The missing planet
In 1596 the German astronomer Johannes Kepler was drawing up a geometrical scheme for explaining the orbital spacing of the planets, and was also establishing his three laws of planetary motion.
Both these activities made him realise that there was a gap in the solar system between the orbits of Mars and Jupiter where he would have expected to find another planet.
However, it took another 200 years before another planet was found, and it wasn’t in the gap between Mars and Jupiter, it was much further away. The planet was discovered by William Herschel in 1781 and was named Uranus.
Even more interesting was the fact that it obeyed a ‘law’ that had been proposed nine years earlier by Johann Daniel Titius and Johann Elert Bode, as a result their proposal was considered to hold true. Their ‘law’ suggested that each planet should be found about 65% further away from the Sun than the previous planet.
Having looked at the distances of the planets then known it was easy to see that the gap between Mars and Jupiter (as Kepler had spotted) was too big – there should have been something in-between.
The celestial police
By the 1790s astronomers had decided to start looking for the missing planet in the Mars-Jupiter gap. On 11 September 1800 a group of European astronomers, jokingly called the Celestial Police, led by Baron Franz von Zach and Johann Schroeter, divided the ecliptic band of the sky (the planets in our solar system orbit the Sun in approximately the same plane, this is called the ecliptic band) into 24 regions and started searching systematically. Before the search had really got underway, the first asteroid was discovered by Giuseppe Piazzi, completely by accident.
Giuseppe Piazzi, an Italian astronomer and the director of the Palermo Observatory in Sicily, was taking full advantage of the benign climate enjoyed by what was then Europe’s most southern observatory to produce an accurate catalogue of the fixed stars. He had some of the best equipment available at the time at his disposal, and he was using a recent star catalogue by William Wollaston as a guide. On the night of January 1st 1801 he was plotting the positions of the stars in the constellation Taurus.
To his surprise he noticed a dim star that had not been recorded by Wollaston. He checked this star on the following night and it had moved. This movement continued over the following three weeks. Piazzi then wrote to Bode and also to Barnaba Oriani (a very close friend and astronomer) hinting that he might have discovered a comet although the lack of a tail or a coma made him think it might be “something better” - and it was! Piazzi had discovered the first asteroid, Ceres, named after the patron goddess of Sicily.
Unfortunately the February weather was poor and Piazzi also fell ill and as a result he was unable to continue his observations. As the new asteroid moved through its orbit during the month it was heading towards the point of being directly behind the Sun (from the perspective of the Earthbound astronomers).
As it continued along its path it gradually became visible earlier and earlier in the evening, until it ‘rose’ so early that the daytime sky was too bright to see it and it disappeared from the astronomer’s view.
Luckily the brilliant mathematician Carl Fredrich Gauss had just devised a method which enabled the orbital parameters of a solar system body to be calculated from three or more observations.
By October 1801 Gauss had fixed the orbit and produced an ephemeris (a list of the positions of key objects in the night sky and a reference of where to find them in the sky at different times) showing where Ceres would be at the end of the year.
Baron Franz von Zach found Ceres on January 1, 1802 and just a matter of weeks later, on March 28, Heinrich Olbers was also observing Ceres when he discovered another asteroid near by that was named Pallas (named after the goddess of wisdom, war and the liberal arts).
Ceres and Pallas were surprisingly small, they were expecting to find a planet, but these new discoveries were only a few hundred kilometres across. Olbers suggested that they were fragments of a much larger planet that once occupied the Mars-Jupiter region, this planet having suffered an internal explosion or a cometary impact many million years before. His theory was produced little controversy and was quickly accepted by the scientific community.
It wasn’t long before another two asteroids were discovered; Juno in September 1804 and Vesta in March 1807. But there then followed a 38 year hiatus until the fifth asteroid Astraea was found.
Technology makes the search wider
As the nineteenth century progressed many more asteroids were discovered as the introduction of larger telescopes and the use of photography enabled fainter and fainter (and therefore smaller and smaller) asteroids to be found.
Photography also helped as it enabled astronomers to make a record of their observations that they could study at their leisure. By 1900 about five hundred asteroids were known and the numbers kept increasing.