The Planet Vulcan, Pluto and New Horizons

United States - Scott #2577 (1991): First Day Cover from the Jet Propulsion Laboratory (NASA) Stamp Club, autographed by stamp designer Ron Miller.
United States – Scott #2577 (1991): First Day Cover from the Jet Propulsion Laboratory (NASA) Stamp Club, autographed by stamp designer Ron Miller.

On January 2, 1860, the discovery of the planet Vulcan was announced at a meeting of the Académie des Sciences in Paris by French mathematician Urbain Le Verrier. This announcement was based on observations made by physician and amateur astronomer Edmond Modeste Lescarbault, who claimed to have seen a transit of a previously-unknown planet in March of the previous year. Not to be confused with Spock’s home planet in the Star Trek TV series and later movies, this Vulcan supposedly revolved about the Sun in a nearly circular orbit at a distance of 13 million miles (0.14 AU or 21 million kilometers) between the Sun and planet Mercury. According to Le Verrier, its 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 from the Sun was 8 degrees.

A number of reputable investigators became involved in the search for Vulcan.  Lescarbault, for his part, was awarded the Légion d’honneur and invited to appear before numerous learned societies. No such planet was ever found, and the peculiarities in Mercury’s orbit have now been explained by Albert Einstein’s theory of general relativity. Searches of data gathered by NASA’s two STEREO spacecraft have failed to find any vulcanoids that could have accounted for claimed observations of Vulcan. It is doubtful that there are any vulcanoids larger than 3,5 miles (5.7 kilometers) in diameter. Other than Mercury, asteroid 2007 EB26, whose orbit has a semi-major axis of 0.55 AU (51,000,000 miles or 82,000,000 km) has the smallest known semi-major axis of any known object orbiting the Sun.

The name and presumed location of planet Vulcan was not new in 1860 and in fact appears on this map of the Solar System dated to 1846. Pictured is the detail at the center of a lithograph titled,
The name and presumed location of planet Vulcan was not new in 1860 and in fact appears on this map of the Solar System dated to 1846. Pictured is the detail at the center of a lithograph titled, “A plan or map of the Solar System projected for schools & academies by Hall Colby, Rochester, N.Y.” it was deposited in the Library of Congress on October 8, 1846. Between Mars and Jupiter, the hypothetical planets of Vesta, Juno, Ceres, and Pallas are also shown.

In 1840, François Arago, the director of the Paris Observatory, suggested to Urbain Le Verrier that he work on the topic of the planet Mercury’s orbital motion around the Sun. The goal of this study was to construct a model based on Sir Isaac Newton’s laws of motion and gravitation. By 1843, Le Verrier published his provisional theory on the subject, which would be tested during a transit of Mercury across the face of the Sun in 1843. As it turned out, predictions from Le Verrier’s theory failed to match the observations.

Le Verrier renewed his work and, in 1859, published a more thorough study of Mercury’s motion. This was based on a series of meridian observations of the planet as well as 14 transits. The rigor of this study meant that any differences from observation would be caused by some unknown factor. Indeed, there still remained some discrepancy. During Mercury’s orbit, its perihelion advances by a small amount each orbit, technically called perihelion precession. The phenomenon is predicted by classical mechanics, but the observed value differed from the predicted value by the small amount of 43 arcseconds per century.

Le Verrier postulated that the excess precession could be explained by the presence of a small planet inside the orbit of Mercury, and he proposed the name “Vulcan” for this object. In Roman mythology, Vulcan was the god of beneficial and hindering fire, including the fire of volcanoes, making it an apt name for a planet so close to the Sun. Le Verrier’s recent success in discovering the planet Neptune using the same techniques lent veracity to his claim, and astronomers around the world attempted to observe a new planet there, but nothing was ever found.

On December 22, 1859, Le Verrier received a letter from French physician and amateur astronomer Edmond Modeste Lescarbault, who claimed to have seen a transit of the hypothetical planet earlier in the year. Le Verrier took the train to the village of Orgères-en-Beauce, some 43 miles (70 km) southwest of Paris, where Lescarbault had built himself a small observatory. Le Verrier arrived unannounced and proceeded to interrogate the man.

Lescarbault described in detail how, on March 26, 1859, he noticed a small black dot on the face of the Sun, which he was studying with his modest 3.75 inches (95 mm) refractor. Thinking it to be a sunspot, Lescarbault was not at first surprised, but after some time had passed he realized that it was moving. Having observed the transit of Mercury in 1845, he guessed that what he was observing was another transit, but of a previously undiscovered body. He took some hasty measurements of its position and direction of motion, and using an old clock and a pendulum with which he took his patients’ pulses, he estimated the duration of the transit at 1 hour, 17 minutes and 9 seconds.

Le Verrier was satisfied that Lescarbault had seen the transit of a previously unknown planet. On January 2, 1860, he announced the discovery of Vulcan. Not everyone accepted the veracity of Lescarbault’s “discovery”, however. 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”.

Numerous reports — all of them unreliable — began to reach 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 could not be properly dated. Nevertheless, Le Verrier continued to tinker with Vulcan’s orbital parameters as each new reported sighting reached him. He frequently announced dates of future Vulcan transits, and when these failed to materialize, he tinkered with the parameters some more.

Among the earlier alleged observers of Vulcan, the following are the most noteworthy:

  • Capel Lofft reported “an opaque body traversing the suns disc” on January 6, 1818.
  • Bavarian physician and astronomer Franz von Gruithuisen reported seeing “two small spots…on the Sun, round, black and unequal in size” on June 26, 1819.
  • Pastorff claimed to have seen two spots on October 23, 1822, July 24 and 25, 1823, six times in 1834, on October 18, 1836, November 1, 1836, and on February 16, 1837; the larger was 3 arcseconds across, and the smaller 1.25 arcseconds.

Shortly after eight o’clock on the morning of January 29, 1860, F.A.R. Russell and three other people saw an alleged transit of an intra-Mercurial planet from London. An American observer, Richard Covington, many years later claimed to have seen a well-defined black spot progress across the Sun’s disk around 1860, when he was stationed in Washington Territory.

Map of a different Vulcan by the United Federation of Planets (Star Trek).
Map of a different Vulcan by the United Federation of Planets (Star Trek).

No “observations” of Vulcan were made in 1861. Then, on the morning of March 22, 1862, between eight and nine o’clock Greenwich Time, another amateur astronomer, a Mr. Lummis of Manchester, England, saw a transit. His colleague, whom he alerted, also saw the event. 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.

On May 8, 1865, another French astronomer, Aristide Coumbary, observed an unexpected transit from Istanbul, Turkey.

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 Ann Arbor Observatory in Michigan, and Lewis Swift, an amateur from Rochester, New York, both claimed to have seen a Vulcan-type planet close to the Sun. Watson, observing from Separation, Wyoming, placed the planet about 2.5 degrees southwest of the Sun and estimated its magnitude at 4.5. Swift, who was observing the eclipse from a location near Denver, Colorado, saw what he took to be an intra-mercurial planet about 3 degrees southwest 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 very nearly in line with the center of the Sun.

Watson and Swift had reputations as excellent observers. Watson had already discovered more than twenty asteroids, while Swift had several comets named after him. Both described the color of their hypothetical intra-mercurial planet as “red”. Watson reported that it had a definite disk — unlike stars, which appear in telescopes as mere points of light — and that its phase indicated that it was approaching superior conjunction.

These are merely the more “reliable observations” of alleged intra-Mercurial planets. For half a century or more, many other observers tried to find the hypothetical Vulcan. Many false alarms were triggered by round sunspots that closely resembled planets in transit. During solar eclipses, stars close to the Sun were mistaken for planets. At one point, to reconcile different observations, at least two intra-mercurial planets were postulated.

Le Verrier died in 1877 convinced to the end of having discovered another planet. With the loss of its principal proponent, however, the search for Vulcan abated. After many years of searching, astronomers were seriously doubting the planet’s existence.

In 1915, Albert Einstein’s theory of relativity, an entirely different approach to understanding gravity from classical mechanics, solved the problem. His equations predicted exactly the observed amount of advance of Mercury’s perihelion without any recourse to the existence of a hypothetical Vulcan. 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.

Observing a planet inside the orbit of Mercury is difficult, since the telescope must be pointed very close to the Sun, where the sky is only dark during a solar eclipse. Also, an error in pointing the telescope can result in damage for the optics, and injury to the observer if viewing directly. The huge amount of light present even quite far away from the Sun can produce false reflections inside the optics, thus fooling the observer into seeing things that do not exist.

The best observational strategy might be to monitor the Sun’s disk for possible transits, but transits would only be seen from Earth provided the object orbits close enough to the ecliptic plane. A small, dark spot might be seen to move across the Sun’s disk, as happens with transits of Mercury and Venus.

Albert Einstein during a 1921 lecture in Vienna.
Albert Einstein during a 1921 lecture in Vienna.

In 1915, when Einstein successfully explained the apparent anomaly in Mercury’s orbit, most astronomers abandoned the search for Vulcan. A few, however, remained convinced that not all the alleged observations of Vulcan were unfounded. Among these was Henry C Courten, of Dowling College, New York. Studying photographic plates of the 1970 eclipse of the Sun, he and his associates detected several objects which appeared to be in orbits close to the Sun. Even accounting for artifacts, Courten felt that at least seven of the objects were real.

Courten believed that an intra-Mercurial planetoid between 80 and 110 miles (130 and 180 km) in diameter was orbiting the Sun at a distance of about 0.1 AU (9,300,000 miles or 15,000,000 km). Other images on his eclipse plates led him to postulate the existence of an asteroid belt between Mercury and the Sun.

None of these claims has ever been substantiated after more than forty years of observation. It has been surmised, however, that some of these objects — and other alleged intra-Mercurial objects — may exist, being nothing more than previously unknown comets or small asteroids. Today, the search continues for these so-called vulcanoid asteroids, which are thought to exist in the region where Vulcan was once sought. None have been found yet and searches have ruled out any such asteroids larger than about 3.7 miles (6 km). Neither SOHO nor STEREO has detected a planet inside the orbit of Mercury.

[url=https://flic.kr/p/234eRMo][img]https://farm5.staticflickr.com/4682/39418059492_9db4925146_o.png[/img][/url][url=https://flic.kr/p/234eRMo]Vulcanoidorbits[/url] by [url=https://www.flickr.com/photos/am-jochim/]Mark Jochim[/url], on Flickr
Vulcanoid orbits

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and culminated at the start of the 20th century with Percival Lowell’s quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.

In the 1840s, Urbain Le Verrier used Newtonian mechanics to analyze perturbations in the orbit of Uranus, and hypothesized that they were caused by the gravitational pull of a yet-undiscovered planet. Le Verrier predicted the position of this new planet and sent his calculations to German astronomer Johann Gottfried Galle. On September 23, 1846, the night following his receipt of the letter, Galle and his student Heinrich d’Arrest discovered Neptune, exactly where Le Verrier had predicted. There remained some slight discrepancies in the giant planets’ orbits. These were taken to indicate the existence of yet another planet orbiting beyond Neptune.

Even before Neptune’s discovery, some speculated that one planet alone was not enough to explain the discrepancy. On November 17, 1834, the British amateur astronomer the Reverend Thomas John Hussey reported a conversation he had had with French astronomer Alexis Bouvard to George Biddell Airy, the British Astronomer Royal. Hussey reported that when he suggested to Bouvard that the unusual motion of Uranus might be due to the gravitational influence of an undiscovered planet, Bouvard replied that the idea had occurred to him, and that he had corresponded with Peter Andreas Hansen, director of the Seeberg Observatory in Gotha, about the subject. Hansen’s opinion was that a single body could not adequately explain the motion of Uranus, and postulated that two planets lay beyond Uranus.

In 1848, Jacques Babinet raised an objection to Le Verrier’s calculations, claiming that Neptune’s observed mass was smaller and its orbit larger than Le Verrier had initially predicted. He postulated, based largely on simple subtraction from Le Verrier’s calculations, that another planet of roughly 12 Earth masses, which he named “Hyperion”, must exist beyond Neptune. Le Verrier denounced Babinet’s hypothesis, saying, “[There is] absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part.”

In 1850 James Ferguson, Assistant Astronomer at the United States Naval Observatory, noted that he had “lost” a star he had observed, GR1719k, which Lt. Matthew Maury, the superintendent of the Observatory, claimed was evidence that it must be a new planet. Subsequent searches failed to recover the “planet” in a different position, and in 1878, CHF Peters, director of the Hamilton College Observatory in New York, showed that the star had not in fact vanished, and that the previous results had been due to human error.

Percival Lowell, originator of the Planet X hypothesis, observing Venus in the daytime from the observer's chair of the 24-inch (61 cm) Alvan Clark & Sons refracting telescope, installed in the summer of 1896 at the Lowell Observatory, which he established in Flagstaff, Arizona. Photo taken on October 17, 1914.
Percival Lowell, originator of the Planet X hypothesis, observing Venus in the daytime from the observer’s chair of the 24-inch (61 cm) Alvan Clark & Sons refracting telescope, installed in the summer of 1896 at the Lowell Observatory, which he established in Flagstaff, Arizona. Photo taken on October 17, 1914.

In 1894, with the help of William Pickering, Percival Lowell, a wealthy Bostonian, founded the Lowell Observatory in Flagstaff, Arizona. In 1906, convinced he could resolve the conundrum of Uranus’s orbit, he began an extensive project to search for a trans-Neptunian planet, which he named Planet X, a name previously used by Gabriel Dallet. The X in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for 10 (at the time, Planet X would have been the ninth planet). Lowell’s hope in tracking down Planet X was to establish his scientific credibility, which had eluded him due to his widely derided belief that channel-like features visible on the surface of Mars were canals constructed by an intelligent civilization

Lowell’s first search focused on the ecliptic, the plane encompassed by the zodiac where the other planets in the Solar System lie. Using a 5-inch photographic camera, he manually examined over 200 three-hour exposures with a magnifying glass, and found no planets. At that time Pluto was too far above the ecliptic to be imaged by the survey. After revising his predicted possible locations, Lowell conducted a second search from 1914 to 1916. In 1915, he published his Memoir of a Trans-Neptunian Planet, in which he concluded that Planet X had a mass roughly seven times that of Earth — about half that of Neptune — and a mean distance from the Sun of 43 AU. He assumed Planet X would be a large, low-density object with a high albedo, like the giant planets. As a result, it would show a disc with diameter of about one arcsecond and an apparent magnitude of between 12 and 13 — bright enough to be spotted.

Separately, in 1908, Pickering announced that, by analyzing irregularities in Uranus’s orbit, he had found evidence for a ninth planet. His hypothetical planet, which he termed “Planet O” (because it came after “N”, i.e. Neptune), possessed a mean orbital radius of 51.9 AU and an orbital period of 373.5 years. Plates taken at his observatory in Arequipa, Peru, showed no evidence for the predicted planet, and British astronomer P. H. Cowell showed that the irregularities observed in Uranus’s orbit virtually disappeared once the planet’s displacement of longitude was taken into account. Lowell himself, despite his close association with Pickering, dismissed Planet O out of hand, saying, “This planet is very properly designated “O”, [for it] is nothing at all.” Unbeknownst to Pickering, four of the photographic plates taken in the search for “Planet O” by astronomers at the Mount Wilson Observatory in 1919 captured images of Pluto, though this was only recognized years later. Pickering went on to suggest many other possible trans-Neptunian planets up to the year 1932, which he named P, Q, R, S, T and U; none were ever detected.

The astronomer Clyde Tombaugh, discoverer of Pluto here shown with his homemade 9-inch telescope in Kansas, circa 1930.
The astronomer Clyde Tombaugh, discoverer of Pluto here shown with his homemade 9-inch telescope in Kansas, circa 1930.

Lowell’s sudden death in 1916 temporarily halted the search for Planet X. Failing to find the planet, according to one friend, “virtually killed him”. Lowell’s widow, Constance, engaged in a legal battle with the observatory over Lowell’s legacy which halted the search for Planet X for several years. In 1925, the observatory obtained glass discs for a new 13-inch (33 cm) wide-field telescope to continue the search, constructed with funds from Abbott Lawrence Lowell, Percival’s brother. In 1929, the observatory’s director, Vesto Melvin Slipher, summarily handed the job of locating the planet to Clyde Tombaugh, a 22-year-old Kansas farm boy who had only just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.

Tombaugh’s task was to systematically capture sections of the night sky in pairs of images. Each image in a pair was taken two weeks apart. He then placed both images of each section in a machine called a blink comparator, which by exchanging images quickly created a time lapse illusion of the movement of any planetary body. To reduce the chances that a faster-moving (and thus closer) object be mistaken for the new planet, Tombaugh imaged each region near its opposition point, 180 degrees from the Sun, where the apparent retrograde motion for objects beyond Earth’s orbit is at its strongest. He also took a third image as a control to eliminate any false results caused by defects in an individual plate. Tombaugh decided to image the entire zodiac, rather than focus on those regions suggested by Lowell.

By the beginning of 1930, Tombaugh’s search had reached the constellation of Gemini. On February 18, 1930, after searching for nearly a year and examining nearly 2 million stars, Tombaugh discovered a moving object on photographic plates taken on January 23 and 29 of that year. A lesser-quality photograph taken on January 21 confirmed the movement. Upon confirmation, Tombaugh walked into Slipher’s office and declared, “Doctor Slipher, I have found your Planet X.” The object lay just six degrees from one of two locations for Planet X Lowell had suggested. It seemed he had at last been vindicated.

After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The new object was later precovered on photographs dating back to March 19, 1915. The decision to name the object Pluto was intended in part to honor Percival Lowell, as his initials made up the word’s first two letters. After discovering Pluto, Tombaugh continued to search the ecliptic for other distant objects. He found hundreds of variable stars and asteroids, as well as two comets, but no further planets.

NASA's New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. The image combines blue, red and infrared images taken by the Ralph/Multispectral Visual Imaging Camera (MVIC). Pluto’s surface sports a remarkable range of subtle colors, enhanced in this view to a rainbow of pale blues, yellows, oranges, and deep reds. Many landforms have their own distinct colors, telling a complex geological and climatological story that scientists have only just begun to decode. The image resolves details and colors on scales as small as 0.8 miles (1.3 kilometers). The viewer is encouraged to zoom in on the image on a larger screen to fully appreciate the complexity of Pluto’s surface features.
NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. The image combines blue, red and infrared images taken by the Ralph/Multispectral Visual Imaging Camera (MVIC). Pluto’s surface sports a remarkable range of subtle colors, enhanced in this view to a rainbow of pale blues, yellows, oranges, and deep reds. Many landforms have their own distinct colors, telling a complex geological and climatological story that scientists have only just begun to decode. The image resolves details and colors on scales as small as 0.8 miles (1.3 kilometers). The viewer is encouraged to zoom in on the image on a larger screen to fully appreciate the complexity of Pluto’s surface features.

To the observatory’s disappointment and surprise, Pluto showed no visible disc; it appeared as a point, no different from a star, and, at only 15th magnitude, was six times dimmer than Lowell had predicted, which meant it was either very small, or very dark. Because Lowell astronomers thought Pluto was massive enough to perturb planets, they assumed that its albedo could be no less than 0.07 (meaning that it reflected only 7% of the light that hit it); about as dark as asphalt and similar to that of Mercury, the least reflective planet known. This would give Pluto an estimated mass of no more than 70% that of Earth. Observations also revealed that Pluto’s orbit was very elliptical, far more than that of any other planet.

Almost immediately, some astronomers questioned Pluto’s status as a planet. Barely a month after its discovery was announced, on April 14, 1930, in an article in the New York Times, Armin O. Leuschner suggested that Pluto’s dimness and high orbital eccentricity made it more similar to an asteroid or comet: “The Lowell result confirms the possible high eccentricity announced by us on April 5. Among the possibilities are a large asteroid greatly disturbed in its orbit by close approach to a major planet such as Jupiter, or it may be one of many long-period planetary objects yet to be discovered, or a bright cometary object.” In that same article, Harvard Observatory director Harlow Shapley wrote that Pluto was a “member of the Solar System not comparable with known asteroids and comets, and perhaps of greater importance to cosmogony than would be another major planet beyond Neptune.” In 1931, using a mathematical formula, Ernest W. Brown asserted (in agreement with E. C. Bower), that the presumed irregularities in the orbit of Uranus could not be due to the gravitational effect of a more distant planet, and thus that Lowell’s supposed prediction was “purely accidental”.

Throughout the mid-20th century, estimates of Pluto’s mass were revised downward. In 1931, Nicholson and Mayall calculated its mass, based on its supposed effect on the giant planets, as roughly that of Earth; a value somewhat in accord with the 0.91 Earth mass calculated in 1942 by Lloyd R. Wylie at the U.S. Naval Observatory, using the same assumptions. In 1949, Gerard Kuiper’s measurements of Pluto’s diameter with the 200-inch telescope at Mount Palomar Observatory led him to the conclusion that it was midway in size between Mercury and Mars and that its mass was most probably about 0.1 Earth mass.

Diameter comparison of Pluto, Moon, and Earth. Scale: Approximately 28.9 km per pixel. Images from NASA, March 20, 2015.
Diameter comparison of Pluto, Moon, and Earth. Scale: Approximately 28.9 km per pixel. Images from NASA, March 20, 2015.

In 1973, Dennis Rawlins conjectured, based on the similarities in the periodicity and amplitude of brightness variation between Pluto and Neptune’s moon Triton, that Pluto’s mass must be similar to Triton’s. This is, in fact, true, and had been argued by astronomers Walter Baade and E. C. Bower as early as 1934. However, because Triton’s mass was then believed to be roughly 2.5% that of the Earth–Moon system (more than ten times its actual value), Rawlins’s determination for Pluto’s mass was similarly incorrect. It was nonetheless a meagre enough value for him to conclude that Pluto was not Planet X. In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii analyzed spectra from Pluto’s surface and determined that it must contain methane ice, which is highly reflective. This meant that Pluto, far from being dark, was in fact exceptionally bright, and thus was probably no more than 0.01 Earth mass.

Pluto’s size was finally determined conclusively in 1978, when American astronomer James W. Christy discovered its moon Charon. This enabled him, together with Robert Sutton Harrington of the U.S. Naval Observatory, to measure the mass of the Pluto–Charon system directly by observing the moon’s orbital motion around Pluto. They determined Pluto’s mass to be 1.31×1022 kg; roughly one five-hundredth that of Earth or one-sixth that of the Moon, and far too small to account for the observed discrepancies in the orbits of the outer planets. Lowell’s “prediction” had been a coincidence: If there was a Planet X, it was not Pluto.

On October 1, 1991, the U.S. Postal Service issued  a set of ten 29-cent Space Exploration commemorative stamps in Pasadena, CA (Scott #2568-2577). The stamps featured the nine planets and the Earth’s Moon along with the unmanned spacecraft that had visited each (with the exception of Pluto which bore the inscription “Not Yet Explored“). The promotion of the stamps was tied to the annual October celebration of National Stamp Collecting Month, with the theme “Journey to a New Frontier…Collect Stamps.” Leonard Nimoy unveiled the stamps aboard the bridge of the USS Enterprise, the starship made famous in the TV series Star Trek.  “Mr. Spock” was the official spokesman for the stamp promotion.

The designer of the stamps was Ron Miller, of Fredericksburg, Virginia. Art director and typographer was Howard Paine, Design Coordinator for the Citizens’ Stamp Advisory Committee. The Postal Service manager was Jack Williams, Program Manager of Philatelic Design. They were printed in the gravure process at the Bureau of Printing and Engraving, and were issued in booklets of 20 (2 panes of 10), perforated 11 on two or three sides, with a total of 333,948,000 stamps issued.

After 1992, Pluto’s status as a planet was questioned following the discovery of many  bodies of similar size orbiting, showing that it is part of a population of objects called the Kuiper belt. Many scientists questioned whether Pluto should be considered together with or separately from its surrounding population. Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the Solar System. The Hayden Planetarium reopened — in February 2000, after renovations — with a model of only eight planets, which made headlines almost a year later.

As objects increasingly closer in size to Pluto were discovered in the region, it was argued that Pluto should be reclassified as one of the Kuiper belt objects, just as Ceres, Pallas, Juno and Vesta lost their planet status after the discovery of many other asteroids. On July 29, 2005, astronomers at Caltech announced the discovery of a new trans-Neptunian object, Eris, which was substantially more massive than Pluto and the most massive object discovered in the Solar System since Triton in 1846. Its discoverers and the press initially called it the tenth planet, although there was no official consensus at the time on whether to call it a planet. Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.

Non-planets of the Solar System
Non-planets of the Solar System

The debate came to a head on August 24, 2006, with an International Astronomical Union (IAU) resolution that created an official definition for the term “planet”. According to this resolution, there are three conditions for an object in the Solar System to be considered a planet:

  • The object must be in orbit around the Sun.
  • The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by hydrostatic equilibrium.
  • It must have cleared the neighborhood around its orbit.

Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth’s mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2 but do not meet criterion 3 would be called dwarf planets. On September 13, 2006, the IAU included Pluto, and Eris and its moon Dysnomia, in their Minor Planet Catalogue, giving them the official minor planet designations “(134340) Pluto”, “(136199) Eris”, and “(136199) Eris I Dysnomia”. Had Pluto been included upon its discovery in 1930, it would have likely been designated 1164, following 1163 Saga, which was discovered a month earlier.

There has been some resistance within the astronomical community toward the reclassification. Alan Stern, principal investigator with NASA’s New Horizons mission to Pluto, derided the IAU resolution, stating that “the definition stinks, for technical reasons”. Stern contended that, by the terms of the new definition, Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded. He argued that all big spherical moons, including the Moon, should likewise be considered planets. He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community. Marc W. Buie, then at the Lowell Observatory petitioned against the definition.

Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said “through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involve

Plot of the positions of all known Kuiper belt objects (green), set against the outer planets (blue). Axes list distances in AU, projected onto the ecliptic, with ecliptic longitude zero being to the right, along the
Plot of the positions of all known Kuiper belt objects (green), set against the outer planets (blue). Axes list distances in AU, projected onto the ecliptic, with ecliptic longitude zero being to the right, along the “x” axis). Radial “spokes” of higher density in this image, or gaps in particular directions are due to observational bias (i.e. where objects were searched for), rather than any real physical structure. The pronounced gap at the bottom is due to obscuration by the band of the Milky Way.

Public reception to the IAU decision was mixed. Many accepted the reclassification, but some sought to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California State Assembly facetiously called the IAU decision a “scientific heresy”. The New Mexico House of Representatives passed a resolution in honor of Tombaugh, a longtime resident of that state, that declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007, was Pluto Planet Day. The Illinois Senate passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was “unfairly downgraded to a ‘dwarf’ planet” by the IAU.” Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.

In 2006, in its 17th annual words-of-the-year vote, the American Dialect Society voted plutoed as the word of the year. To “pluto” is to “demote or devalue someone or something”.

Researchers on both sides of the debate gathered on August 14–16, 2008, at the Johns Hopkins University Applied Physics Laboratory for a conference that included back-to-back talks on the current IAU definition of a planet. Entitled “The Great Planet Debate”, the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet. On June 11, 2008, the IAU had announced in a press release that the term “plutoid” would henceforth be used to refer to Pluto and other objects that have an orbital semi-major axis greater than that of Neptune and enough mass to be of near-spherical shape.

Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this global view of Pluto. The lower right edge of Pluto in this view currently lacks high-resolution color coverage. The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away, show features as small as 1.4 miles (2.2 kilometers). The north polar region is at top, with bright Tombaugh Regio to the lower right of center and part of the dark Cthulhu Regio at lower left. Part of the dark Krun Regio is also visible at extreme lower right. Images from NASA on July 14, 2015, released July 25, 2015.
Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this global view of Pluto. The lower right edge of Pluto in this view currently lacks high-resolution color coverage. The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away, show features as small as 1.4 miles (2.2 kilometers). The north polar region is at top, with bright Tombaugh Regio to the lower right of center and part of the dark Cthulhu Regio at lower left. Part of the dark Krun Regio is also visible at extreme lower right. Images from NASA on July 14, 2015, released July 25, 2015.

With its reclassification, Pluto is the largest and second-most-massive known dwarf planet in the Solar System, and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small — about one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.5 AU).

Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body.

On July 14, 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. The release of the 1991 Space Exploration stamps, in particular Scott #2577, actually inspired the mission.  The set featured a stamp for all planets, displaying an image of the planet and highlighting an associated spacecraft which was sent to it. The stamp for Pluto, however, depicted a featureless sphere, presented with the phrase “Not Yet Explored” in place of the name of a spacecraft. The stamps were unveiled in a ceremony at the Jet Propulsion Laboratory. Two scientists who attended the event, World Space Foundation president Robert Staehle and JPL scientist Stacy Weinstein, were inspired by Pluto’s status on the stamp, such that they started to inquire about the feasibility of sending a spacecraft to Pluto. Engineers at the Jet Propulsion Laboratory, inspired by the “Not Yet Explored” status of Pluto, also started to put forward ideas about a mission to Pluto.

In August 1992, Staehle telephoned Pluto’s discoverer, Clyde Tombaugh, requesting permission to visit his planet. “I told him he was welcome to it”, Tombaugh later remembered, “though he’s got to go one long, cold trip”. That year, Staehle, with the help of JPL engineers and students from the California Institute of Technology, formed the Pluto Fast Flyby project. The mission heralded the same ideology as the Pluto 350 concept: small in size and cost-effective in scope, so that the spacecraft would be able to get to Pluto faster and be affordable to develop and launch. Described as a “radical” mission concept, the mission would see two spacecraft being sent to Pluto. Both spacecraft were to weigh only around 35-50 kilograms each (including 7 kg worth of scientific instruments), and the project would cost less than US $500 million to develop, excluding launch costs. Described by Staehle as a “faster, better, [and] cheaper” approach than the Pluto 350 and Mariner Mark II projects, it caught the attention of then-NASA Administrator Daniel S. Goldin, who ordered all work on both Pluto 350 and Mariner Mark II to cease and shift all resources to the new Pluto Fast Flyby project instead.

New Horizons space probe
New Horizons space probe

New Horizons was launched successfully on January 19, 2006. The mission leader, S. Alan Stern, confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft. The stamp itself was also on board, Stern explained, “For many years, people had waved that stamp around as sort of a call to arms, as a motivating graphic: ‘Not yet explored’. That stamp had been in so many presentations by that point, I knew it would please people to have it go along.”

New Horizons captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager. The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft’s ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a gravity assist from Jupiter.

On February 4, 2015, NASA released new images of Pluto (taken on January 25 and 27) from the approaching probe. New Horizons was more than 126,000,000 miles (203,000,000 km) away from Pluto when it began taking the photos, which showed Pluto and its largest moon, Charon. On March 20, 2015, NASA invited the general public to suggest names for surface features that will be discovered on Pluto and Charon. On April 15, 2015, Pluto was imaged showing a possible polar cap. Between April and June 2015, New Horizons began returning images of Pluto that exceeded the quality that the Hubble Space Telescope could produce.

Pluto’s small moons, discovered shortly before and after the probe’s launch, were considered to be potentially hazardous, as debris from collisions between them and other Kuiper belt objects could have produced a tenuous dusty ring. If New Horizons had travelled through such a ring system, there would have been an increased risk of potentially disabling micrometeoroid damage.

Artist's concept of New Horizon's fly-by of Pluto and Charon on July 14, 2015.
Artist’s concept of New Horizon’s fly-by of Pluto and Charon on July 14, 2015.

New Horizons had its closest approach to Pluto on July 14, 2015 — after a 3,462-day journey across the Solar System. Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter. New Horizons used a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyze Pluto’s neutral atmosphere and its escape rate. New Horizons also photographed the surfaces of Pluto and Charon.

Photographs of Pluto taken on July 14, 2015, taken 15 minutes after New Horizon‘s closest approach, from a distance of 18,000 kilometers and sent to Earth on September 13, 2015, show a near-sunset on Pluto with details of the surface and a haze in the atmosphere. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and other gases released from the atmosphere of Pluto and transferred about 12,000 miles (19,000) to the orbiting moon.

On May 31, 2016, the United States Postal Service released a new set of stamps under the title “Views of Our Planets”, portraying striking images of each of the eight planets (Scott #5069=5076). These were accompanied by two se-tenant stamps, one portraying Pluto and the other picturing the New Horizons space probe (Scott #5077-5078). The 47-cent self-adhesive stamps were designed by art director Antonio Alcala. The USPS called this set “Pluto Explored!”

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