Ceres (dwarf planet)

Discovered in 1801, Ceres, named for the Roman goddess of agriculture, is the largest of the main-belt asteroids. It was believed to be the eighth planet in the solar system for a short time. Still, discovering additional large main-belt asteroids influenced astronomers to revoke its planetary status. Discoveries of Pluto-sized objects in the Kuiper Belt have once again brought Ceres back into the discussion of what constitutes the definition of a planet.

Overview

On the night of January 1, 1801, the Italian astronomer Giuseppe Piazzi was observing the heavens when he noted a faint object that did not appear on his star charts. At first, he thought it might be a comet, but it did not have the typical “fuzzy” appearance associated with comets. If not a comet, what could it be? By observing its motion over the next several weeks, Piazzi could determine that its orbital speed was greater than that of Mars but slower than that of Jupiter. This suggested to him that the object must lie between the orbits of Mars and Jupiter.

Additional help came from the German mathematicianCarl Friedrich Gauss, who had perfected a means of calculating orbital motion based on limited observations. When he applied his method to Piazzi’s observations, Gauss could calculate where and when this mysterious object should next appear, and it did just as he predicted. Within one year of Piazzi’s discovery, Heinrich Olbers and Franz von Zach were able to relocate Ceres and refine its 4.6 Earth-year orbit. Later, scientists determined that it has a spherical shape with a 930-kilometer diameter. Compared to the Moon, Ceres is one-third its size, with less than 2 percent of its mass, giving it a much lower density of 2.1 grams per cubic centimeter.

Piazzi’s observations and the calculations of Gauss led many of the leading scientists of that time to believe that a new planet had been discovered. This conclusion seemed logical based on an earlier idea first suggested by Johann Daniel Titius of Wittenberg and later championed by Johann Elert Bode. In 1792, Bode pointed out an apparent mathematical relationship between the distances of the various planets to the Sun. He suggested that the planets were positioned at specific distances from each other based on a mathematical ratio that would later be referred to as Bode’s law. This worked reasonably well for all the planets from Mercury through Uranus, except for an apparent gap between Mars and Jupiter. When Piazzi found his mystery object positioned in this gap where Bode suggested a planet should be, this seemed to be the observational confirmation of Bode’s law. Even though modern science treats Bode’s law as more of an exciting coincidence than scientific law, it did serve a purpose then. It contributed to the eventual discovery of Neptune.

Although initially proclaimed the eighth planet in 1801, Ceres did not long retain its planetary status. The excitement created in the astronomical community by the discovery of Ceres led to a systematic search of the heavens, which centered on the ecliptic plane. Scientists believed that many new and interesting objects would soon be found, and they were right. Within the next six years, three new asteroids—Pallas, Juno, and Vesta—were found within the same general vicinity as Ceres. With four minor bodies occupying the same region of space, scientists concluded that no one planet would fill the gap in Bode’s law. A new theory would have to be created to explain the presence of so many small bodies occupying planetary positions.

Since then, many theories have been created to explain the presence of the main-belt asteroids. One of the more popular but incorrect theories described a large planet exploding and creating many smaller bodies ranging from the largest, Ceres, down to tiny meteoroid-sized fragments. Perhaps the most widely accepted theory describes a “planet that never formed.” This theory can be supported by the generally accepted nebular hypothesis of planetary formation, which envisions a final stage of accretion during which many smaller bodies are attracted to each other and form a much larger object. In the case of Ceres and the other main-belt asteroids, that final stage was interrupted, and they never fully accreted into a single large object.

By the late twentieth century, the study of Ceres and its family of asteroids was no longer regulated by the limitations of Earth-based telescopic observations. The Hubble Space Telescope, operating well above Earth’s atmosphere, revealed details never before seen by surface-based telescopes. In addition, modern astronomers can send their spacecraft-borne instruments directly to the asteroids to get close-up views of their surfaces and analyze their mineralogical compositions. Several flyby spacecraft missions have investigated smaller asteroids, revealing previously unimagined surface conditions. One in particular, the NEAR Shoemaker probe, first orbited and then actually landed on the asteroid Eros, giving scientists their first detailed images from the surface of an asteroid. The Japanese probe Hayabusa is believed to have landed on the surface of the asteroid Itokawa and collected a sample for return to Earth. Data returned by these missions have rewritten the textbooks on what is known about asteroids. The Dawn spacecraft, en route to both Vesta and Ceres, was designed first to orbit Vesta in 2011 and then leave orbit and go on to rendezvous with and orbit Ceres in 2015 and to collect sufficient data to help scientists gain a better understanding of the conditions present at the initial accretion stage of planetary formation in the solar system. This mission continued until 2018.

Ceres is very different from Vesta. Studies based on a comparison of densities and surface reflectivity have shown that Ceres may have what is considered a “wet” surface, composed of water-bearing minerals, as opposed to Vesta's “dry” surface minerals. Some scientists speculate that Ceres may have a total amount of water locked up in its interior that could rival Earth’s surface, while Vesta is more comparable to Earth’s Moon. Other Dawnexperiments should shed light on these two primordial bodies' internal structure, shape, composition, and mass. Scientists have determined that a particular class of meteorites, the HED achondrites, is probably derived from Vesta. Most meteorites are believed to be fragments of crustal material blasted off an asteroid’s surface during the accretion process or from later impacts. This is based on reflectivity studies of minerals present on the surface of Vesta and from radioisotope chronology studies of the HED meteorites. All evidence seems to point to Vesta. No such data exists for Ceres. This is why a spacecraft mission to Ceres is so essential.

Knowledge Gained

The discovery and later scientific study of the asteroid Ceres profoundly affected our early view of the solar system and our subsequent understanding of the origin and nature of planetary bodies. At its discovery in 1801, Ceres represented another “new” object in the heavens unknown to previous astronomers. It had been less than fifty years since the return of Halley’s Comet (1758), which had galvanized the concept of gravity and its effects on motion, and only twenty years after the discovery of Uranus (1791). The discovery of three other asteroids would soon follow, but after 1807, no other asteroid was detected until 1845.

The following year, astronomers discovered Neptune and moved into the modern era with bigger and better telescopes, spectroscopy and photography technology, and a more scientific perspective of the universe. With Neptune recognized as the eighth planet, the asteroids fell into their proper place within the solar system's structure. The study of asteroids remained within the domain of observational astronomers until scientists and engineers could develop the technology to send their scientific instruments to the planets and minor bodies. Once this happened, the outpouring of data changed our view of the nature and origin of the planets.

To the early astronomers, Ceres was only a faint speck of light in the night sky. Modern astronomers see it quite differently, primarily due to the Hubble Space Telescope’s Advanced Camera for Surveys observations. Astronomers observed Ceres through a nine-hour revolution in the early twenty-first century, taking 267 photographic images. From these observations, astronomers could determine that Ceres has a spherical shape with a diameter slightly wider at the equator than at the poles. This suggests that it has a differentiated internal structure with denser materials forming a core and lighter materials closer to the surface. Scientists suspect that because Ceres’s density is much lower than Earth’s, large amounts of water ice may exist either on the surface of Ceres or buried within its crust. This surmise is supported by spectral evidence for water-bearing minerals that may be present on the surface that are not representative of Ceres’s crystal rocks. Additional microwave studies suggest that this surface material might be dry clay. Ceres could be the most Earth-like body in the solar system and may even be a haven for primitive life forms.

Context

The asteroids hold many vital clues to unraveling the mysteries surrounding the formation of the planets. Based on their respective sizes, densities, and chemical compositions, the planets in our solar system are divided into three major groups: the terrestrial (Earth-like) planets, the Jovian (Jupiter-like) planets, and the dwarf planets. The third group can be further divided into rocky objects like Ceres and ice bodies like Pluto. These dwarf planets, rocky or ice, most likely represent a fundamental primordial stage in forming planets.

Scientists can better understand their formative processes by studying these early remnants of planetary formation. Each group will have its distinctive secrets to reveal. The rocky dwarf planets positioned between Mars and Jupiter formed under higher temperatures, higher density, and higher velocity than the icy worlds at the edge of the solar system in the Kuiper Belt. It is believed that at this distance from the Sun, objects have remained essentially unchanged over the last 4.6 billion years. In 2015, the New Horizons spacecraft began visiting Pluto and sent back images and data, giving science its first close-up look at this unknown world. In that same year, the Dawn spacecraft orbited Ceres several times, completing its final orbit in 2018. Scientists were able to fill many gaps in our understanding of planetary formation.

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