January 10 1964 - Ghosts of the Solar System

As a change from the normal programmes, it was suggested that people would be interested in the various bodies of the Solar System which have been reported from time to time but which appear to have no real existence. These include the planet Vulcan, the satellite of Venus, and the ring of Neptune. Whether all these are in fact’ ghosts' remains to be seen.

So far, no further information is available about the Jovian ring reported by S. Vsekhvyatsky, but the evidence seems to be rather against its existence, at least in the form suggested by Vsekhvyatsky.

The solar system contains bodies of various types. As well as the Sun, Moon, and planets, there are the asteroids, comets, satellites, and meteoroids, together with a tremendous amount of fine inter­planetary material. We must also consider several objects whose existence is open to serious doubt, and which are often called 'ghosts'. The most celebrated of them is undoubtedly the planet Vulcan.

The inner group of planets consists of Mercury, Venus, the Earth, and Mars, whose respective distances from the Sun are 36, 67, 93 and 1411 million miles. (These values are not constant, since the orbits are somewhat eccentric; at perihelion, on January 3, the Earth was only 91 1/2 million miles from the Sun. Generally speaking, however, the departures from circularity are not very marked.) Mercury has been known from the earliest times, but it is never far from the Sun in the sky, and is none too easy to see without a telescope. A planet still closer in would be almost impossible to detect in ordinary circumstances. However, the famous French astronomer U. J. J. Le Verrier, Director of the Paris Observatory for many years during the nineteenth century, believed such a planet to exist. It was known as Vulcan, and the name of the blacksmith of the gods was certainly appropriate for it!

Naturally, Le Verrier had what he considered to be sound reasons for his view, and he did not lack experience, since he had already been personally responsible for the discovery of one new planet. In 1781 William Herschel had detected Uranus, which can just be seen without optical aid, and which moves in an orbit far beyond that of Saturn. Before long it became clear that the new­comer was not moving as it ought to have done; something was pulling it out of its predicted path, and that 'something' was presumably an unknown planet still more remote from the Sun. Le Verrier investigated the problem, and was able to work out just where the perturbing body must be. He then notified Galle and D'Arrest, at Berlin, who found the planet within a few days of receiving the message; it was named Neptune. J. C. Adams, in England, had made similar calculations and had reached an almost identical result, but this does not detract in the least from Le Verrier's triumph

Neptune was found in 1846. In 1859 Le Verrier announced that he had been studying similar discrepancies in the motion of Mercury, and that here too an unknown planet was presumably responsible. Unfortunately, confirmation would be far from easy. The planet (Vulcan) would be hopelessly lost in the Sun's glare, and the only real hope of seeing it would be during its transit across the solar disk.

Both Mercury and Venus, the two 'inferior planets', can transit at times; Venus will next do so in 2004, Mercury in 1970. At such times the planet in transit may be seen as a well-defined black spot against the Sun, moving across the disk in a matter of a few hours. Vulcan, too, should occasionally be visible in transit, and not long after Le Verrier had made his statement he received a letter from a certain M. Lescarbault that Vulcan had actually been seen on March 26, 1859.

Lescarbault was not a professional scientist; he was in fact the typical 'eccentric amateur'. He was a physician at Orgeres, in the Department of Eure-et-Loire, and was also the local carpenter; he used to record his observations upon wooden planks, planing them off when he had no further need of them. His timekeeper was an old watch which lacked its second hand, and his telescope was extremely small. Le Verrier, on the other hand, was the most famous astronomer of his day - and he is also said to have been one of the rudest men who has ever lived; he was once compelled to retire from the directorship of the Paris Observatory on account of his irritability, though he was reinstated when his successor, Delaunay, met a tragic death by drowning.

Le Verrier decided to go to Orgeres and see Lescarbault personally. The beginning of the interview must have been some­what strained, and it is on record that Le Verrier began as follows: 'It is then you, sir, who pretend to have observed the intra- Mercurial planet, and who have committed the grave offence of keeping your observation secret for nine months. I warn you that I have come here with the intention of doing justice to your pre­tensions, and of demonstrating that you have been either dishonest or deceived.' Lescarbault replied meekly, showing the drawings and explaining that he had followed Vulcan for four hours as the black dot passed from one side of the Sun to the other. Amazingly, Le Verrier was convinced, and 'became perfectly satisfied that a new planet had really been discovered. He congratulated the doctor, and left with the intention of making the fact thus obtained the subject of fresh calculations.'

Le Verrier at once calculated an orbit, using his own theoretical work together with Lescarbault's observation. Vulcan was sup­posed to be 13,082,000 miles from the Sun, and to have a period of 19 days 17 hours, as against the 88 days of Mercury. The greatest possible elongation was 8 degrees, so that Vulcan would normally be unobservable, particularly as its diameter was estimated at only 1,000 miles, roughly one-third that of Mercury. Then, on March 20, 1862, Lummis at Manchester, using a magnification of 80 on a 2 f-inch refractor, sent in another report of a moving spot against the Sun; this also was identified with Vulcan, and when Lummis' observation was taken into account the calculated orbit was similar to that derived by Le Verrier.

All this seemed rational, even though Liais, in Brazil, stated that he had been observing the Sun at the same time as Lescarbault and had seen nothing at all. However, Vulcan refused to show itself again, and although a careful watch was kept during the times of predicted transits the results were completely negative. Le Verrier continued to believe in the existence of Vulcan up to his death in 1877, but he found few supporters.

There was one other possible way of detecting an intra-Mercurial planet. During a total solar eclipse the bright face of the Sun is hidden by the Moon, and the sky becomes dark enough for stars to be seen, so that Vulcan also might be expected to show itself. During the eclipse of 29 July 1878, two American astronomers, J. G. Watson and L. Swift, decided to make a serious search, and their reports reopened the whole matter. Watson stated that he had seen two bright reddish stars which could not be identified, while Swift described two objects, 'each having a round red disk, and being free from twinkling'. Unfortunately Watson's observa­tions did not agree with Swift's, while neither agreed with the star maps. This would mean that between them Watson and Swift had detected four Vulcans, which seemed wildly improbable.

There have been many total eclipses since 1878, and Vulcan has been looked for now and then, but with no success. Since the 1878 objects were said to have been bright (third or fourth magni­tude) it is only reasonable to suppose that Watson and Swift were guilty of human error. It is also true that little reliance can be placed in Lescarbault or Lummis — and it is worth noting that objects in the Earth's atmosphere are sometimes seen silhouetted against the Sun; in 1880, for instance, M. Ricco at Palermo observed what he at first thought to be a swarm of meteors in transit, though closer examination revealed that they were flapping (they turned out to be cranes). Perhaps I may be allowed to give a personal experience here. Once, in 1960, I was projecting the Sun with a 4-inch refractor when I saw a prominent black spot which showed perceptible movement after five or six minutes. When it eventually moved clear of the disk, it revealed itself in its true guise of a weather balloon.

Since Vulcan has never been seen again, and since the irregu­larities in the movements of Mercury have been satisfactorily explained by relativity theory without the need of introducing a new planet, we may now conclude that Vulcan does not exist. There may of course be various smaller bodies in those inner regions, and one asteroid, Icarus, is known to have a perihelion distance of less than 20,000,000 miles, but Le Verrier's Vulcan may safely be rejected as a ghost.

Equally interesting is the phantom satellite of Venus, which was first reported in the mid-seventeenth century and which made its final exit in 1764. It, too, provides a good example of how even experienced observers can be misled.

Venus is about the same size as the Earth, and might therefore be expected to have a satellite. Obviously, no attendant compar­able with the Moon can exist, or it would be visible with the naked eye, but in 1686 Cassini, at Paris, using one of the long-focus refractors common in that period, observed 'a luminous appear­ance, of a shape not well defined, that seemed to have the same phase with Venus, which was then gibbous on the western side. The diameter of this object was nearly one-quarter that of Venus'. In 1740 James Short, using one of his well-known Gregorian reflectors, made a similar observation, estimating the diameter of the satellite as one-third that of Venus and of the same phase.

Further confirmation came in May 1761 from Montaigne of Limoges, as a result of which the French Academie des Sciences was informed that the discovery could be regarded as definite. The new satellite was said to be roughly one-quarter of a million miles from Venus, and to have a period of 9 days 7 hours. Frederick the Great of Prussia proposed to name it 'D'Alembert' in honour of his old friend Jean d'Alembert, but the prudent mathematician declined the honour with thanks.

In March 1764 the object was again reported by three independent observers: Rodkiaer and Horrebow at Copenhagen, and Montbaron at Auxerre. And thenceforth it disappears from the records. Astronomers of the calibre of Schroter and Herschel were unable to find it, despite close searches, and all later observers have been similarly unsuccessful.

Attempts have been made to explain the observations made before 1764. Admiral Smyth, author of the famous Cycle of Celestial Objects, suggested that the satellite was patchy, so that only parts of it were capable of reflecting light; the German astronomer Schorr believed that the total brightness varied so much that the satellite was normally invisible. Yet theories of this kind seem inadmissible, and there is no escape from the conclusion that Venus has no attendant more than a few miles in diameter. Venus itself is extremely brilliant; when observed with an imperfect telescope, ghost images would be produced, which would, naturally, be of the same phase as the planet. Admittedly it is strange that Short, in particular, fell into so elementary a trap, but there seems to be no alternative. We have no proof that Venus has even a tiny satellite; like Mercury, it is a solitary traveller in space.

Our next ghost is of a rather different type. After the discovery of Neptune in 1846, as a result of the calculations made by Le Verrier and Adams, the new planet was studied with the largest available telescopes. Several astronomers, notably Lassell and Ghallis in England and Bond in America, reported that they could see indications of a ring similar to Saturn's but much fainter. Later work with much more efficient telescopes did not confirm the Neptunian ring, which seems to be definitely absent. (In this connection, it is interesting to note that in early 1964 S. Vsekhsvyatsky, of the Ukraine, announced the detection of a very faint ring associated with Jupiter. This, too, may well prove to be a ghost, though as yet it is too early to say.)

So far as Saturn is concerned, various observers from 1908 onward have described a dusky ring outside the bright Ring A. There is no theoretical bar to its existence, but the evidence is slender. I have looked for the ring under good conditions with large telescopes, notably the 33-inch refractor at Meudon (Paris), without seeing a trace of it; this has also been the experience of many observers with far greater skill than mine. And we must note, too, the tenth satellite of Saturn, reported by W. H. Pickering in 1904 and named Themis. Pickering recorded it several times, but it has not been seen since, and its real existence is dubious.

Finally, let us turn to the ghost of a comet - Biela's, so named because it was discovered by the Austrian astronomer von Biela in 1826. When its orbit was computed, the comet was found to be identical with comets observed on two previous occasions. The period was almost seven years, and until 1846 nothing remarkable happened. In that year, however, Biela's Comet broke into two fragments. The twins returned in 1852, were missed in 1858 because of their unfavourable position in the sky, and were expected once more in 1866. To the surprise of all concerned, nothing was seen, and it was clear that the comet had ceased to exist as such. At the next predicted return, that of 1872, there came a shower of meteors, which could only represent the debris of the dead comet. Even now we still see a few 'Bieliid' meteors each November. Other former short-period comets, notably those of Brorsen and Holmes, have also vanished from the scene.

We must be wary of jumping to conclusions, and it is always possible that we have not heard the last of Vulcan, Themis, the satellite of Venus, and the elusive Neptunian ring; but we must agree that while the Solar System contains many objects of extreme interest, it also includes its full quota of’ ghosts'.