Editing Vulcan (hypothetical planet)

{{Short description|Hypothetical planet between the Sun and Mercury}}
{{redirect|Vulcan (planet)|the fictional planet|Vulcan (Star Trek)}}
[[File:Planet-Vulcan 1846 003790.jpg|thumb|Vulcan in a lithographic map from 1846<ref>{{cite web|url=https://www.loc.gov/resource/g3180.ct003790|title=LOC file of the solar system, as seen in 1846|website=[[Library of Congress]]|access-date=2 January 2019}}</ref>]]

'''Vulcan''' ({{IPAc-en|'|v|V|l|k|@n}})<ref>{{OED|Vulcan}}</ref> was a proposed [[planet]] that some pre-20th century astronomers thought existed in an [[orbit]] between [[Mercury (planet)|Mercury]] and the [[Sun]]. Speculation about, and even purported observations of, intermercurial bodies or planets date back to the beginning of the 17th century.

The case for their probable existence was bolstered by the support of the French mathematician [[Urbain Le Verrier]], who had predicted the existence of [[Neptune]] using disturbances in the orbit of [[Uranus]]. By 1859, he had confirmed unexplained peculiarities in Mercury's orbit and predicted that they had to be the result of the gravitational influence of another unknown nearby planet or series of [[asteroid]]s. A French amateur astronomer's report that he had observed an object passing in front of the Sun that same year led Le Verrier to announce that the long sought after planet, which he gave the name Vulcan, had been discovered at last.

Many searches were conducted for Vulcan over the following decades but, despite several claimed observations, its existence could not be confirmed. The need for the planet as an explanation for Mercury's orbital peculiarities was later rendered unnecessary when [[Albert Einstein|Einstein]]'s 1915 theory of [[general relativity]] showed that Mercury's departure from an orbit predicted by [[Newtonian Physics|Newtonian physics]] was explained by effects arising from the curvature of [[spacetime]] caused by the Sun's mass.<ref name="Clemence"/><ref>[https://aether.lbl.gov/www/classes/p10/gr/PrecessionperihelionMercury.htm Precession of the perihelion of Mercury, aether.lbl.gov]</ref>

== Hypotheses and observations ==

Celestial bodies interior to the orbit of Mercury had been hypothesized, searched for, and were even claimed to have been observed, for centuries.{{citation needed|date=January 2023}}

Claims of seeing objects passing in front of the Sun included those made by the German astronomer [[Christoph Scheiner]] in 1611 (which turned out to be the discovery of [[sunspot]]s),<ref>{{cite journal | last = Drobyshevskii | first = E. M. | title = Impact Avalanche Ejection of Silicates from Mercury and the Evolution of the Mercury / Venus System | journal = Soviet Astr | volume = 36 | issue = 4 | pages = 436–443 | date=1992| bibcode = 1992SvA....36..436D}}</ref> British lawyer, writer and amateur astronomer [[Capel Lofft]]'s observations of 'an opaque body traversing the sun's disc' on 6 January 1818,<ref>{{Cite journal|url=https://babel.hathitrust.org/cgi/pt?id=njp.32101064253428;view=2up;seq=112;skin=mobile|title=The Monthly magazine. v.45 (1818). - Full View {{!}} HathiTrust Digital Library {{!}} HathiTrust Digital Library|journal=Monthly Magazine and Critical Register of Books|year=1796|language=en|access-date=2017-07-04|last1=Blake|first1=William}}</ref> and Bavarian physician and astronomer [[Franz von Paula Gruithuisen]]'s 26 June 1819 report of seeing "two small spots...on the Sun, round, black and unequal in size".<ref name="articles.adsabs.harvard.edu">{{Cite journal |title=The Supposed New Planet Vulcan|journal=Astronomical Register|volume = 7|page=164|last=Elger|first=T.G.E.|date=May 4, 1869|bibcode=1869AReg....7..164E}}</ref> German astronomer {{ill|Johann Wilhelm Pastorff|lt=J. W. Pastorff|de}} reported many observations also claiming to have seen two spots, with the first observation on 23 October 1822 and subsequent observations in 1823, 1834, 1836, and 1837; in 1834 the larger spot was recorded as 3 [[Minute and second of arc|arcseconds]] across, and the smaller 1.25 arcseconds.<ref name="articles.adsabs.harvard.edu"/>

Proposals that there could be planets orbiting inside Mercury's orbit were put forward by British scientist [[Thomas Dick (scientist)|Thomas Dick]] in 1838<ref name=Baum>{{cite book
| last1=Baum
| first1=Richard P.
| last2=Sheehan
| first2=William
| title=In Search of Planet Vulcan: The Ghost in Newton's Clockwork
| date=August 2003
| publisher=[[Basic Books]]
| location=New York
| isbn=0738208892}}</ref>{{rp|page=264}} and by French physicist, mathematician, and astronomer [[Jacques Babinet]] in 1846 who suggested there may be "incandescent clouds of a planetary kind, circling the Sun" and proposed the name "Vulcan" (after the god [[Vulcan (mythology)|Vulcan]] from [[Roman mythology]]) for a planet close to the Sun.<ref name=Baum/>{{rp|page=156}}

As a planet near the Sun would be lost in its glare, several observers mounted systematic searches to try to catch it during "[[astronomical transit|transit]]", i.e. when it passes in front of the Sun's disc. German amateur astronomer [[Heinrich Schwabe]] searched unsuccessfully on every clear day from 1826 to 1843 and [[Yale]] scientist [[Edward Claudius Herrick]] conducted observations twice daily starting in 1847, hoping to catch a planet in transit.<ref name=Baum/>{{rp|page=264}} French physician and amateur astronomer [[Edmond Modeste Lescarbault]] began searching the Sun's disk in 1853, and more systematically after 1858, with a 3.75 inch (95&nbsp;mm) [[refractor]] in an observatory he set up outside his surgery.<ref name=Baum/>{{rp|page=146}}

== Le Verrier's prediction ==
In 1840, [[François Arago]], the director of the [[Paris Observatory]], suggested to mathematician Urbain Le Verrier that he work on the topic of [[Mercury (planet)|Mercury]]'s orbit around the [[Sun]]. The goal of the study was to construct a model based on Sir [[Isaac Newton]]'s laws of [[Newton's laws of motion|motion]] and [[Newton's law of universal gravitation|gravitation]]. By 1843, Le Verrier published his provisional theory regarding Mercury's motion, with a detailed presentation published in 1845, which would be tested during a [[transit of Mercury]] across the face of the Sun in 1848.<ref name=hsu>{{cite book
 | first=Jong-Ping | last=Hsu |author2=Fine, Dana  | date=2005
 | title=100 Years of Gravity and Accelerated Frames: The Deepest Insights of Einstein and Yang-Mills
 | publisher=[[World Scientific]] |page=479 | isbn=978-981-256-335-4 }}</ref><ref>{{cite book
 | first=Urbain J. | last=Le Verrier | date=1845
 | title=Théorie du mouvement de Mercure | trans-title=Theory on the movement of Mercury | language=French
 | url=https://books.google.com/books?id=GdPdG14SNawC
 | location=Paris 
 | publisher=Bachelier
 |via=Google }}</ref> Predictions from Le Verrier's theory failed to match the observations.<ref name=hsu/>

Despite that, Le Verrier continued his work and, in 1859, published a more thorough study of Mercury's motion. That was based on a series of meridian observations of the planet and 14 transits. The study's rigor meant that any differences between the motion predicted and what was observed would point to the influence of an unknown factor. Indeed, some discrepancies remained.<ref name=hsu/> During Mercury's orbit, its [[perihelion]] advances by a small amount, something called [[perihelion precession]]. The observed value exceeds the classical mechanics prediction by the small amount of 43 [[arcsecond]]s per century.<ref>{{Cite web|url=http://www.math.toronto.edu/~colliand/426_03/Papers03/C_Pollock.pdf |archive-url=https://web.archive.org/web/20040808214429/http://www.math.toronto.edu/%7Ecolliand/426_03/Papers03/C_Pollock.pdf |archive-date=2004-08-08 |url-status=live|title=Mercury's Perihelion (2003). - |language=en|access-date=2018-11-14}}</ref>

Le Verrier postulated that the excess precession could be explained by the presence of some unidentified object or objects inside the orbit of Mercury. He calculated that it was either another Mercury-sized planet or, since it was unlikely that astronomers were failing to see such a large object, an unknown asteroid belt near the Sun.<ref>Thomas Levenson, The Hunt for Vulcan. . . And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe, Random House Publishing Group, 2015, page 80</ref>

The fact that Le Verrier had predicted the existence of the planet [[Neptune]] in 1846,<ref>{{cite journal
 | first=J. G. | last=Galle
 | bibcode=1846MNRAS...7..153G
 |title=Account of the discovery of the planet of Le Verrier at Berlin
 | journal=[[Monthly Notices of the Royal Astronomical Society]]
 | volume=7 | issue=9
 | page=153 | date=November 13, 1846
 | publisher=Blackwell Publishing 
 | doi = 10.1093/mnras/7.9.153 | doi-access=free
 }}</ref> using the same techniques, lent veracity to his claim.{{primary source inline|date=March 2022}}{{citation needed|date=March 2022}}

=== Claimed discovery ===
On 22 December 1859, Le Verrier received a letter from Lescarbault, saying that he had seen a transit of the hypothetical planet on March 26 of that year. Le Verrier took the train to the village of [[Orgères-en-Beauce]], some {{convert|70|km}} south-west of [[Paris]], to Lescarbault's home-made observatory. Le Verrier arrived unannounced and proceeded to interrogate the man.<ref name="Levenson">{{cite book |last1=Levenson |first1=Thomas |title=The hunt for Vulcan: ... and how Albert Einstein destroyed a planet, discovered relativity, and deciphered the universe |year=2015 |publisher=Random House |isbn=9780812998986 |edition=First |url-access=registration |url=https://archive.org/details/huntforvulcanand0000leve }}</ref>

Lescarbault described in detail how, on 26 March 1859, he observed a small black dot on the face of the [[Sun]].<ref>{{citation|url=https://books.google.com/books?id=1cohAQAAMAAJ&pg=PA336 |title=A Promised Transit of Vulcan|newspaper=[[The Spectator]]|volume=52|page=336|date=15 March 1879}}</ref> After some time had passed, he realized that it was moving. He thought it looked similar to the transit of [[Mercury (planet)|Mercury]] which he had observed in 1845. He estimated the distance it had already traveled, made some measurements of its position and direction of motion and, using an old clock and a pendulum with which he took his patients' pulses, estimated the total duration of the transit (coming up with 1 hour, 17 minutes, and 9 seconds).<ref name="Levenson" />

Le Verrier was not happy about Lescarbault's crude equipment but was satisfied the physician had seen the transit of a previously unknown planet. On 2 January 1860, he announced the discovery of the new planet with the proposed name from mythology, "Vulcan",<ref name=dumezil>{{cite book
 | first=Georges | last=Dumézil | author-link=Georges Dumézil
 | others=trans. Philip Krapp
 | title=Archaic Roman Religion: Volume One | orig-year=1966
 | date=1996 | publisher=[[Johns Hopkins University Press]]
 | location=Baltimore | isbn=978-0-8018-5482-8
 | pages=320–321 }}</ref> at the meeting of the [[Académie des Sciences]] in Paris. Lescarbault, for his part, was awarded the [[Légion d'honneur]] and invited to appear before numerous learned societies.<ref>{{cite news|last1=Proctor|first1=Richard A.|title=Leverrier and the Discovery of Neptune|url=https://www.nytimes.com/1877/09/30/archives/leverrier-and-the-discovery-of-neptune-tune.html|newspaper=[[The New York Times]]|date=September 30, 1877}}</ref>

However, not everyone accepted the veracity of Lescarbault's "discovery". An eminent French astronomer, [[Emmanuel Liais]], who was working for the Brazilian government in [[Rio de Janeiro]] in 1859, claimed to have been studying the surface of the Sun with a telescope twice as powerful as Lescarbault's, at the very moment that Lescarbault said he observed his mysterious transit. Liais, therefore, was "in a condition to deny, in the most positive manner, the passage of a planet over the sun at the time indicated".<ref>''Popular Science'', Volume 13, pages 732-735, 1878.</ref>

Based on Lescarbault's "transit", Le Verrier computed Vulcan's orbit: it supposedly revolved about the Sun in a nearly circular orbit at a distance of {{Convert|21|e6km|AU mi}}. The period of revolution was 19 days and 17 hours, and the orbit was inclined to the [[ecliptic]] by 12 degrees and 10 minutes (an incredible degree of precision). As seen from the Earth, Vulcan's greatest [[Elongation (astronomy)|elongation]] from the Sun was 8 degrees.<ref name="Levenson" />

=== Attempts to confirm the discovery ===
Numerous reports reached Le Verrier from other amateurs who claimed to have seen unexplained transits. Some of these reports referred to observations made many years earlier, and many were not dated, let alone accurately timed. Nevertheless, Le Verrier continued to tinker with Vulcan's orbital parameters as each newly reported sighting reached him. He frequently announced dates of future Vulcan transits. When these failed to materialize, he tinkered with the parameters some more.<ref>Jay B. Holberg, Sirius - Brightest Diamond in the Night Sky, Springer New York · 2007, pages 53-54</ref>

Shortly after 08:00 on 29 January 1860, F.A.R. Russell and three other people in London saw an alleged transit of an intra-Mercurial planet.<ref>''Nature'', 5 October 1876.</ref> Many years later, an American observer, Richard Covington, claimed to have seen a well-defined black spot progress across the Sun's disk around 1860 when he was stationed in [[Washington Territory]].<ref>''Scientific American'', 25 November 1876.</ref>

No observations of Vulcan were made in 1861. Then, on the morning of 20 March 1862, between 08:00 and 09:00 [[Greenwich Mean Time|Greenwich Time]], another amateur astronomer, a Mr. Lummis of Manchester, England, saw a transit. His colleague, whom he alerted, also saw the event.<ref>{{Cite web|url=http://adsbit.harvard.edu/full/seri/MNRAS/0022//0000232.000.html|title=1862MNRAS..22..232H Page 232|website=adsbit.harvard.edu|access-date=2019-04-10}}</ref> Based on these two men's reports, two French astronomers, [[Benjamin Valz]] and [[Rodolphe Radau]], independently calculated the object's supposed orbital period, with Valz deriving a figure of 17 days and 13 hours and Radau a figure of 19 days and 22 hours.<ref name=Baum/>{{rp|page=168}}

On 8 May 1865 another French astronomer, [[Aristide Coumbary]], observed an unexpected transit from [[Istanbul]], [[Turkey]].<ref name="Coumbary" />

Between 1866 and 1878, no reliable observations of the hypothetical planet were made. Then, during the total [[solar eclipse of July 29, 1878]], two experienced astronomers, Professor [[James Craig Watson]], the director of the [[Detroit Observatory|Ann Arbor Observatory]] in [[Michigan]], and [[Lewis Swift]], from [[Rochester, New York]], both claimed to have seen a Vulcan-type planet close to the Sun. Watson, observing from [[Separation Point, Wyoming]], placed the planet about 2.5 degrees south-west of the Sun and estimated its [[apparent magnitude|magnitude]] at 4.5. Swift, observing the eclipse from a location near [[Denver, Colorado]], saw what he took to be an intra-mercurial planet about 3 degrees south-west of the Sun. He estimated its brightness to be the same as that of [[Theta Cancri]], a fifth-magnitude star which was also visible during totality, about six or seven minutes from the "planet". Theta Cancri and the planet were nearly in line with the Sun's centre.{{citation needed|date=December 2022}}

Watson and Swift had reputations as excellent observers. Watson had already discovered more than twenty [[asteroid]]s, while Swift had several [[comet]]s named after him. Both described the colour of their hypothetical intra-mercurial planet as "red". Watson reported that it had a definite disk&mdash;unlike stars, which appear in telescopes as mere points of light&mdash;and that its phase indicated that it was on the far side of the Sun approaching [[superior conjunction]].<ref>John C. Barentine, Mystery of the Ashen Light of Venus - Investigating a 400-Year-Old Phenomenon, Springer International Publishing · 2021, page 211</ref>

Both Watson and Swift had observed two objects they believed were not known stars, but after Swift corrected an error in his coordinates, none of the coordinates matched each other, nor known stars. The idea that ''four'' objects were observed during the eclipse generated controversy in scientific journals and mockery from Watson's rival [[C. H. F. Peters]]. Peters noted that the margin of error in the pencil and cardboard recording device Watson had used was large enough to plausibly include a bright known star. A skeptic of the Vulcan hypothesis, Peters dismissed all the observations as mistaking known stars as planets.<ref name="baron" /> {{rp|pp=215–217}}

Astronomers continued searching for Vulcan during total solar eclipses in 1883, 1887, 1889, 1900, 1901, 1905, and 1908.<ref name="baron">{{cite book |title=American Eclipse |author=David Baron |year=2017 |publisher=Liveright |isbn=9781631490163}}</ref>{{rp|p=219}} Finally, in 1908, [[William Wallace Campbell]], Director, and [[Charles Dillon Perrine]], Astronomer, of the [[Lick Observatory]], after comprehensive photographic observations at three solar eclipse expeditions in 1901, 1905, and 1908, stated: "In our opinion, the work of the three Crocker Expeditions ... brings the observational side of the intermercurial planet problem{{mdash}}famous for half a century{{mdash}}definitely to a close."<ref>{{cite journal |last=Campbell |first=W. W. |title=Report of the Lick Observatory |journal=[[Publications of the Astronomical Society of the Pacific]] |date=1909 |volume=21 |issue=128 |pages=213–214}}</ref>

== Hypothesis disproved ==
In 1915 [[Albert Einstein|Einstein]]'s [[theory of relativity]], an approach to understanding gravity entirely differently from [[classical mechanics]], removed the need for Le Verrier's hypothetical planet.<ref name="Clemence"/> It showed that the peculiarities in Mercury's orbit were the results of the curvature of spacetime caused by the mass of the Sun.<ref>[https://www.oddsalon.com/jan-2-1860-the-discovery-of-planet-vulcan/ Kurt Larson, Jan 2, 1860: The Discovery of Planet Vulcan, oddsalon.com]</ref> This added a predicted 0.1 arc-second advance of Mercury's perihelion each orbital revolution, or 43 arc-seconds per century, exactly the observed amount (without any recourse to the existence of a hypothetical Vulcan).<ref>William Sheehan, Richard Baum, Vulcan Chase - The Search for an Inside Planet, Astronomy Magazine, December 1997, page 47</ref>

The new theory modified the predicted orbits of all planets, but the magnitude of the differences from Newtonian theory diminishes rapidly as one gets farther from the Sun. Also, Mercury's fairly eccentric orbit makes it much easier to detect the perihelion shift than is the case for the nearly circular orbits of [[Venus]] and [[Earth]]. Einstein's theory was empirically verified in the [[Eddington experiment]] during the [[solar eclipse of May 29, 1919]], during which photographs showed the curvature of spacetime was bending starlight around the Sun. Most astronomers quickly accepted that a large planet inside the orbit of Mercury could not exist, given the corrected equation of gravity.<ref name="baron" />{{rp|p=220}}

The [[International Astronomical Union]] has reserved the name '[[Vulcanoid]]" for asteroids that may exist inside the orbit of the planet Mercury. So far, however, earth- and space-based [[telescope]]s and the [[NASA]] [[Parker Solar Probe]] have detected no such asteroids.<ref>[https://www.iau.org/news/pressreleases/detail/iau1303/ Names for New Pluto Moons Accepted by the IAU After Public Vote, Press Releases - iau.org, 2 July 2013]</ref>

== See also ==
* [[Atira asteroid]]
* [[Fictional planets of the Solar System]]
* [[Hypothetical moon of Mercury]]
* [[Nemesis (hypothetical star)]]
* [[Planet Nine]]
* [[Planets beyond Neptune]]
* [[John H. Tice]], weather forecaster who based predictions on supposed movements of Vulcan
* [[Tyche (hypothetical planet)]]
* [[Vulcan (Star Trek)|Vulcan (''Star Trek'')]]
* [[Vulcanoid]]
* {{mpl|2021 PH|27}}, an [[Atira asteroid]] with an intra-Mercurian perihelion, the smallest semi-major axis and the shortest orbital period of all asteroids
* {{mpl|(434326) 2004 JG|6}}, an [[Atira asteroid]] with an intra-Mercurian perihelion
* [[Atira asteroid#Members|{{mp|2010 XB|11}}]], an [[Atira asteroid]] with an intra-Mercurian perihelion

== References ==
{{reflist|30em
| refs =

<ref name="Clemence">{{cite journal
 | first=G. M. | last=Clemence
 | title=The Relativity Effect in Planetary Motions
 | journal=Reviews of Modern Physics | volume=19
 | issue=4 | pages=361–364 | date=1947
 | doi=10.1103/RevModPhys.19.361 | bibcode=1947RvMP...19..361C}} [http://www.mathpages.com/rr/s6-02/6-02.htm (math)]</ref>

<ref name="Coumbary">{{cite journal
 | first=Aristide | last=Coumbary
| author2=Chambers, G. F.
 | title=Observation of a Supposed New Inferior Planet
 | journal=Astronomical Register | volume=3
 | page=214 | date=1865
 |bibcode=1865AReg....3..214C}}</ref>

}}

== Further reading ==
* {{Cite book |last=Baum |first=Richard |author2=Sheehan, William |date=1997 |title=In Search of Planet Vulcan, the Ghost in Newton's Clockwork Machine |url=https://archive.org/details/insearchofplanet0000baum |url-access=registration |location=New York |publisher=Plenum Press |isbn=978-0-306-45567-4}}
* {{Cite book |last=Levenson |first=Thomas |date=2015 |title=The Hunt for Vulcan: How Albert Einstein Destroyed a Planet and Deciphered the Universe |url=https://archive.org/details/huntforvulcanhow0000leve_j7g6 |url-access=registration |location=London |publisher=Head of Zeus |isbn=978-0812998986}} Originally published as [https://books.google.com/books?id=6cucDAAAQBAJ ''The Hunt for Vulcan: ... And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe''].
* The subject was also featured on an episode of ''[[Arthur C. Clarke's Mysterious World]]'' entitled "Strange Skies", originally broadcast on November 18, 1980.

== External links ==
{{Wikisource|Littell's Living Age/Volume 131/Issue 1690/The Planet Vulcan|1876 article on Vulcan in ''Littell's Living Age''}}
* [[Isaac Asimov|Asimov, Isaac]] (1975). [http://members.tripod.com/~geobeck/frontier/planet.htm "The Planet That Wasn't"], ''[[The Magazine of Fantasy and Science Fiction]]''
* Schlyter, Paul (2006).  [http://www.nineplanets.org/hypo.html#vulcan "Vulcan, the intra-Mercurial planet, 1860–1916, 1971"], ''The <s>Nine</s>8 Planets: A Multimedia Tour of the Solar System'' (Appendix 7: "Hypothetical Planets") converted to HTML by Bill Arnett.
* "[https://books.google.com/books?id=p4o9AQAAIAAJ&pg=PA128 The Planet Vulcan]", ''[[Scientific American]]'', 31 August 1878, p.&nbsp;128, columns 2–3

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