How big is Ceres?

Ceres is the largest object in the asteroid belt between Mars and Jupiter, and it is about 940 kilometers (584 miles) across on average. It is also a dwarf planet, and the only one in the inner solar system. It orbits the Sun about 2.8 times as far out as Earth does, taking 4.6 years to go around once.

Size

NASA's Dawn spacecraft studied Ceres from orbit between 2015 and 2018 and mapped its shape. Ceres is not a perfect ball. It measures about 964 kilometers across at its equator but only 892 kilometers from pole to pole, so it is slightly flattened. A sphere with the same volume would be 939 kilometers across, which is the figure given as its diameter.

Some NASA pages give a radius of 476 kilometers, which makes a diameter of 952 kilometers. That is close to the 950 kilometers shown above. The 939-kilometer figure, from a 2016 study of Ceres's shape, is precise to within a fraction of a kilometer and is the better measurement.

Size comparisons

If Earth were shrunk to the size of a basketball, the Moon would be about the size of a tennis ball, and Ceres would be a bead about 18 millimeters (0.7 inches) across.
The Moon is about 3.7 times as wide as Ceres, and Earth is about 14 times as wide.
Ceres is about twice as wide as the Grand Canyon is long. The canyon runs 446 kilometers (277 miles) along the Colorado River.
Pluto, the best-known dwarf planet, is about 2.5 times as wide as Ceres.

From planet to asteroid to dwarf planet

The Italian astronomer Giuseppe Piazzi discovered Ceres on January 1, 1801. It was the first object found in the main asteroid belt. At first it was called a planet, but as many more bodies were found in the same region it was reclassified as an asteroid.

In 2006 Ceres was designated a dwarf planet, along with Pluto and Eris. Under the rules of the International Astronomical Union (IAU), a dwarf planet orbits the Sun and has enough mass for its own gravity to pull it into a nearly round shape. Unlike a planet, it has not cleared other objects from around its orbit, and many asteroids come close to Ceres's path. As of 2026, the IAU recognizes five dwarf planets: Ceres, Pluto, Eris, Haumea and Makemake.

Ceres holds a large share of the asteroid belt's mass. NASA's estimates range from about a quarter to more than a third of the belt's total. Even so, Pluto is 14 times as massive as Ceres.

What Dawn found

Even before Dawn arrived, Ceres's low density showed that it is about 25 percent ice by mass. That makes it the most water-rich body in the inner solar system after Earth, counting the total amount of water. Dawn reached Ceres in March 2015, making it the first dwarf planet to be visited by a spacecraft. Its images showed that Ceres's bright spots, the brightest of them in Occator Crater, are deposits of carbonates and other salts.

Measurements of Ceres's gravity show that it gets denser toward its center. Early in its history, rock sank inward and a crust rich in water formed above it. Dawn also found signs of recent and possibly ongoing activity on the surface. Salts may help drive this activity by lowering the temperature at which water freezes, keeping some of it liquid.