In 2006, the International Astronomical Union reclassified Pluto from a planet to a dwarf planet. Many perceived the change in status as an attempt to disregard the former ninth planet, and there is still a debate as to whether the IAU should modify its definitions to once again include Pluto as a planet.
But Pluto is not the first to lose its planetary status.
In the late 18th century, German astronomer Johann Elert Bode theorized the existence of a planet at roughly 260 million miles from the Sun — between the orbits of Mars and Jupiter. By 1801, Italian astronomer Giuseppe Piazzi discovered Ceres in 1801 in the orbit predicted by Bode. Ceres became the eighth planet — Uranus had been confirmed just twenty years earlier, and it would be an additional twenty years before orbital observations led to speculation of another massive object beyond Uranus, now known to be Neptune… and twenty years more until Neptune was directly observed and confirmed in 1846.
“2006-16-a-full-1-” by NASA, ESA, and A. Schaller (for STScI) – Hubblesite STScI-2006-16. Licensed under Creative Commons Attribution 3.0 via Wikimedia Commons
Another astronomer, Heinrich Olbers, also spotted Ceres in late December 1801, based on its predicted orbital path. Within just a few months, in March 1802, Olbers noticed an additional object near Ceres; he had just discovered Pallas, the ninth named planet. In September of 1803, after just eighteen months, another object was discovered by astronomer Karl Ludwig Harding: Juno, the tenth planet. Olbers continued his observations of the night sky, and in March of 1807 discovered Vesta, the eleventh named planet.
For nearly forty years, the world knew eleven named planets, until in 1845, amatuer astronomer Karl Ludwig Hencke discovered a twelfth object between the orbits of Mars and Jupiter, subsequently named Astraea. With the confirmation of Neptune’s existence in 1846, there were then thirteen planets. Beginning within months of Neptune’s discovery, from 1847 through 1851 another eight objects were confirmed in orbits between Mars and Jupiter: Hebe, Iris, Flora, Metis, Hygiea, Parthenope, Victoria and Egeria.
In 1851, with the number of named planets already in the dozens and continuing to climb, astronomer Johann Franze Encke made a change to the notation for the “star-like” objects that had been found between Mars and Jupiter. And in the 1854 Berlin Astronomical Yearbook, Encke introduced the use of a circle (the traditional symbol for a star) with a number inscribed in the center noting order of discovery, and assigned the first symbol for an asteroid to (1) Astraea.
The new notation caught-on quickly, and by 1855 the number for Astraea changed from 1 to 5, making room for (1) Ceres, (2) Pallas, (3) Juno and (4) Vesta. The new classification of asteroids reduced the count of planets back to a manageable total of eight, and kept the asteroids — those objects found between Mars and Jupiter — neatly arranged into their own category, with eight planets (and their satellites) then also neatly arranged into their own category.
For seventy-five more years, the world knew a solar system with eight planets (some with their own moons), and hundreds of smaller objects orbiting the Sun in a ring of debris found between Mars and Jupiter — a.k.a. the asteroid belt.
Towards the end of the 19th century, businessman-turned-astronomer Percival Lowell hypothesized the existence of a planet beyond the orbit of Neptune. From his observatory in Flagstaff, Arizona, Lowell set out to discover the object, observing and photographing the night sky in search of Planet X. Before finding the distant object, Lowell died of a stroke in 1916.
In 1920, astronomer Walter Baade had discovered yet another asteroid, designated (944) Hidalgo. But this asteroid was different than the others — its orbit is located about 50 million miles beyond Jupiter’s orbit, making it unlike the previous asteroids that had been found closer to the Sun. The discovery of Hidalgo indicated that asteroids could be found beyond Jupiter’s orbit, which presented a puzzle for the scientific community. The orbits of the asteroids between Mars and Jupiter were stable, but how could an object located within the orbits of the massive outer planets — Jupiter, Saturn, Uranus and Neptune — maintain its orbit?
Although Percival Lowell passed away before finding his theoretical Planet X, astronomers continued their search, and in 1930 astronomer Clyde Tombaugh discovered the distant world. The name Pluto was chosen, and the object was given a planetary symbol that incorporates the letters “P” and “L” in reference to Percival Lowell.
But in the same way that (944) Hidalgo challenged asteroid classification, Pluto presented incongruity when compared with the other planets. For starters, the outer planets are all massive gas giants; Pluto is very small by comparison. Nearly a third smaller, in fact, than the Earth’s moon. Pluto also has a very high inclination of orbit, compared to the other planets. Pluto orbits at more than 17° inclination — only the asteroids held such drastic inclinations to the ecliptic. At just more than 7°, Mercury holds the next-largest inclination for planets. Additionally, the eccentricity of Pluto’s orbit (0.24) is elongated more than any other planet (0.20 for Mercury) — only comets and asteroids held such eccentric orbits.
Despite its irregularities in comparison with the other planets, Pluto became the ninth planet.
In the late 1960’s, while primarily searching for supernova remnants, Palomar Observatory staff astronomer Charles Kowal catalogued a number of additional asteroids, and discovered the 13th and 14th moons of Jupiter. In early October 1977, Kowal compared images of the night sky and found a very distant object moving between the stars. The object presented yet another challenge to the neat model of planets and asteroids. Later named (2060) Chrion, the 140-mile wide asteroid was discovered at aphelion just beyond the orbit of Uranus — definitely not in the neatly compartmentalized belt of asteroids between Jupiter and Mars. The elongated eccentricity (0.379) of its orbit brings the asteroid closer to the Sun than the orbit of Saturn, but not closer than Jupiter — the object moved between the orbits of the outer giant planets.
Kowal had just discovered the second object in a class of icy solar system objects now known as centaurs; the first was (944) Hidalgo, discovered in 1920 by Walter Baade. Centaus are found in unstable orbits between the outer planets. They are like asteroids in size, but traits of their composition are more comet-like; in fact, some centaurs sublimate gases and produce comas around them, just like comets.
The orbits of centaurs are greatly influenced by the gravitational pull of the massive planets with which they share their orbits. This prevents their orbits from being stable, so centaurs only exist between the outer planets for a few million years before they are either ejected entirely from the solar system, or merge with another planetary object. It stands to reason then, that if these icy objects exist only temporarily in orbit between the outer planets, they must be drawn-in from some distant place beyond the gas giants.
The discovery of centaurs revealed objects orbiting the Sun in unexpected locations, some with elongated eccentricities and highly tilted inclinations — much like Pluto. If there were some source for these objects beyond Neptune’s orbit, wouldn’t it be possible that there were additional icy worlds to be discovered beyond Neptune — much like Pluto? If additional trans-Neptunian objects (TNO’s) could be found, they would perhaps show that Pluto was not an object formed like the other planets in the solar system, but in fact a TNO that had been drawn-into a closer orbit, like (944) Hidalgo and (2060) Chiron.
With Pluto’s discovery, Lowell and Tombaugh proved at least one world exists beyond Neptune. After Pluto’s discovery, other astronomers believed that additional objects would be found beyond Neptune’s orbit. Apart from Pluto, trans-Neptunian objects had yet to be discovered. In 1951 Gerard Kuiper proposed that a disk of debris would be found beyond Neptune’s orbit; other astronomers proposed that objects similar to Pluto could still be found, and continued to search for decades after Pluto’s discovery.
In 1992, more than sixty years after Pluto had been discovered, two astronomers working for the department of astronomy at the University of Hawaii located object (15760) 1992 QB1 and determined its mean orbit distance from the Sun at 43.7 AU — more than 11% further from the Sun than Pluto. Astronomers David Jewitt and Jane Luu discovered the first trans-Neptunian object beyond Pluto, and continued to discover dozens more through the end of the century.
Still, the trans-Neptunian objects that were located by Jewitt and Luu were comparatively small — from a few miles to a several dozen miles in width, but none as large as Pluto (735 miles radius). Perhaps it would be just a matter of time before another large object would be found, and the question to be answered was: if a trans-Neptunian object the size of Pluto, or larger, were to be discovered, would that indicate a tenth planet? Would additional discoveries lead us to name more planets? Through the end of the 20th century, the world had become very comfortable knowing a relatively neat system of nine planets, an asteroid belt, and the centaur anomolies found between Jupiter and Neptune. With the discovery of so many dozens of additional TNO’s, and the theory that centaurs likely are TNO’s pulled into their current orbits from beyond the gas giants, the International Astronomical Union realized that clearer definitions for solar system bodies would be required, in order to finely differentiate and categorize the objects in our solar system.
In 2003, after analyzing data from a newly-discovered TNO named Sedna, the team who made its discovery realized that they may have been discounting additional data from their observations that could reveal additional TNO’s. Astronomers Michael Brown, Chad Trujillo and David Rabinowitz began to re-analyze images taken at the Palomar Observatory, and in January of 2005 discovered (136199) Eris at ~96AU from the Sun. Upon determining that Eris could be as large, or possibly larger than Pluto, the media and even NASA dubbed Eris the “tenth planet.” The issue had been forced: the IAU now needed a clear definition for what constituted a planet, and to determine whether Eris and other TNO’s, centaurs and asteroids fit into that definition.
The next year, the IAU announced in 2006 what it had determined to be three requirements for an object to be considered a planet:
The object must orbit the Sun.
The object must have sufficient mass to be nearly round in shape (hydrostatic equilibrium).
The object has “cleared its neighborhood” of orbit of other objects.
The third requirement leaves Pluto out of the picture. As part of his ongoing research in the 1990’s, David Jewitt had discovered many objects in a similar orbit as Pluto, and coined the term “plutinos” to describe them. Like Pluto, they orbit the Sun in a 2:3 resonance with Neptune; thus, Pluto does not fit the requirement. Nor does Eris, as it is part of the scattered disk of icy debris beyond Neptune.
Thousands of solar system objects meet only the first requirement, and these are known as small solar system bodies (SSSB’s). These include most of the asteroids in the main asteroid belt located between Mars and Jupiter; centaurs, those objects in orbit around the Sun located between the orbits of the outer gas giant planets; trojans; and most trans-Neptunian objects including those that become comets.
The objects that meet the first two requirements are now termed dwarf planets. They are large enough that their own gravity pulls them into a spherical shape. The IAU currently recognizes five dwarf planets in the solar system: the former asteroid (1) Ceres, the scattered disk trans-Neptunian object (136199) Eris, and the Kuiper Belt Objects (136108) Haumea, (136472) Makemake and finally (134340) Pluto.
So as of 2006, the Solar System holds one star, eight planets with hundreds of satellites, five dwarf planets several with their own satellites, and thousands of small solar system bodies including asteroids, trojans, centaurs, comets and other TNO’s, KBO’s, and SDO’s. And, as astronomers keep collecting and reviewing data from the night sky, many more dwarf planets and SSSB’s will very likely be found and catalogued.
But for now we can say with a large degree of certainty — there are eight planets in the solar system.
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