The Search for Vulcan and other hypothetical solar system objects

Guest Speaker

Thursday, 4th January 2024 (19:45 - 22:00)

Venue: The J.A. Jones Hoober Observatory

Discussing the hypothetical planet Vulcan. People believed that such an object existed between Mercury and the Sun because Mercury does not move according to Newtonian laws; its orbit is ‘wrong’ by 43 arcseconds per century. 

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An 1846 diagram of the solar system, including a possible orbit for the hypothetical planet Vulcan.An 1846 diagram of the solar system, including a possible orbit for the hypothetical planet Vulcan. 

The mathematician Le Verrier, at the suggestion of François Arago, published a report in 1843 on the orbit of Mercury. In 1845, observations were made during a transit of this planet across the Sun, which showed that Le Verrier’s prediction was wrong and needed more work. His calculations were, however, used in the discovery of Neptune. In 1859, he did a particularly in-depth study of Mercury’s orbit, which still did not match observations. He then proposed the existence of a new planet or multiple smaller bodies inside the orbit of Mercury. This had been suggested before, but he was the first to give a mathematical justification. It is unclear who suggested the name of Vulcan, which entered general usage. Edmond Modeste Lescarbault, a doctor and amateur astronomer, recorded a slow-moving object transiting the Sun, lasting for 1h 17min 9s on 1859 Mar 26. He wrote to Le Verrier on 1859 Dec 22, after hearing about the announcement of a possible new planet. Le Verrier was excited but suspicious, and made an unannounced visit to Lescarbault on Dec 31, accompanied by a witness. Le Verrier was informed of how Lescarbault used a simple clock to make these observations, and a homemade pendulum for timing. It was also revealed that he often recorded his observations by writing them on a plank of wood. Despite this, Le Verrier became satisfied that the observation was correct and proceeded to announce this to the world. Lescarbault was awarded Légion d’honneur for his observation. Not everybody was convinced. Emmanuel Liais reported that he had observed the Sun at the same time as this supposed discovery and that he had not seen Vulcan. Le Verrier produced orbital predictions for Vulcan based on the observations, giving it a period of 19d 17h and a distance of 0.14au from the Sun. Prediction times for future transits were produced. These transits were looked for but with no success. The well-known astronomer, James Craig Watson, and the comet discoverer Lewis Swift made independent observations of a Vulcan-like object during the total eclipse of 1878. However, neither of these observations matched predicted Vulcan positions. With the death of Le Verrier in 1877, the interest in Vulcan waned and finally died following new predictions of Mercury’s orbit using Einstein’s equations instead of Newton’s. However, following the 1970 solar eclipse, Henry Courten reported several objects near the Sun. This led him to propose the existence of an asteroid belt between Mercury and the Sun, since named the Vulcanoids, but this was never confirmed. Modern observations using the SOHO and MESSENGER spacecraft were made, but no objects resembling Vulcanoids were found. The search for Vulcan did lead to another discovery, however. The 19th-century German astronomer Heinrich Schwabe observed the Sun for 17 years, looking for planets. He did not find any planets, but he noted a regular variation in sunspot numbers, with a period of approximately 10 years.

It was suspected for a short period that Mercury had a moon. On 1974 Mar 27, Mariner 10 went past Mercury and detected strong UV radiation which then disappeared, only to return three days later. The source appeared to have moved at 4km/s, the sort of speed expected for a moon. The object was finally identified as the hot star 31 Crateris. This made astronomers realise that UV radiation can penetrate the interstellar medium further than had been thought. For a while it was also thought that Venus had a moon, called Neith(e). In 1672, Giovanni Domenico Cassini noted a companion while observing the planet, but did not report it. In 1686 he saw it again, and word started to spread. He estimated that the object had the same phase as Venus and about a quarter of its diameter. This object was observed 18 times by a total of five observers during 1761. During the transit of Venus in 1671, some observers claimed to see a dot following the planet, but others did not. Then in 1766, Father Hell of Vienna published a report stating that all these observations were optical illusions. Not everyone, however, was convinced. In 1884, Jean-Charles Houzeau said this object was not a moon but another planet, which orbits the Sun every 283 days and comes in conjunction with Venus every 1,080 days. The skilled observer Edward Emerson Barnard saw a proposed satellite in 1892 at 7th magnitude. It has since been suggested that this observation could have been of a nova. In 1846, Frédéric Petit thought that the Earth had another moon, with a period of 2h 44min and an elliptical orbit, at a distance of between 3,570 and 11.4km. Le Verrier heard this report and was dismissive of it. However, Petit’s proposal received more widespread awareness as it was referred to in Jules Verne’s book From the Earth to the Moon. We do of course have a temporary moon called Cruithne, which orbits the Sun but interacts with the Earth’s orbit at times. The hypothetical gas giant Tyche was proposed to be located in the Oort Cloud by John Matese in 1999. This unseen object was an explanation for the orbital behaviour of long-period comets. The WISE space telescope did not find it which, if it exists, puts a limit on its possible size. Themis is the name given to the tenth satellite of Saturn, which was ‘discovered’ by William Henry Pickering on 1905 Apr 28. However, it did not exist. Pickering had already discovered the ninth satellite, so he had some credibility for his observations. He even got awarded the Lalande Prize for this discovery, and that of the non-existent tenth satellite. Planet X was the label for a proposed planet affecting the orbits of Uranus and Neptune. The work on this by Percival Lowell led to the discovery of Pluto. Nemesis is a hypothetical star proposed in 1984, which could be a red or brown dwarf. It would be close to our solar system, orbiting the Sun at a distance of 1.5ly. Its existence could explain the 30-million-year cycles of mass extinctions on Earth; its interactions with the Ort Cloud would result in comets being sent Earthwards. Unfortunately, this object has not been found. Does Planet Nine exist in our solar system? It could be causing manipulation of minor planets’ orbits. Its proposed orbit takes it out to 400–800au, with a period of between 10,000 to 200,000 years. Surveys are looking for it to the east of Orion. It is interesting to speculate what other theories could one day be abandoned. Patrick Moore, who had plenty of alternative proposals sent to him, referred to their proponents as ‘independent thinkers’.

Speaker: Dr. Ann Bonell

Dr. Ann Bonell is President of the Leicester Astronomical Society with a lifelong and broad interest in all aspects of the subject.   She is a chemist by profession.  Many years ago, she worked in the Chemistry Department at Wuhan University in China - a long time before anyone in this country had ever heard of Wuhan!