seen at all, even in a good telescope. Since Saturn revolves about the Sun in just under thirty years, the rings disappear from view every fifteen years.

When Galileo, back in the 1610s, was looking at the sky with his primitive telescope, he turned if on Saturn and found that there was something odd about it. He seemed to see two small bodies, one on either side of Saturn, but couldn't make out what they were. Whenever he returned to Saturn, it was harder to see them until, finally, he saw only the single sphere of Saturn and nothing else.

"What!" growled Galileo, "does Saturn still swallow his children?" and he never looked at the planet again. It was another forty years before the Dutch astronomer Christiaan Huygens, catching the rings as they were further and further (and with a telescope better than Galileo's), worked out what they were.

Could the Greeks, then, in working out their myth of Kronos swallowing his children, have referred to the planet Saturn, its rings, the tilt of its equatorial plane, and its orbital relationship to Earth?

No, I always say to people asking me this question, unless we can't think up some explanation that is simpler and more straightforward. In this case we can--coincidence.

People are entirely too disbelieving of coincidence. They are far too ready to dismiss it and to build arcane structures of extremely rickety substance in order to avoid it. I, on the other hand, see coincidence everywhere as an inevitable consequence of the laws of probability, according to which having no unusual coincidence is far more unusual than any coincidence could possibly be.

And those who see purpose in what is only coincidence don't usually even know the really goad coincidences--something I have discussed before (1). In this case what about other correspondences between planetary names and Greek mythology? How about the planet that the Greeks named Zeus and the Romans named Jupiter? The planet is named for the chief of the gods and it turns out to be more massive than all the other planets put together. Could it be that the Greeks knew the relative masses of the planets?

The most amazing coincidence of all, however, deals with a planet the Greeks (you would think) had never heard of.


Consider Mercury, the planet closest to the Sun. It has the most eccentric orbit of any known in the nineteenth century. Its orbit is so eccentric that the Sun, at the focus of the orbital ellipse, is markedly off-center.

When Mercury is at that point in its orbit closest to the Sun ("perihelion"), it is only 46 million kilometers away and is moving in its orbit at a speed of fifty-six kilometers a second. At the opposite point in its orbit, when if is farthest from the Sun ("aphelion"), it is 70 million kilometers away and has, in consequence, slowed down to thirty-seven kilometers a second. The fan that Mercury is sometimes half again as far from the Sun as it is at others, and that it moves half again as quickly at some times than at others makes it somewhat more difficult to plot its movements accurately than those of the other, more orderly, planets.

This difficulty arises most noticeably in one particular respect--

Since Mercury is closer to the Sun than Earth Is, it occasionally gets exactly between Earth and Sun and astronomers can see its dark circle move across the face of the Sun.

Such "transits" of Mercury happen in rather irregular fashion because of the planet's eccentric orbit and because that orbit is tilted by seven degrees to the plane of Earth's orbit. The transits happen only in May or November (with November transits the more common in the ratio of 7 to 3) and at successive intervals of thirteen, seven, ten, and three years.

In the 1700s, transits were watched very eagerly because they were one thing that could not be seen by the unaided eye and yet could be seen very well by the primitive telescopes of the day. Furthermore, the exact times at which the transit started and ended and the exact path it took across the solar disc changed slightly with the place of observation on Earth. From such changes, the distance of Mercury might he calculated and, through that, all the other distances of the Solar System.

It was very astronomically embarrassing, then, that the prediction as to when the transit would take place was sometimes off by as much as an hour. It was a very obvious indication of the limitations of celestial mechanics at the time.

If Mercury and the Sun were all that existed in the Universe, then whatever orbit Mercury followed in circling the Sun, it would follow it exactly in every succeeding revolution. There would be no difficulty in predicting the exact moments of transits.

However, every other body in the Universe also pulls at Mercury, and the pull of the nearby planets--Venus, Earth, Mars, and Jupiter--while very small in comparison to that of the Sun, is large enough to make a difference.

Each separate pull introduces a slight modification in Mercury's orbit (a "perturbation") that must be allowed for by mathematical computations that take into account the exact mass and motion of the object doing the pulling. The resulting set of complications is very simple in theory since it is entirely based on Isaac Newton's law of gravitation, but is very complicated in practice since the computations required are both lengthy and tedious.

Still if had to he done, and more and more careful attempts were made to work out the exact motions of Mercury by taking into account all possible perturbations.

In 1843, a French astronomer, Urbain Jean Joseph Leverrier, published a careful calculation of Mercury's orbit and found that small discrepancies persisted. His calculations, carried out in inordinate detail, showed that after all conceivable perturbations had been taken into account, there remained one small shin that could not be accounted for. The point at which Mercury reached its perihelion moved forward in the direction of its motion just a tiny bit more rapidly than could be accounted for by all the perturbations.

In 1882, the Canadian-American astronomer Simon Newcomb, using better instruments and more observations, corrected Leverrier's figures very slightly. Using this correction, it would seem that each time Mercury circled the Sun, its perihelion was 0.104 seconds of are farther along than it should be if all pafurbations were taken into account.

This isn't much. In one Earth century, the discrepancy would amount to only forty-three seconds of are. It would take four thousand years for the discrepancy to mount up to the apparent width of our Moon and three million years for it to amount to a complete turn about Mercury's orbit.

But that's enough. If the existence of this forward motion of Mercury's perihelion could not be explained, then there was something wrong with Newton's law of gravitation, and that law had worked out so perfectly in every other way that to have it come a cropper now was not something an astronomer would cheerfully have happen.

In fact, even as Leverrier was working out this discrepancy in Mercury's orbit, the law of gravitation had won its greatest victory ever. And who had been the moving force behind that victory? Why, Leverrier, who else?

The planet Uranus, then the farthest known planet from the Sun, also displayed a small discrepancy in its motions, one that couldn't be accounted for by the gravitational pull of the other planets. There had been suggestions that there might be still another planet, farther from the Sun than Uranus was, and that the gravitational pull of this distant and still unknown planet might account for the otherwise unaccounted-for discrepancy in Uranus's motions.

An English astronomer, John Couch Adams--using the law of gravity as his starting point--had, in 1843, worked out a possible orbit for such a distant planet. The orbit would account for the discrepancy in Uranus's motions and would predict where the unseen planet should be at that time.

Adams's calculations were ignored, but a few months later, Leverrier, working quite independently, came to the same conclusion and was luckier. Leverrier transmitted his calculations to a German astronomer, Johann Gottfried Galle, who happened to have a new star map of the region of the heavens in which Leverrier said there was an unknown planet. On September 23, 1846, Galle began his search and, in a matter of hours, located the planet, which we now call Neptune.

After a victory like that no one (and Leverrier least of all) wanted to question the law of gravity. The discrepancy in Mercury's orbital motions had to be the result of some gravitational pull that wasn't being taken into account.

For instance, a planet's mars is most easily calculated if it has satellites moving around it at a certain distance and with a certain period. The distance-period combination depends upon the planetary mass, which can thus be calculated quite precisely. Venus, however, has no satellites. Its mass could only be determined fuzzily, therefore, and a might be that it war actually ten percent more massive than the astronomers of the mid-nineteenth century had thought. If it were, that additional mass, and the additional gravitational pull originating from if, would just account for Mercury's motion.

The trouble is that if Venus were that much more massive than was supposed, that extra mass would also effect the orbit of its other neighbor, Earth--and disturb it in a way that is not actually observed. Setting Mercury to rights at the cost of upsetting Earth is no bargain, and Leverrier eliminated the Venus solution.

Leverrier needed some massive body that was near Mercury but not too disturbingly near any other planet, and by 1859 he suggested that the gravitational source had to come from the far side of Mercury. There had to be a planet inside Mercury's orbit, close enough to Mercury to account for the extra motion of its perihelion, but far enough from the planets farther out from the Sun to leave them substantially alone.

Leverrier gave to the suggested intra-Mercurial planet the name Vulcan. This was the Roman equivalent of the Greek god Hephaistos, who presided over the forge as the divine smith. A planet that was forever hovering near the celestial fire of the Sun would be more appropriately named in this fashion.

If an intra-Mercurial planet existed, however, why was it that it had never been seen? This isn't a hard question to answer, actually. As seen from Earth, anybody that was closer to the Sun than Mercury is would always be in the neighborhood of the Sun, and seeing it would be very difficult indeed.

In fact, there would only be two times when it would be easy to see Vulcan. The first would be on the occasion of a total solar eclipse, when the sky in the immediate neighborhood of the Sun is darkened and when any object that is always in the immediate neighborhood of the Sun could be seen with an ease that would, at other times, be impossible.

In one way, this offers an easy out, since astronomers can pinpoint the times and places at which total solar eclipses would take place and be ready far observations then. On the other hand, eclipses do not occur frequently, usually involve a large amount of traveling, and last only a few minutes.

What about the second occasion for easy viewing of Vulcan? That would be whenever Vulcan passes directly between Earth and Sun in a transit. Its body would then appear like a dark circle on the Sun's orb, moving rapidly from west to east in a straight line.

Transits should be more common than eclipses, be visible over larger areas far longer times, and give a far better indication of the exact orbit of Vulcan--which could then be used to predict future transits, during which further investigations could be made and the properties of the planet worked out.

On the other hand, the time of transit can't be predicted surely until the orbit of Vulcan is accurately known, and that can't be accurately known until the planet is sighted and followed for a while. Therefore, the first sighting would have to be made by accident.

Or had that first sighting already been made? Such a thing was possible, and even likely. The planet Uranus had been seen on a scare of occasions prior to its discovery by William Herschel. The first astronomer royal of Great Britain, John Flamsteed, had seen it a century before its discovery, had considered it an ordinary star, and had listed it as "34 Tauri." Herschel's discovery did not consist in seeing Uranus for the first time, but in recognizing it as a planet for the first time.

Once Leverrier made his suggestion (and the discoverer of Neptune carried prestige at the time), astronomers began searching for possible previous sightings of strange objects that would now be recognized as Vulcan.

Something showed up at once. A French amateur astronomer, Dr. Lescarbault, announced to Leverrier that in 1845 he had observed a dark object against the Sun which he had paid little attention to at the time, but which now he felt must have been Vulcan.

Leverrier studied this report in great excitement, and from it he estimated that Vulcan was a body circling the Sun at an average distance of 21 million kilometers, a little over a third of Mercury's distance. This meant its period of revolution would be about 19.7 days.

At that distance, it would never be more than eight degrees from the Sun. This meant that the only time Vulcan would be seen in the sky in the absence of the Sun would be during, at most, the half-hour period before sunrise or the half-hour period after sunset (alternately, and at ten-day intervals). This period is one of bright twilight, and viewing would be difficult, so that it was not surprising that Vulcan had avoided detection so long.

From Lescarbault's description, Leverrier also estimated the diameter of Vulcan to be about two thousand kilometers, or only a little over half the diameter of our Moon. Assuming the composition of Vulcan to be about that of Mercury, it would have a mass about one-seventeenth that of Mercury or one-fourth that of the Moon. This is not a large enough mass to account for all of the advance of Mercury's perihelion, but perhaps Vulcan might be only the largest of a kind of asteroidal grouping within Mercury's orbit.

On the basis of Lescarbault's data, Leverrier calculated the times at which future transits ought to take place, and astronomers began watching the Sun on those occasions, as well as the neighborhood of the Sun whenever there were eclipses.

Unfortunately, there were no clear-cut evidences of Vulcan being where it was supposed to be on predicted occasions. There continued to be additional reports as someone claimed to have seen Vulcan from time to time. In each case, though, it meant a new orbit had to be calculated, and new transits had to be predicted--and then these, too, led to nothing dear-cut. It became more and more difficult to calculate orbits that included all the sightings, and none of them successfully predicted future transits.

The whole thing became a controversy, with some astronomers insisting that Vulcan existed and others denying it.

Leverrier died in 1877. He was a firm believer in the existence of Vulcan to the end, and he missed by one year the biggest Vulcan flurry. In 1878, the path of a solar eclipse was to pass over the western United States and American astronomers girded themselves for a mass search for Vulcan.

Most of the observers saw nothing, but two astronomers of impressive credentials, lames Craig Watson and Lewis Swift, reported sightings that seemed to be Vulcan. From the reports, it seemed that Vulcan was about 650 kilometers in diameter and only one fortieth as bright as Mercury. This was scarcely satisfactory, since it was only the size of a large asteroid and could not account for much of the motion of Mercury's perihelion, but it was something.

And yet even that something came under attack. The accuracy of the figures reported for the location of the object was disputed and no orbit could be calculated from which further sightings could be made.

As the nineteenth century closed, photography was coming into its own. There was no more necessity to make feverish measurements before the eclipse was over, or to try to make out clearly what was going on across the face of the Sun before it was all done with. You took photographs and studied them at leisure.

In 1900, after ten years of photography, the American astronomer Edward Charles Pickering announced there could be no intra-Mercurial body that was brighter than the fourth magnitude.

In 1909, the American astronomer William Wallace Campbell went further, and stated categorically that there was nothing inside Mercury's orbit that was brighter than the eighth magnitude. That meant that nothing was there that was larger than forty-eight kilometers in diameter. It would take a million bodies of that size to account for the movement of Mercury's perihelion (2).

With that, hope for the existence of Vulcan flickered nearly to extinction. Yet Mercury's perihelion did move. If Newton's law of gravitation was correct (and no other reason for supposing ifs incorrectness had arisen in all the time since Newton) there had to be some sort of gravitational pull from inside Mercury's orbit.

And, of course, there was, but it originated in a totally different way from that which anyone had imagined. In 1915, Albeit Einstein explained the matter in his General Theory of Relativity.

Einstein's view of gravitation was an extension of Newton's--one that simplified itself to the Newtonian version under most conditions, but remained different, and better, under extreme conditions. Mercury's presence so close to the Sun's overwhelming presence was an example of the extreme condition that Einstein could account for and Newton not.

Here's one way of doing it. By Einstein's relativistic view of the Universe, mass and energy are equivalent, with a small quantity of mass equal to a large quantity of energy in accordance with the equation e=mc2.

The Sun's enormous gravitational field represents a large quantity of energy and this is equivalent to a certain, much smaller, quantity of mass. Since all mass gives rise to a gravitational held, the Sun's gravitational held, when viewed as mass, must give rise to a much smaller gravitational field of its own.

If is this second-order pull, the small gravitational pull of the mass-equivalent of the large gravitational pull of the Sun, that represents the additional mass and the additional pull from within Mercury's orbit. Einstein's calculations showed that this effect just accounts for the motion of Mercury's perihelion, and accounted further for much smaller motions of the perihelia of planets farther out.

After this, neither Vulcan nor any other Newtonian mass was needed. Vulcan was hurled from the astronomical sky forever.

Now to get back to coincidences--and a much more astonishing one than that which connects Kronos's swallowing of his children with the rings of Saturn.

Vulcan, you will remember, is the equivalent of the Greek Hephaistos, and the most famous myth involving Hephaistos goes as follows--

Hephaistos, the son of Zeus and Hera, at one time took Hera's side when Zeus was punishing her for rebellion. Zeus, furious at Hephaistos's interference heaved him out of heaven. Hephaistos fell to Earth and broke both his legs. Though he was immortal and could not die, the laming was permanent.

Isn't if strange, then, that the planet Vulcan (Hephaistos) was also hurled from the sky. It couldn't die, in the sense that the mass which supplied the additional gravitational pull had to be there, come what may. It was lamed, however, in the sense that it was not the kind of mass that we are used to, not mass in the form of planetary accumulations of matter. It was the mass-equivalent, instead, of a large energy field.

You are not impressed by the coincidence? Well, let's carry if further.

You remember that in the myth about Kronos swallowing his children, Zeus was saved when his mother substituted a stone in the swaddling clothes. With a stone serving as a substitute for Zeus, you would surely be willing to allow the phrase "a stone" to be considered the equivalent of Zeus."

Very well, then, who flung Hephaistos (the mythical Vulcan) from the heavens? Zeus!

And who flung the planetary Vulcan from the heavens? Einstein!

And what does ein stein mean in Einstein's native German? "A stone!"

I rest my case.

We can say that the Greeks must have foreseen the whole Vulcanian imbroglio right down to the name of the man who solved it--Or we can say that coincidences can be enormously amusing--and enormously meaningless.

The Planet That Wasn't originally appeared in the May 1975 issue of The Magazine of Fantasy and Science Fiction.



1. See "Pompey and Circumstance" in The Left Hand of the Electron (Doubleday, 1972).

2. This is correct as far as we know. To this day, the only objects known to have approached the Sun more closely than Mercury does have been an occasional comet of negligible mass and the asteroid Icarus, which is only a kilometer or two across.