Why Poor Pluto Is
No Longer A Planet?
T V Venkateswaran
IT is just about 75 years since the discovery of Pluto; hitherto cute little baby planet of our solar system. With the raising of quite a few yellow cards in Prague, on August 24, 2006, by 3000 and odd astronomers, Pluto was demoted from full-fledged planet to dwarf planet. Moreover, under the revised classification, the object 2003 UB313 which in many ways precipitated this final debate, sometimes called Xena, and much touted as the “10th planet, becomes the largest known dwarf planet. Indeed, it is an irony that Pluto was a planet when the New Horizon space mission was launched in January 2006; but will no longer be: as it would reach Pluto-Charon in 2015.
Demotion of stellar bodies from the initial status assigned to it is not new to astronomy, or for that matter science. In fact when Galileo Galilee famously turned his telescope towards the heaven he was blessed with the enchanting sight of rings of Saturn, mountains on the moon-landscape and extraordinary sight of stars rotating about the planet Jupiter. Yes, indeed Galileo unhesitatingly asserted that that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun and named them initially medicean stars, in honour of Medic family who provided patronage to him during his years of trial and tribulations. However they were subsequently reclassified as medicean planets because naming them as stars was deemed inappropriate. In those days moon was the proper name for the natural satellite of Earth and many other satellites of planets such as Rhea of Saturn were also called as planets. It was only around 1700s that astronomers commenced the practice of calling the satellites of planets as moons.
It was on March 13, 1781 William Herschel along with his sister Caroline discovered the planet Uranus. After careful study to rule out the possibility that this object could be a comet, Herschel concluded that he has indeed discovered a new planet; a planet about which no sacred text had any inkling. Obviously such momentous discovery inspired many other astronomers to search the skies for other planets that may be lurking around.
Meanwhile, Johann Elert Bode, a German astronomer observed that if you divide the distance from the Sun to Saturn into 100 lengths, then: Mercury is at 4 lengths; Venus is at 7 lengths (4+3); Earth is at 10 lengths (4+6); Mars is at 16 lengths (4+12); Nothing was seen at 28 lengths (4+24); Jupiter is at 52 lengths (4+48); Saturn is at 100 lengths (4+96). Discovery of Uranus too seemed to provide credence to this claim; as per the above rule the next planet beyond Saturn was to be at 196 (4+192); although Uranus was actually at 192 lengths. It was so near, it was thought to verify the law. Encouraged by this, in 1800 Hungarian Baron, Franz Xavier von Zach, set up a group of astronomers called the Celestial Police, to find the missing planet. They divided the Zodiac into zones and allocated different areas to different astronomers in the group.
The Lady luck had a different plan; she smiled on Rev Father Giuseppe Piazzi, an Italian astronomer. Piazzi was compiling a star catalogue at Palermo Observatory in Sicily. He accidentally discovered a planet between the orbits of Mars and Jupiter on January 1, 1801 and named it after Sicilian God of harvest Ceres. Thus at the beginning of the 19th century there were 8 planets, including Ceres and Uranus. However Zachs Celestial Police was not in vain. Heinrich Olbers, a member of the Celestial Police discovered Pallas, which too was in the same orbital area of Ceres, on March 28, 1802. Karl Harding, another member of the Celestial Police, found Juno during September 1804. Heinrich Olbers discovered Vesta in March 1807. All these were classified as planets, and the number of planets rose to 11 including Vesta. However there was something fishy in the discoveries, all these planets seemed to occupy the same orbital area as that of Ceres.
Things appeared to cool off. The fifth planet in the same orbital area Astraea was discovered only in 1845, nearly 39 years after the discovery of Vesta. However, soon the floodgate of discovery of planets burst at its seam in 1847 with the discovery of three new asteroids. By the end of 1851 there were 15 asteroids, although astronomers were disquieted, they braved themselves and still listed all of them as planets solar system then consisted of 23 planets including Neptune (discovered in 1845), Uranus and 15 planets in the orbital region of Ceres. By 1668 more than 100 stellar objects were discovered in that region. Surely things were unseemly; in fact due to their star like appearance, Herschel had declared even as early as 1802 that from their asteroidal appearance, I shall take my name, and call them Asteroids; reserving for myself, however, the liberty of changing that name, if another, more expressive of their nature should occur.” Astronomers who had not heeded to this perceptive counsel at that time had to but agree. At one go 15 planets were reclassified as minor planets or asteroids around 1850s.
Discovery of Pluto is intimately linked to that of Neptune. Once Uranus was discovered astronomers observed that there was perturbations in its path; as if there is another massive body beyond Uranus giving it a jab. Convinced that there is a planet beyond Uranus, the search began. Adams in England and Levarrier in France spearheaded the search. They calculated the possible position of Neptune and with much drama the planet indeed was discovered by Galle and DArrest of Berlin Observatory on September 23, 1845.
Neptune accounted for the observed perturbations in the path of Uranus, nonetheless Neptune itself showed perturbations as if being shoved around by a planet beyond it. Ah said astronomers lets go for the Planet X. Nonetheless it was soon evident to astronomers that treading the same path will not lead them to the Planet X. Calculations after calculations were made and yet the elusive planet was not to be seen. Where calculations and prediction failed, perseverance triumphed. With the invention of photography it was possible to record the position of stellar objects weeks apart and compare them. Clyde Tombaugh in fact did exactly that; and on March 13, 1930 Lowell Observatory announced the discovery of the ninth planet of the Solar system thereby bringing to an end a search, which went on for about 25 years. When Tombaugh discovered Pluto, astronomers welcomed it as the long sought Planet X, which would account for residual perturbations in the orbit of Neptune. In a curious twist to the tale those perturbations proved to be illusory, and the discovery of Pluto was fortuitous.
Right from the day of its discovery Pluto has been an odd ball. Pluto’s orbit deviates significantly from a perfect circle while the major planets have quasi-circular orbits. It is so elongated that it crosses the orbit of Neptune. Due to this, from 1979 to 1999 for twenty years in fact Pluto was rather closer than Neptune. Pluto’s orbit is also considerably tilted whopping 17 degrees compared to the orbits of the major planets. At the time of its discovery Pluto was estimated to be the size of Earth; later downgraded to size of Mars. Further, in 1978, Plutos companion, Charon was discovered. Charon was estimated to be half its size; earlier the size of Pluto was estimated by including the size of Charon as it was not possible to resolve them separately. This led to further downsizing of Pluto; now we know that Pluto is much smaller than the major planets just about 2320 km across smaller than seven moons of Solar system including our moon. Its mass is only 0.2 per cent of the Earth’s mass, and 100,000 times less than the mass of Jupiter. Pluto rotates in the opposite direction from most of the other planets. Pluto has been an irritant as a planet for astronomers quite long; astronomers were indeed puzzled what to make of this small, frigid world.
2005 witnessed the 75th anniversary celebrations of the discovery of Pluto. Clyde Tombaugh was hailed. Nevertheless even before the din and dust settled down poor Pluto has been striped of its planethood. As a matter of fact, even at an earlier occasion, astronomers had suggested that Pluto does not really belong to the category of planets. In 1999, an inadvertent suggestion, that Pluto may be termed the 10000th minor planet giving it “dual citizenship” of sorts as both a major and a minor planet was made by Brian Marsden of the Minor Planet Center of IAU. This sparked off a major debate that spread onto the streets; passionate editorials were written and email campaign was conducted fearing that Pluto might be “demoted” to non-planet status, in particular in USA. Taken aback, putting at rest the rumours, IAU then emphatically declared that there were no plans to change Pluto’s planetary status.
However, for astronomers it was evident for long that the fate of Pluto was sealed; sooner or later it would lose its status as planet. The need for a strict definition was deemed necessary after new telescope technologies, especially Hubble Telescope and use of computer technologies to compare photographs, began to reveal far-off objects in the region of Pluto. The first Kuiper belt object (KBOs) 1992 QB1 was discovered in 1992. 2001 KX76 an icy, reddish world over a thousand kilometres across was soon discovered. Two is a company, but surely three is a crowd. Soon many more objects were discovered in the Kuiper Belt area going around the sun in the region of Pluto. In 2002, Quaoar (1280 km diameter) was discovered, making it a bit more than half the size of Pluto. Another discovery, Orcus, is probably even larger. In 2004 Sedna, an extremely distant object beyond the Kuiper belt, was discovered with estimated 1800 km diameter, close to Pluto’s 2320 km. As of now more than 783 objects have been discovered in the same region as that of Pluto. The proverbial last straw on the camel was the discovery of Trans-Neptunian object 2003 UB313 (popularly called as Xena). Announced on July 29, 2005 it rekindled the debate as to whether to classify Pluto as a planet or not. It is estimated to be at least as large as Pluto; and is the largest object yet discovered in the solar system since Neptune in 1846 and has caused some to refer to it as the “10th planet” of the solar system. It was clear that Pluto is embedded in a vast swarm of small bodies, just like the asteroid Ceres in the asteroid belt between Mars and Jupiter. Pluto has many friends orbiting nearby and at least one of them is larger than Pluto. There have been another important developments in our knowledge of “planetary systems” in the last decade or so: the discovery of celestial bodies orbiting stars exoplanets other than the sun. These recent developments make it pressing to arrive at a proper definition for the word “planet”.
(To be continued)