The Story of Ceres, the Not-Quite-a-Planet Planet

There are signs that Ceres contains large amounts of water ice beneath its surface and water vapor perhaps rooted in sublimating ice.

We're all fascinated by the outward appearance of foreign planets—Saturn's rings, Jupiter's largesse—but often the most interesting thing about a world is how it became known as a planet in the first place. Ceres is a perfect example. This dwarf planet, which NASA's Dawn spacecraft will go into orbit around on March 6, has been classified as a planet, an asteroid, and a dwarf planet in the 200 or so years we've known about it.

Scientists consider Ceres an embryonic planet. When the planets in our Solar System were forming the cosmic swirl of gas and dust, Ceres wanted to be a planet, but Jupiter had other ideas. The massive gas giant managed to stop Ceres from amassing enough material to become a planet, leaving it as the largest object in the debris ring that orbits between Mars and Jupiter; Ceres measures about 590 miles across and accounts for one quarter of the asteroid belt's total mass.

There are signs that Ceres contains large amounts of water ice beneath its surface and water vapor perhaps rooted in sublimating ice. If it turns out that Ceres is made of just 25 percent water, it will have more fresh water than the Earth. Within the huge gap between Mars and Jupiter, Ceres stands out.

It was Johannes Kepler, the 17th century German astronomer for whom the Kepler Space Telescope is named, who first addressed the significant distance between Mars and Jupiter. He joined the tradition of astronomers who tried to make the orbits of the planets fit into some perfect arrangement to reflect divine perfection. First forced to add a hypothetical planet between Mars and Jupiter to maintain the ratio of the distances between planets, he eventually explained the gap by arranging the cosmos with the planetary spheres nestled between Platonic solids.

Kepler's Platonic solid model of the Solar System from Mysterium Cosmographicum (1596). Image: Wikipedia

Later astronomers tried to work this gap into the overall dynamics of the Solar System. Isaac Newton chalked the gap up to a divine plan for a clockwork universe, while Immanuel Kant described the gap as worthy of holding a great planet with a large mass.

But these awkward attempts to explain the gap became obsolete as telescopes became an integral part of astronomy. In 1610, Italian astronomer Galileo Galilei turned his telescope on Jupiter and found its four major moons. Almost two centuries later, in 1801, Sicilian astronomer Father Giuseppe Piazzi found a new, small body using a telescope.

He named it Ceres after the Roman goddess of corn and harvests. It was the first planet discovered that couldn't be seen with the naked eye, and it closed the gap between Mars and Jupiter. Ceres was given a symbol—all the planets have one to make shorthand notation simple and universal—and entered into what few astronomical books were being printed at the time.

It wasn't long before astronomers found other bodies in the vicinity of Ceres. Three fairly large objects called Pallas, Juno, and Vesta were discovered in 1802, 1804, and 1807 respectively. They were the first evidence that Ceres wasn't unique, and because all four orbited the Sun so close to one another and were similarly small in size, they became known collectively as asteroids or minor planets. Ceres, in any case, remained considered a planet.

The Dawn spacecraft observed Ceres for an hour on Jan. 13, 2015, from a distance of 238,000 miles. Image: ​NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

But as astronomers found more small bodies orbiting in the same general area as Ceres, things started to get crowded and the symbols became more complicated. In the 1850s, American astronomer Benjamin Apthorp Gould suggested simplifying the shorthand to identify the small bodies in this growing population: do away with the symbols and use numbers corresponding to the order of discovery inside circles. The simpler notation was quickly adopted, solidifying the small bodies as one type of object.

Ceres and the other larger of these small bodies were classified as small planets, asteroids, and planets in turn in a variety of astronomical publications. Then, in the 1860s, Ceres, along with Vesta, Juno, and Pallas, was lumped into the same categories with all the other small bodies that lie between Mars and Jupiter. This objects that make up this group were briefly classified as planets in the late 19th century before they were reclassified as asteroids in the 1890s.

So Ceres, the first planet discovered in the modern age, was an asteroid, and it stayed that way until 2006.

When CalTech astronomer Mike Brown discovered an object larger and brighter than Pluto orbiting well beyond the accepted last planet in our solar system, it threw the notation system into chaos; the body was appropriately named Eris. The question of whether Brown found a planet or not had bigger implications. If Eris was a planet, then all the similarly-sized bodies should be planets, including objects like Ceres and the distant Sedna. We'd go from nine planets to possibly hundreds, depending on what defined a "planet."

Determining Eris's fate forced a redefinition of what a planet is, and eventually saw the introduction of the term dwarf planet. A dwarf planet is a body that is round and orbits the Sun but is too small to have cleared its orbit of debris. Eris fell into this new category, taking Pluto and Ceres with it.

We're sure to find some amazing things when Dawn reaches Ceres in a couple of months. For one little rock in space, it's got a really big story to tell.