1801

Ceres

Giuseppe Piazzi (1746–1826)

The discoveries of Uranus and a dozen new solar system moons in the late eighteenth century spurred astronomers into even more detailed cataloging and searching of the skies. Among the most meticulous and methodical of those sky mappers was the Italian priest, mathematician, and astronomer Giuseppe Piazzi of the Palermo Astronomical Observatory. Between 1789 and 1803 Father Piazzi oversaw the creation of the Palermo star catalog, a survey of nearly eight thousand stars.

As part of his catalog work, on January 1, 1801, Piazzi noticed a dim (8th magnitude) star near the head of the constellation Cetus that hadn’t appeared in any previous catalogs. He observed that it moved against the fixed stars over the following nights, and thought that he might have discovered a comet. But the lack of any comet-like coma or tail led him to suspect that it might be “something better.”

Indeed, follow-on observations by Piazzi and others led to the discovery late in 1801 that the starlike object was instead orbiting the Sun as though it were a planet, between Mars and Jupiter (near 2.7 astronomical units). Piazzi named it Ceres Ferdinandea, after the Roman goddess of plants and his patron, King Ferdinand III of Sicily. Astronomers weren’t sure how to classify Ceres—too small and dim to be a planet but clearly not a star, comet, or moon. In 1802 the astronomer William Herschel coined the term asteroid (meaning “starlike”) to describe Ceres and the newly discovered Pallas.

Modern telescopic observations have shown that Ceres is the largest of the main belt asteroids, with a diameter of around 590 miles (950 kilometers) and a density around 2 grams per cubic centimeter, suggesting the possible presence of an ice-rich interior. Because Ceres is large enough to have been formed into a round shape by its own gravity, astronomers today classify it as a dwarf planet (like Pluto). We learned an enormous amount about the surface and interior of Ceres when the NASA Dawn mission finally reached it in March 2015. Highlights include the discovery of ancient ice volcanoes and simple organic molecules on the asteroid’s surface.

As of mid-2018, astronomers have determined the orbits of more than 750,000 asteroids, and that number is expected to grow much larger as more detailed surveys discover fainter objects. The solar system is teeming with minor planets!

This image of Ceres, the largest asteroid in the solar system, was taken from NASA’s Dawn spacecraft shortly after entering Ceres’s orbit in 2015. The colors approximate how Ceres would appear to the human eye.