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It is easy to imagine the excitement astronomers felt when the telescope was invented early in the seventeenth century. Until that time, mankind’s observations had been confined to five bright planets, moving against a starry background. Nevertheless 150 more years passed until another planet was discovered. In England in 1781, German-born musician and telescope maker, Sir William Herschel, announced that he had discovered a new planet. This body was named Uranus after the most ancient of the Greek gods.

By the beginning of the nineteenth century, astronomers had long since realized that Uranus did not follow a predictable orbit. There was always a discrepancy between its predicted position and its actual position. Some astronomers suggested that a planet, more distant yet, might be exerting a gravitational tug on Uranus.

In the 1840s, two astronomers began to search for another planet. Both men shared the honours when the discovery of another planet was announced on September 23, 1846. This one was named Neptune, for the Greek and Roman gods of the sea.

Few people however believed that the search for planets was at an end. Even after astronomers allowed for Neptune’s gravitational effect, Uranus still seemed off course. They deduced that there must be another planet farther out, which was causing this very slight extra wobble.

It was not possible to draw conclusions about the new planet’s orbit however because its progression around the sun is so slow: 165 years for Neptune compared to only 84 years for Uranus. The astronomers had quite a lot of information about Uranus’ orbit, so these were the data upon which They based their conclusions.

The search was on for a new planet, called Planet X for purposes of discussion. In the early 1900s, American astronomer Percival Lowell (1855-1916) began the search for Planet X. Based on the wobbles of Uranus, he predicted that the new planet would be found 6.4 billion km from the sun (actual 6 billion km) and that its annual orbit would take 282 years (actual 248 years). He never found the ninth planet, but before his death he set up a trust fund for a continued search.

With additional donated money, a new camera was purchased in 1929 for the Lowell Observatory in Flagstaff, Arizona. A local youth was hired to make a systematic search for the new planet. Thus Clyde William Tombaugh (1906-1997) achieved fame and fortune when the discovery of the new planet was announced March 13, 1930. This was the 149th anniversary of the dis- covery of Uranus.

The new planet was named Pluto after the Greek god who was ruler of the dark underworld. The choice seemed apt, the first two letters of the name were P and L (for Percival Lowell) and the planet was extremely far away from the sun in a place of perpetual darkness. Pluto however turned out to be something of a disappointment.

Computer models, based on modern ideas of planet formation, suggest that Pluto should have been as large as Neptune (about 50,000 km in diameter). Instead it seemed to be only about the size of Earth (13,000 km in diameter). Moreover, since its discovery, the size of Pluto has been modified downward several times. It is now estimated to be only 2300 km in diameter.

In addition, it seems to consist largely of ice, unlike the four inner rocky planets and the four outer gaseous giants. Certainly such a distant and light-weight body could not be expected to exert an effect on Uranus. Once the rejoicing about Pluto’s discovery was over, Clyde Tombaugh spent a further thirteen years searching for another planet. He never found anything.

Recently astronomers have concluded that they do not need a planet beyond Nep- tune to account for the wobble in Uranus. Analyses of the space probes Pioneer 10 and 11, launched in the 1970s, revealed that the trajectories of these probes were not affected by a tenth planet. Further calculations of the orbits of these two planets then revealed that mutual effects by these two bodies adequately explained their wobbling. If astronomers of the past had known what we now know, they would never have looked for Pluto.

Attitudes toward Pluto’s status as a planet began to change in 1992. Increasingly sophisticated digital technology and fancy software in high speed computers, have enabled scientists to detect small bodies moving in orbits farther and farther away.

In August of 1992, two astronomers identified an icy object 240 km across, occupying an orbit 8 billion km from our sun. (Pluto lies only 6 billion km away). Officially named 1992 QB1, this was the first object identified beyond the orbit of Pluto.

Soon after, a similar object was found. As of 2006, more than 1000 such objects have been documented beyond Pluto.

These discoveries suggested that there must not be a large planet in this region where so many small objects exist. Also, many astronomers decided that Pluto could better be grouped with these fragments than with the conspicuous planets.

The most compelling discovery was revealed in 2005. An object dubbed 2003UB313 was discovered 14 billion km from the sun. Indirect studies of the light spectrum reflected from this tiny dot led astronomers to conclude that the object is 3100 km in diameter.

As 2006 dawned, there were hints of controversy in astronomical circles. Some people suggested that Pluto be demoted. Others suggested that 2003UB313 (called Xena for a while but now officially dubbed Eris, Greek goddess of chaos and strife), and similar objects, all be given planetary status.

Many American astronomers staunchly defended Pluto’s status as a planet, while international astronomers led the charge for the other side. The showdown took place in August at the International Astronomical Union meeting in Prague. Some determined scientists declared that a planet must ‘dominate’ its local orbital zone, or in other words, sweep other objects out of its path. According to these individuals, this criterion would disqualify Pluto as a planet. On August 24 that indeed is what the delegates voted to accept.

Later, one scientist pointed out that the accepted definition was not clear. In that Pluto twice crosses Neptune’s orbit in one circuit of the sun, the definition should disqualify Neptune as well as Pluto since neither has cleared its orbit. In addition, Jupiter should be disqualified since it shares its orbit with certain as- teroids. Nobody wants to disqualify Jupiter or Neptune however, only Pluto.

Most likely Pluto will not disappear from public view. New Horizons space probe to Pluto was launched on January 19, 2006. The mission is expected to arrive at its target in 2015. The whole event has been an object lesson in the importance of definitions in science. In this case, our perception of what bodies there are in space, depends on what scientists want to see out there. It can’t hurt to critically evaluate scientific information. Don’t accept everything the experts say.

April 2007

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