Ceres (dwarf planet)

Ceres is a dwarf planet sitting in the main asteroid belt, between Mars and Jupiter. It was the first object ever spotted in that belt, discovered on January 1, 1801, by Giuseppe Piazzi at the Palermo Astronomical Observatory in Sicily. At the time, it was announced as a new planet. Later, it got reclassified as an asteroid, and then, more recently, as the only confirmed dwarf planet inside the asteroid belt. It is also the largest object in the belt that has no moon, and it is the only recognized dwarf planet in the solar system whose orbit stays inside Neptune’s. In the minor planet numbering system, it goes by 1 Ceres or (1) Ceres.

Its diameter is roughly one-quarter that of the Moon. Because it is so small, even at its brightest, you cannot see it with the naked eye unless the sky is extremely dark. Its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition—when it is closest to Earth—every 15 to 16 months. As a result, even the most powerful telescopes barely reveal any surface features, and very little was known about it until NASA’s Dawn spacecraft arrived for its orbital mission in 2015.

Dawn found that Ceres’s surface is a mix of water, ice, and hydrated minerals like carbonates and clay. Gravity data suggest it is partially differentiated: it has a muddy mantle and core made of ice and rock, topped by a less dense but stronger crust that is at most 30 percent ice by volume. Although Ceres likely has an internal ocean of liquid water, brines still flow through the outer mantle and reach the surface. This allows cryovolcanoes like Ahuna Mons to form roughly every 50 million years, making Ceres the closest known cryovolcanically active body to the Sun. It also has an extremely thin, temporary atmosphere of water vapor, vented from localized sources on its surface.

**History** In the years between the acceptance of heliocentrism in the 1700s and the discovery of Neptune in 1846, several astronomers argued that mathematical laws predicted a hidden or missing planet between Mars and Jupiter. In 1596, Johannes Kepler thought the ratios between planetary orbits would match “God’s design” only if two extra planets existed: one between Jupiter and Mars, and another between Venus and Mercury. Other thinkers, like Immanuel Kant, wondered if Jupiter’s gravity had created the gap. In 1761, Johann Heinrich Lambert asked, “And who knows whether already planets are missing which have departed from the vast space between Mars and Jupiter? Does it then hold of celestial bodies as well as of the Earth, that the stronger chafe the weaker, and are Jupiter and Saturn destined to plunder forever?”

In 1772, Johann Elert Bode, citing Johann Daniel Titius, published a formula later called the Titius–Bode law. It seemed to predict the orbits of the known planets, except for an unexplained gap between Mars and Jupiter. The formula said there should be another planet with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun. The law gained more support when William Herschel discovered Uranus in 1781, close to the predicted distance for a planet beyond Saturn. In 1800, a group led by Franz Xaver von Zach, editor of the German journal *Monatliche Correspondenz*, sent requests to 24 experienced astronomers—whom he called the “celestial police”—asking them to work together and systematically search for the expected planet. They did not find Ceres, but later discovered the asteroids Pallas, Juno, and Vesta.

**Discovery** One of the astronomers chosen for the search was Giuseppe Piazzi, a Catholic priest at the academy of Palermo, Sicily. Before he even received his invitation to join the group, Piazzi discovered Ceres on January 1, 1801. He was looking for “the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille,” but found that “it was preceded by another.” Instead of a star, Piazzi had spotted a moving, star-like object, which he first thought was a comet. He observed Ceres 24 times, with the last sighting on February 11, 1801, when illness cut his work short. He announced his discovery on January 24, 1801, in letters to two fellow astronomers: his compatriot Barnaba Oriani of Milan and Bode in Berlin. He reported it as a comet, but added, “since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet.” In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the *Monthly Correspondence*.

By then, Ceres’s apparent position had shifted (mostly because of Earth’s motion around the Sun), and it was too close to the Sun’s glare for other astronomers to confirm Piazzi’s observations. Toward the end of the year, Ceres should have been visible again, but after such a long gap, predicting its position was difficult. To recover Ceres, the mathematician Carl Friedrich Gauss—then 24 years old—developed an efficient method of orbit determination. He predicted Ceres’s path within a few weeks and sent his results to von Zach. On December 31, 1801, von Zach and fellow celestial policeman Heinrich W. M. Olbers found Ceres near the predicted position and continued to record its location. At 2.8 AU from the Sun, Ceres seemed to fit the Titius–Bode law almost perfectly. When Neptune was discovered in 1846, eight AU closer than the law predicted, most astronomers concluded the law was a coincidence.

The early observers could only calculate Ceres’s size to within an order of magnitude. In 1802, Herschel underestimated its diameter at 260 km (160 mi); in 1811, German astronomer Johann Hieronymus Schröter overestimated it.