For centuries, the number of known “wandering stars,” or planets, that trailed through the night sky was five. Together with the sun and moon, that brought the total of major celestial bodies visible from Earth to seven—a number imbued with mystical significance. Then, in 1781, William Herschel spotted Uranus out beyond the orbit of Saturn, which forced astronomers to rethink this number. However, when the new planet’s orbit was placed in an updated plan of the solar system, it revealed another number conundrum.

Finding a gap

In 1766, a German astronomer named Johann Titius discovered a mathematical link between the orbital distances of the planets. He divided the orbital distance of Saturn by 100 to create a unit to measure all the other orbits. Mercury’s orbit was 4 units from the sun, and every other planet’s position from there was linked to a doubling of 3, or the number sequence 0, 3, 6, 12, 24, 48, and 96. So Mercury was located at 4 + 0 units from the sun, Venus at 4 + 3, Earth at 4 + 6, and Mars was at 4 + 12. Jupiter was at 4 + 48 and Saturn was at 4 + 96. There was no known planet in the sequence at 4 + 24 = 28, so there appeared to be a gap in the solar system between Mars and Jupiter. Titius proposed that the gap must be occupied by an unknown body. However, his findings seemed too good to be true—and the results for Mars and Saturn were slightly out, so few astronomers paid them much heed.

A few years later, in 1772, a fellow German named Johann Bode published a slightly modified version of Titius’s work, which met with greater acclaim. As a result, the theory is best remembered as Bode’s law. When Uranus was discovered, Bode’s law predicted that it would be 196 units from the sun. It was finally shown to be nearer to 192 units, but that seemed close enough. Surely, it meant the 28-unit gap must also contain a planet.

In 1800, a group of German-based astronomers led by Franz Xaver von Zach, Heinrich Olbers, and Johann Schröter decided to launch a search of the gap. Their plan was to divide up the zodiac—the strip of sky in which all the planets move—and ask Europe’s top 24 astronomers to patrol one zone each, searching for planetlike motion. The team they put together was dubbed the Celestial Police. But in the end it was straightforward luck, not efficiency, that filled the gap.

“From Mars there follows a space of 4 + 24 = 28 such parts, but so far no planet was sighted there. But should the Lord Architect have left that space empty? Not at all.” Johann Titius

Photographed by NASA’s Dawn spacecraft in 2015, Ceres is the largest object in the asteroid belt, and the only object large enough to have been made spherical by its own gravity.

Surveying telescope

One of the astronomers among the Celestial Police was Giuseppe Piazzi, who was based in Palermo, Sicily. Like most astronomers at the time, Piazzi was mainly concerned with creating accurate star maps. To that end, he had acquired a surveying telescope now known as the Palermo Circle. Although it was not the most powerful telescope of its day, its altazimuth mounting could move both vertically and horizontally, enabling it to make very accurate measurements of stellar positions, a feature that would pay rich dividends.

On the evening of New Year’s Day 1801, the instructions from the Celestial Police were still en route to Piazzi so he spent the evening surveying stars and recorded a new, faint object (with a magnitude of eight) in the constellation of Taurus. The following night, Piazzi checked his measurements and found that the object had moved slightly. This meant it was definitely not a star.

Piazzi watched the object for 24 days before informing Bode. He thought at first that it was a comet—a relatively common discovery—but his observations soon suggested otherwise. He could see no fuzzy coma, or tail, and while comets sped up as they approached the sun, Piazzi’s discovery took a more stable, circular path. In his letter to Bode, Piazzi made his suspicions clear: this could be the missing planet everyone was looking for.

On hearing the news in late March, Bode wasted no time in announcing the discovery of a new planet, which he named Juno. (He had recently chosen the name for Uranus, and clearly felt confident in his right to do so.) Other astronomers preferred the name Hera, but Piazzi, still the only person who had actually seen the object, had opted for Ceres, after the Roman goddess of agriculture.

By June, Ceres’s orbit had taken it into the glare of the sun. Piazzi had been sick in the interim and so had not had the chance to map anything but the simplest orbital arc. He calculated that his discovery would be visible again in the fall. But, try as they might, neither Piazzi nor anyone else could find Ceres.

Piazzi’s telescope, the Palermo Circle, was built by Jesse Ramsden. Its precision mounting allowed it to measure stellar positions with an accuracy of a few seconds of an arc.

Mathematical hunch

Von Zach decided to follow a hunch, and sent the details of Ceres’s orbit to the mathematician Carl Friedrich Gauss. In less than six weeks, Gauss calculated all the places Ceres was likely to be. It took von Zach most of December to search through Gauss’s predictions, but on the evening of New Year’s Eve 1801, almost exactly a year to the day after it was first seen, he found Ceres once again.

The orbital distance of Ceres was 27.7 Bode units, a remarkably close fit to the predicted location. However, the orbital data showed that this new member of the solar system was far smaller than the known planets. William Herschel’s early estimate put Ceres at just 160 miles (260 km) across. A few years later, Schröter proposed a diameter of 1,624 miles (2,613 km). The actual figure is 588 miles (946 km), which means it would be a comfortable fit over the Iberian Peninsula or Texas.

The Celestial Police kept up the search and, in March 1802, Olbers discovered a second body like Ceres located at the same distance from the sun, calling it Pallas. In 1804, Karl Harding found a third, named Juno, while it was Olbers again who spotted the fourth, Vesta, in 1807. All these bodies were later shown to be smaller than Ceres—Vesta and Pallas were slightly more than 300 miles (500 km) wide and Juno was half that size.

“The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. I waited till the evening of the fourth, when I had the satisfaction to see it had moved at the same rate as on the preceding days.” Giuseppe Piazzi

Asteroid belt

The Celestial Police called their discoveries minor planets, but William Herschel chose another name—asteroid, which means starlike. Herschel reasoned that, unlike true planets, these small objects had no discernible features, or at least none that could be made out with the telescopes of the day, so would be indistinguishable from starlight were it not for the fact that they moved. Perhaps still smarting from his failure to name the planet he had found 20 years earlier, Herschel qualified his suggestion by reserving for himself “the liberty of changing that name, if another, more expressive of their nature, should occur.”

Nothing more expressive did occur, and after the Celestial Police was disbanded in 1815, a steady trickle of asteroid discoveries continued. By 1868, their number stood at 100; by 1985, it was 3,000. The advent of digital photography and image analysis has now boosted the number of recorded asteroids to more than 50,000, spread around the 28-Bode-unit gap. Olbers and Herschel had discussed the possibility that the asteroids were the remains of a planet that once orbited in the gap before being smashed by an astronomical cataclysm. Today, it is thought that the gravitational disruption of nearby Jupiter prevented the asteroids from accreting into a planet in the first place, as similar disks had done elsewhere in the primordial solar system.

Under constant influence from the cumulative gravity of other asteroids, about 80 percent of known asteroids have unstable orbits. The 13,000 or so bodies that come particularly close to Earth—the Near Earth Asteroids (NEAs)—are monitored in the hope of predicting and preventing devastating future impacts.

“They resemble small stars so much as hardly to be distinguished from them. From this, their asteroidal appearance, if I take my name, and call them asteroids.” William Herschel

The asteroid Vesta was visited by the Dawn spacecraft from 2011–12. Its orbit lies within that of Ceres, and it is the brightest asteroid as seen from Earth.


There are also asteroids known as trojans, which travel in the same orbits as planets, gathering far from their host in gravitationally stable “libration points.” Most of these are in the Jupiter system, where they form two clusters: the “Trojan Camp” and “Greek Camp.” Mars and Neptune have trojans, and the first Earth trojan was discovered in 2011.

In 2006, the International Astronomical Union gave Ceres the status of dwarf planet, the only one in the asteroid belt. At the same time, Pluto was reclassified as a dwarf planet. The orbits of neither Neptune nor Pluto match the predictions of Bode’s law. Despite the fact that it was instrumental in the discovery of Ceres, Bode’s law is now viewed as a mathematical coincidence, and not a key to unlocking the formation of the solar system.


As was common for younger sons in wealthy Italian families, Guiseppe Piazzi’s career began in the Catholic Church. By his mid-20s, it was obvious that his abilities lay in academia. In 1781, he was appointed math professor at a newly founded academy in Palermo, Sicily, but soon switched to astronomy. His first task in this role was to build a new observatory, which he equipped with the Palermo Circle, a telescope built in London with a 5-ft (1.5-m) wide altitude scale. It was the most accurate telescope in the world at the time. Piazzi was famed for his diligence, and would take measurements on at least four consecutive nights to average out errors. In 1806, he recorded the large proper motion of the star 61 Cygni. This prompted several astronomers to use the parallax of that star to measure the distance between stars.

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