The asteroid belt between Mars and Jupiter offers an excellent platform to survey the origins of our solar system. Therein lies Ceres, a dwarf planet and the target of an ongoing NASA mission. This dramatically cratered and inviting world has piqued the interest of astronomers and astrobiologists alike. In 2007, NASA’s Dawn spacecraft was launched in order to reconnoiter its surface. Images beamed back in March 2015, at the spacecraft’s closest approach of 390 kilometres above the surface, yielded intriguing features, bright spots, and chaotic terrains and as such engendered much discussion and speculation. New details and sharper images of the craters have very recently emerged, heightening curiosity about this intriguing world even further.

The Discovery of Ceres

Ceres was first discovered by Italian astronomer Giuseppe Piazzi in 1801 by accident as he went about conducting his usual nightime sky observations, cataloging stars at his observatory in Palermo. Initially, Piazzi thought that it was some faint star but later noticed it moving along the observation field in a very predictable pattern. The discovery elicited a great feeling of euphoria among some of the astronomers of the time. That’s because by 1766, the German astronomer Johann Titius had formulated the controversial Titius-Bode law, which related the spacing of planetary distances with a mathematical description. The law maintained that planetary systems are regularly spaced in a predictable pattern. It’s derived by the empirical formula a = (n +4)/10 where a was to be the distance of the planets from the sun and the values of n were to be substituted from the sequence (0,3,6,12,24,18…). The numbers that resulted by inputting the values of this sequence into the formula quite intriguingly yielded the distances from the sun of the known planets of the time in astronomical units (AU). However, there was a gap between Mars and Jupiter, where the law predicted there should be a planet at 2.8 AU from the sun. “But should the Lord Architect have left that space empty?” wrote Titius. The discovery of Ceres, as Piazzi would name it, fulfilled the prediction and evoked raised excitement. After observing Ceres for about 6 weeks, Piazzi would finally report his discovery to a number of astronomers. He had been largely unaware of their growing search for a planet between Mars and Jupiter.

“On the 1st of January, I have observed in the shoulder of Taurus a star of the 8th magnitude which, on the following night, that is the 2nd, advanced by about 3’30” northwards and about 4′ towards Aries’ section. I did verify my observations on the 3rd and 4th, and found approximately the same motion. On 5, 6, 7, 8, 9, the sky was covered. I did see the star again January 10 and 11, and then on 13, 14, 17, 18, 19, 21, 22, and 23. On the first observation its R.A. was 51°47′ and its northern declination was 16°8′. From 10 to 11 it turned from retrograde to direct motion, and on the observation of 23, it had R.A. 51°46′, Dec. 17°8′. I have announced this star as a comet, but since it shows no nebulosity, and moreover, since it had a slow and rather uniform motion, I surmise that it could be something better than a comet. However, I would not by any means advance publicly this conjecture. As soon as I shall have a larger number of observations, I will try to compute its elements….I beg of you to let me know if it has already been observed by other astronomers; in this case, I should save myself the trouble of computing its orbit.”

As Ceres went behind the sun in February, Piazzi would lose track of it, which would bring the legendary mathematician, Carl Friedrich Gauss, into the scene. Gauss would devise a statistical method to predict where Ceres would exactly reappear later in the year on December 31, 1801. He accomplished this feat by analyzing the measurements of Ceres and pinpointing the errors that resulted from those measurements. In doing so, Gauss would develop an error distribution and what we know of today as the Gaussian (normal) distribution curve – the basis of modern statistics! He would set out his methods in his seminal 1809 work Theoria Motus Corporum Coelestium in Sectionibus Conicis.

The discovery of Ceres by Piazzi and the earlier discovery of Uranus by the English astronomer, William Herschel, seemed to fulfill the predictions made by the Titius-Bode law. However, the law was not really grounded in any theoretical explanation and was dismissed by some astronomers as just an intriguing coincidence. Indeed, soon enough, it would later be shown that the “law” is quite flawed. As the German astronomer, Heinrich Olbers would later discover, another celestial object, Pallas, had an orbit close to Ceres. Later discoveries of the asteroids Juno and Vesta (also at this region between Mars and Jupiter) in the years that would ensue and of the planet Neptune, which does not obey the law, would cause it to collapse as just a chance occurrence. Thousands of asteroids would later be cataloged between Mars and Jupiter and current estimates put the number of asteroids therein at about 10,000.

Largest Asteroid…Smallest Dwarf Planet

When Ceres was first discovered by Piazzi, it was designated planetary status and remained so for 50 more years. It was later, however, relegated as an asteroid as similar objects were spotted nearby, before being finally classed as a dwarf planet in 2006. At 940 kilometres across, the largest of the asteroids became the smallest of the dwarf planets. The International Astronomical Union defines a dwarf planet as a celestial body that “is in direct orbit around the sun” and that has attained enough mass such that gravity would crush it into a spherical form. Indeed, Ceres is only spherical planetoid in the asteroid belt.

The Dawn Mission: a Puzzling World Unraveled

Launched in 2007, the Dawn spacecraft, employing ion propulsion technology, finally arrived at Ceres after a journey of over 7.5 years and 5 billion kilometres. In mid-2015, very fascinating close-up images were beamed back to Earth and engendered much speculation, as they revealed mysterious white spots in one of the craters, the Occator crater. Very recently this past December, a paper was published in Nature and it announced results that the mysterious bright spots are very likely magnesium sulfate deposits – left behind as ice sublimated away from the surface as a result of, hinting at the possibility of subsurface icy water. A separate study also announced the presence of ammonia-rich clay, that carpets much of Ceres’s surface, which pushes back the origin of Ceres beyond the asteroid belt and raises the suggestion that it might have actually formed in the outer fringes of the solar system beyond Neptune, where ammonia is abundant, but later settled further inwards.

Video showing full rotation of Ceres:

New Details Dawn In

Further images of Ceres’s surface and of its fascinating craters have very recently emerged a few days ago. The Kupalo crater, a relatively young crater at 26 kilometres across, is shown in exquisite detail in the latest pictures from NASA. The images expose an intriguing bright material streaking through its rims. However, it is not known with certainty whether such material is related to the white spots that are subsumed by the Occator crater and which were previously discussed.

However, the Dawn spacecraft will continue to hover at this close altitude until the end of its mission in June 2016, by which time it will have beamed back even more fascinating results and more astounding images and will have refined our understanding of our early solar system. From Piazzi’s early forays into cataloging the night sky and unraveling this astonishing celestial world to our sense-extending technologies projecting stunning close-up images from its surface into our minds, the human endeavour to understand the universe has never failed to fascinate.

Bibliography

Clifford J., Marsden, B., and Brian G. (2011) “Giuseppe Piazzi: the controversial discovery and loss of Ceres in 1801.” Journal for the History of Astronomy 42: 283-306.

Featured image courtesy of NASA: Dwarf planet Ceres. Letter from Piazzi in January 1801 courtesy of Brera Observatory archives.