Celestial Police: Stargazers on a Cosmic Beat

In 1800, the architecture of the Solar System was not fully understood. There was a yawning gap between the orbits of Mars and Jupiter. A band of stargazers from Germany and Austria decided to patrol these uncharted territories, looking for hidden worlds. They called themselves the Celestial Police, and divided up the sky into 24 zones for the purpose of producing detailed star maps. This unique collaboration reshaped our understanding of the Solar System. The Celestial Police owe their existence to a now debunked mathematical curiosity.

The German astronomer Johann Daniel Titus proposed in 1766 that the Solar System had an underlying mathematical pattern, that each subsequent planet was at twice the distance from the Sun as the previous one. Mercury, Venus, Earth, Mars, Jupiter and Saturn, the visible-eye planets all fit nearly into this sequence. Johann Elert Bode, another German astronomer popularised and refined the mathematical sequence. The pattern predicted the existence of a world between Mars and Jupiter. Bode urged astronomers to search for this missing world. The Celestial Police were formed to hunt down this missing world.

At the first European astronomical congress in Gotha in 1798, the French mathematician Joseph Jerome de Lalande proposed a systematic sky patrol. The planets roamed among the millions of stars that peppered the zodiac. He proposed dividing the work of charting the skies among multiple observers. The seed took root, and at the second congress in Lilienthan, Germany, the Celestial Police were formed on 20 April, 1800. Johann Schröter became the president, with Franz Xaver von Zach as the Director, spearheading the effort. The founding group of six astronomers included Adolf von Ende, Karl Harding, Johann Gildemeister and Wilhelm Olbers, who hatched a daring plan to recruit 24 astronomers assigned a slice of the zodiacal band, or the ecliptic plane in which the planets orbit the Sun.

In 1781, Wilhelm Herschel discovered Uranus, a planet that fit neatly into the pattern. An Italian monk and astronomer who was not a part of the Celestial Police discovered Ceres on 1 January, 1801, and named it after the patron goddess of Sicily. The discovery galvanised the Cosmic Police, who were convinced that more worlds lurked in the gap between the orbits of Mars and Jupiter. Invitations were sent across Europe to recruit astronomers in Denmark, France, Poland, Sweden, Italy, Russia and England. The Celestial Police was a rare international collaboration during a period wracked by wars and fragmented science.

While tracking Ceres from a rooftop observatory, Olbers discovered Pallas moving through Virgo. The object was smaller than Ceres, but unmistakably planetary. The discovery of two worlds in the same orbit stunned the astronomers. Olbers proposed a daring new theory, anticipating discoveries that would only be confirmed centuries later by sophisticated modern spacecraft. He suggested that the region between Mars and Jupiter was not occupied by the fragments of a world battered to bits. This exploded planet hypothesis gained traction. These scientists were smart enough to anticipate the existence of the asteroid belt after only two objects were discovered in the region.

The Celestial Police continued their concentrated observations. On 1 September, 1894 Karl Harding was patrolling his assigned sector when he spotted another moving body in Pisces. This was the third asteroid to be discovered, Juno. Then on 29 March, 1807 Olbers made his second asteroid discovery by spotting Vesta moving through the constellation of Virgo. This 525 kilometre-wide object is a protoplanet frozen on the cusp of becoming a planet, with a differentiated interior and a metal core. Vesta is the brightest asteroid ever discovered, and is occasionally visible to the naked eye in sufficiently dark skies.

The Celestial Police were careful about announcing their discoveries as planets. Four planets were discovered in six years, Ceres, Pallas, Juno and Vesta. The Celestial Police disbanded by the 1810s, with astronomers studying these worlds for a few decades, when the Solar System was made up of Mercury, Venus, Earth, Mars, Juno, Vesta, Pallas, Ceres, Jupiter, Saturn, Uranus and Neptune, which was discovered in 1846 much closer to the Sun than predicted by the Titius-Bode law.

At that time, the minor planets was a phrase reserved for Mercury, Venus, Earth and Mars. The newly discovered worlds between Mars and Jupiter were initially called minor planets and new planets, before astronomers settled on the word asteroid coined by Herschel in 1802. The Celestial Police mapped thousands of stars to spot interlopers, and investigated stars that appeared to vary in brightness. Von Zach published coordinates and theories in the Monatliche Correspondenz bulletin.

The Celestial Police were responsible for the modern concept of an asteroid belt. The efforts of the group debunked the Titius-Bode law as a fluke of nature, but affirmed that the asteroid belt was a fossil of planetary formation. Most of the belt was made up of raw material that never coalsced into a full world. However, there were the fragments of differentiated bodies as well, worlds that had molten interiors that squeezed igneous lava out to a rocky surface through volcanoes.

Ceres (NASA). 

The vigilance of the Celestial Police set the stage for future focused searches. The discovery of Astraea in 1845 sparked a second wave of discoveries. By 1852, 20 objects were known in the main belt between Mars and Jupiter. Humans have never stopped discovering asteroids since then. In the latter half of the 19th century, astronomers bemoaned that the sport of asteroid hunting had been ruined by the advent of photography, where the asteroids showed up as streaks in long exposure images of the night sky.

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All the asteroids were expected to be discovered within 20 years, but that did not happen, and neither did the pace of discoveries slow down. Automated surveys such as LINEAR discovered thousands of new objects. Even automation and crowdsourcing are not sufficient to process the vast amount of data gathered by instruments. Scientists, in fact, need your help to hunt down asteroids, and you can participate in the search for asteroids by joining The Daily Minor Planet initiative or partnering with at least one teammate to work with the International Asteroid Discovery Project. The tradition of the Celestial Police continues to this day, and those who discover the asteroids get to name the object once its discovery has been confirmed, and is assigned a serial number.