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Discovering Ceres

How the 8th planet was discovered

4-Minute Read

An artist's view of Ceres

Between Mars and Jupiter, somewhere in the asteroid belt, you can find the dwarf planet Ceres. But this celestial object wasn’t always considered to be a dwarf planet.

In the 18th century, an astronomer named Johann Bode was looking at the distances between each planet and the Sun. He noticed that the planets seemed to be following a geometric law. In other words, he found a mathematical link between these distances: If you know the distance from the Sun to Mercury, add one unit to get the distance to Venus. Then add two units to the distance of Venus and you get the distance from the Sun to the Earth. Then add 4, 8, 16, 32… that is, all the powers of 2.

This mathematical link doesn’t quite work for every power of 2, though. For 4, you get the distance to Mars and for 16 you get the distance to Jupiter. But for 8, there weren’t any planets at that distance from the Sun. Johann Bode reasoned that if all the known planets followed this geometrical law, then there must be a planet between Mars and Jupiter yet to be discovered. Which begs the question: Where did the missing planet go???

Many astronomers organized to try and find this mysterious planet, scouring the sky with their telescopes. In 1901, Guiseppe Piazzi, an astronomer who wasn’t trying to find a new planet at all noticed a “star” that was moving in the sky. At first, he thought it was a comet. But as he observed this new celestial body, he realized that is was a planet in a roughly circular orbit around the Sun.

Piazzi named this new planet Ceres Ferdinand, to honor his benefactor king Ferdinand. But other astronomers from other countries weren’t all so fond of the italian king and simply called the new planet Ceres instead.

In the following years, many other planets were discovered: Pallas, Juno, Vesta, Astrea, Hebe, Iris. The list goes on. By the mid 19th century, a couple dozen new planets had been discovered. These new planets shared many similarities, yet were fairly different from the “classical” planets. So astronomers decided to classify all of these new objects into their own new category called asteroids.

In 2006, astronomers decided to create a new category of objects again, and called them the dwarf planets. And Ceres checks all the boxes to be part of this new category. So Ceres was reclassified again. The International Astronomical Union – a group of scientists who decide, among other things, how to name celestial bodies – decided that Ceres is both the largest asteroid and a dwarf planet.

Our knowledge of Ceres has evolved over time. It’s interesting to notice that when Ceres was thought to be a planet, its diameter was more often overestimated. Other than Herschel (who had measured Ceres’ diameter at 260 kilometers in 1802), most astronomers at the time thought that the diameter of Ceres was half that of Mercury. The different estimates of Ceres’ diameter only started to converge towards the correct value after Ceres had been reclassified as an asteroid. Today, we know that Ceres is roughly 1000 kilometers wide.

It makes you wonder if the fact that astronomers were expecting to see a planet influenced their readings. The measurements they made at the time had large uncertainties, so if they thought that they were looking at a planet, that easily could have biased their estimates in favor of a diameter closer to that of a planet.

In the case of the diameter of Ceres, these biases don’t have any serious consequences. But in medicine for instance, this kind of bias is why it is so important to have double blind studies. That way, results aren’t influenced by what researchers and participants expect.

You can listen to this story in French here.

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PhD, astrophysicist. I play with keyboards and telescopes whenever I can.