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A Brief History of Ceres: “The Missing Planet”

New Year’s Day, 1801, the dawn of the 19th century, was a historic moment for astronomy, and for a space mission called Dawn more than 200 years later. That night, Giuseppe Piazzi pointed his telescope at the sky and observed a distant object that we now know as Ceres.Today, NASA’s Dawn mission allows us to see Ceres in exquisite detail. From the images Dawn has taken over the past year, we know Ceres is a heavily cratered body with diverse features on its surface that include a tall, cone-shaped mountain and more than 130 reflective patches of material that is likely salt. But on that fateful evening in 1801, Piazzi wasn’t sure what he was seeing when he noticed a small, faint light through his telescope.

“When Piazzi discovered Ceres, exploring it was beyond imagination. More than two centuries later, NASA dispatched a machine on a cosmic journey of more than 3 billion miles to reach the distant, mysterious world he glimpsed,” said Marc Rayman, mission director and chief engineer for Dawn at NASA’s Jet Propulsion Laboratory, Pasadena, California.

Piazzi was the director of the Palermo Observatory in Sicily, Italy, which has collected documents and instruments from the astronomer’s time, and published a booklet on the discovery of Ceres. According to the observatory, Piazzi had been working on a catalog of star positions on January 1, 1801, when he noticed something whose “light was a little faint and colored as Jupiter.” He looked for it again on subsequent nights and saw that its position changed slightly.

What was this object? Piazzi wrote to fellow astronomers Johann Elert Bode and Barnaba Oriani to tell them he had discovered a comet.

“I have presented this star as a comet, but owing to its lack of nebulosity, and to its motion being so slow and rather uniform, I feel in the heart that it could be something better than a comet, perhaps. However, I should be very careful in passing this conjecture to the public,” Piazzi wrote to Oriani.

A Missing Planet?

Piazzi didn’t entirely keep this secret. He told the press that this object was a comet, but did not provide data from his observations, which generated criticism from other astronomers. Piazzi then became sick for a time, and said he could not observe the object any more.

As newspapers spread the word that a comet had been found, astronomer Jerome de Lalande, based in Paris, wrote to Piazzi requesting relevant data in February. The Italian astronomer obliged in April, after recovering from his illness. One of Lalande’s students, Johann Karl Burckhardt, performed calculations that revealed Piazzi’s discovery did not have an orbit consistent with a comet’s orbit. Instead, the data appeared to better fit a circular orbit.

Of course, there was no email in those days, and letters that Piazzi wrote to his friends Bode and Oriani about the so-called comet were delayed due to the Napoleonic Wars. They finally reached the astronomers in March.

The news was especially interesting to Bode because he had championed the Titius-Bode hypothesis: that the positions of planets in our solar system follow a specific pattern, which predicts each planet’s distance from the sun. Uranus, discovered in 1781, fit the prediction, too. But the pattern also demanded that there be a planet, yet undiscovered, between Mars and Jupiter.

To find this missing planet, a group of German astronomers had established a society called the “Celestial Police” (Himmelspolizei in German), with Franz Xaver von Zach as its secretary, in 1800. There were 24 astronomers who each scoured a 15-degree piece of zodiacal sky for the missing object. However, Piazzi did not receive his invitation to join this group until after he had spotted Ceres.

Bode calculated an orbit based on Piazzi’s data, and he believed that the object Piazzi saw was the missing planet that fit his formula (which was later discredited). Oriani, meanwhile, also calculated an orbit, and on April 7 asked von Zach to publish the news in his well-known astronomy journal, Monatliche Correspondenz, that such a planet may have been discovered.

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