On August 24, 2006, the International Astronomical Union stripped Pluto of its status as a planet.
FOR decades schoolchildren have recited the names of the `nine planets in the solar system'. Tiny, distant Pluto was in some respects different from the others but was still accepted as one of them. On August 24, 2006, however, Pluto was stripped of its status as a planet, when 2,500 astronomers from 75 countries redefined it as a dwarf planet, leaving just eight classical planets in the solar system, namely, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The International Astronomical Union (IAU) is the highest official body of astronomers and is made up of astronomers elected from the international community. It is the overseeing body for all aspects of astronomy. It gets the final say on everything, from naming celestial objects to deciding their classification. The 26th general assembly of the IAU was held in Prague, the Czech Republic capital, from August 14 to 25, 2006. The decision on Pluto was made there.
Before analysing the reasons for this monumental decision, let us have a look at how Pluto was discovered. In ancient times Saturn was thought to be the most distant planet as this was the outermost planet visible to the unaided eye. Nobody imagined that there could be more distant planets in existence. The English astronomer and musician William Herschel constructed a telescope seven feet (2.1 metres) long, with a mirror 6.2 inches (15.7 cm) in diameter, and using it discovered Uranus on the night of March 13, 1781. Since antiquity, astronomers had known the five planets of the solar system - Mercury, Venus, Mars, Jupiter and Saturn - and they supposed that the universe was complete. Herschel's discovery extended the classical universe. Mathematicians analysed the deviations of Uranus (then the outermost-known planet) from the orbit it would follow if gravity from only the sun and the other known planets were acting on it. Astronomers John C. Adams of England and Urbaine J. Joseph Leverrier of France predicted independently in 1845 the position of the yet-to-be-observed planet, and Johann Galle of the Berlin Observatory enthusiastically began searching and ultimately discovered Neptune on September 23, 1846.
By the end of the 19th century, observations of the orbits of Uranus and Neptune suggested that Neptune's influence was not sufficient to account for all the irregularities in Uranus' motion. Further, it seemed that Neptune itself might be affected by some other unknown body. Following the success of perturbation theory in guiding observers to the discovery of Neptune, astronomers hoped to pinpoint the location of this new perturber using similar techniques. One of the most ardent searchers was Percival Lowell, a capable and persistent observer and one of the best-known American astronomers of his day. Lowell set about calculating where the supposed ninth planet should be. He searched for it without success in the decade preceding his death in 1916. The direction in the sky where Lowell expected to find the new planet lay in a high star density region of the Milky Way galaxy, which made it very difficult to detect the slow night-to-night motion of a small speck of light. Fourteen years after Lowell's death, the American astronomer Clyde Tombaugh, working with improved equipment and photographic techniques at the Lowell Observatory, finally succeeded in finding Lowell's ninth planet, only 6o away from the position Lowell had predicted. The new planet was named Pluto for the Roman god of the netherworld, who presided over eternal darkness. Its discovery was announced on March 13, 1930, Lowell's birthday.
Pluto is very difficult to observe from the earth. Only a bit larger than 0.1 second of arc in diameter, it is only 65 per cent the diameter of the earth's moon and shows little surface detail. Most planetary orbits in the solar system are nearly circular, but Pluto's is quite elliptical. On the average, it is the most distant planet, but it can come closer to the sun than Neptune. The planets will never collide, however, because Pluto's orbit is inclined at an angle with the plane of the ecliptic (the apparent path of the sun, relative to the stars on the celestial sphere). The average distance of Pluto from the sun is 39.44 AU (astronomical unit, which is the average distance of the earth from the sun and equals 149,603,500 km). On Pluto, the sun appears a thousand times fainter than it does to us on the earth. We would need a telescope to see the solar disc, which would be about the same size that Jupiter appears from the earth.
Pluto's only known moon was discovered on photographs in 1978 by James Christy, a United States naval observatory astronomer. The moon was named Charon. This discovery was important because astronomers could use it to find the mass of Pluto. Pluto's mass turns out to be only 1/400th the mass of the earth. The equational diameter of Pluto was measured directly for the first time in 1979 and was found to be 2,280 km. Pluto is thus much smaller than Mercury (4,800 km). Its sidereal period (time taken to make one full orbit) is 248.6 years.
Spectra of Pluto reveal that its surface is composed of frozen gases such as methane, carbon monoxide and nitrogen. These are volatile in that they vaporise easily. Infrared observations made in the 1990s confirmed the earlier report that nitrogen ice also exists on Pluto's surface. The solid nitrogen is apparently mixed with small amounts of frozen methane and carbon dioxide. The surface temperature of Pluto has been measured with the James Clerk Maxwell Telescope on Mauna Kea, Hawaii, in the submillimetre region of the spectrum. David Jewitt of the University of Hawaii and his collaborators found temperatures lower than the infrared spacecraft had, showing that Pluto's surface is not uniform in appearance or colour. He suggests that the surface may be part cold regions (close to 34 K) coated by nitrogen ice and part warmer regions (closer to 50 K), which match the dark patches on Pluto's surface, without nitrogen ice ( 0oC = 273 K). Images of Pluto taken by the Hubble Space Telescope, of course, provide our present peak of knowledge of Pluto's surface.
Let us now see why the IAU stripped Pluto of its status as a planet. Pluto has the most eccentric orbit of all planets in the solar system. It is so elliptical that Pluto is sometimes closer to the sun than Neptune, as occurred from January 21, 1979, to March 14, 1999. It is now farther away from the sun than Neptune and will continue to be so for about the next 230 years. In this sense, Pluto has not been able to clear the neighbourhood around its orbit. This is one of the main reasons it was demoted.
Pluto has the most unusual orbit of all the planets in the solar system in another sense as well. Except for Pluto, all planets orbit the sun almost in the plane of the earth's orbit (the ecliptic), but Pluto's orbit is inclined 17o to this plane. This is very unusual and is the second reason for its loss of status.
Since Charon was discovered, the Hubble Space Telescope has detected two more moons of Pluto, and they have been named Nix and Hydra. Usually, the common centre of gravity of a planet and its moon lies below the surface of the planet. For instance, the common centre of gravity of the earth and its moon lies below the surface of the earth. Here also Pluto has a peculiarity of its own because the common centre of gravity of Pluto and Charon lies in the space between them. This is another factor behind the decision to remove Pluto from the list of classical planets.
In the 26th general assembly of the IAU, one topic dominated everyone's mind: what is a planet, how many are there and should Pluto still be described as one? The Oxford English Dictionary describes a planet as "a large round object in space that moves around a star (such as the sun) and receives light from it". Pluto fitted this bill when Tombaugh discovered it in 1930. Unfortunately, the solar system is not as simple as the dictionary describes it. When Christy discovered Charon, this appeared to bolster Pluto's planetary status. Surely, only planets had moons. Pluto appeared to be holding its own until 1992 when the astronomers Jewitt and Jane Luu spotted the first Kuiper Belt Object (KBO), called 1992 QB1. But what is the Kuiper Belt? Over the last few years, more than a hundred icy objects with orbital radii between 30 and 50 AU have been found in Pluto's part of the solar system. These objects match predictions of the existence of a band of incipient comets in that part of the solar system. This band was named the Kuiper Belt after the planetary scientist Gerard Kuiper. It is perhaps 10 AU thick and extends from about the orbit of Neptune to about twice as far out. The icy objects are known as Kuiper Belt Objects. Before 1992, the Kuiper Belt was a hypothetical realm of icy objects located beyond the orbit of Pluto and the source of short-period comets. When Jewitt and Luu found 1992 QB1, the Kuiper Belt suddenly became a reality. Pluto was not alone.
Kuiper Belt discoveries continued, with larger and larger objects being found. This included the 1,000-kilometre-wide Varuna. Shortly after that came Quaoar. At 1,250 km, its diameter is half that of Pluto. In 2004, the discovery of the 1,800-kilometre-wide Sedna was announced. Slowly but surely KBOs were getting bigger, and Pluto was nervously looking over its shoulder. Then, on July 29, 2005, the National Aeronautics and Space Administration (NASA) announced what it called a major discovery in the outer solar system, the most serious threat yet to Pluto's reign. This newly discovered KBO was named 2003 UB313. Dr. Michael Brown of the California Institute of Technology, and co-discoverer of 2003 UB313, said: "Get out your pens and start rewriting the history books today; we are calling this planet 10." The whole world stood up and took notice when NASA announced that it had discovered the `tenth planet' of the solar system. Unofficially it was dubbed `Xena', and its diameter was computed to be 2,398 km, which is slightly greater than Pluto's. This `tenth planet' even had a moon, as did several other KBOs and asteroids. It meant that Pluto's moon Charon was nothing special.
As exciting and daring as this announcement was, however, many in the astronomical community did not see 2003 UB313 as a planet at all. If it was not a planet, could the even smaller Pluto still be considered one? And what is a planet anyway? Does it even matter? The discovery of 2003 UB313 forced the IAU's hand somewhat, with the Pluto issue now playing a major part in a wider problem that the IAU was tackling, namely, how to categorise something as a planet. At the very end of the general assembly, there was a heated debate on the issue of Pluto among some 2,500 astronomers, and a resolution was voted on. The final decision was to draw a clear distinction between Pluto and the other eight planets. Astronomers agreed that to be called a planet, a celestial body must be in orbit around a star while not itself being a star. It must also be large enough in mass for its own gravity to pull it into a nearly spherical shape and it should have cleared the neighbourhood around its orbit. The new definition meant that a second category called "dwarf planets" was created, as well as a third category for all other objects, except satellites, known as small solar system bodies.
The need to define what it takes to be a planet was driven by technological advances that enabled astronomers to look farther into space and to measure more precisely the size of celestial bodies in the solar system.
In the usual process of taking photographs of any celestial object, only 5 per cent of the light striking a photographic plate succeeds in triggering the chemical reaction in the photographic emulsion that is needed to produce an image. Thus, roughly 95 per cent of the light falling on a photographic plate is wasted. On the contrary, in the newly invented charge-couple device (CCD) process of taking photographs of celestial objects, the image is transferred from a computer to a television monitor, and over certain wavelength ranges, nearly 75 per cent of the light falling on a CCD can be recorded. This is a highly innovative process of taking photographs of very distant celestial objects.
According to the IAU's new definition of a planet, Pluto will now be classified as a "dwarf planet", leaving only eight classical planets in the solar system. Astronomers now know that Pluto is simply a very large member of the Kuiper Belt. Had Pluto been found today, many experts agree it would never have been classified as a planet. And, as one would expect, no other KBO found to date has been given such an honour.
Professor Amalendu Bandyopadhyay is a senior scientist at the M.P. Birla Institute of Fundamental Research, M.P. Birla Planetarium, Kolkata.
