Haumea: Fast-Spinning Dwarf Planet
Artist's concept of the dwarf planet Haumea and its two satellites (Hi’iaka and Namaka).
Credit: SINC/José Antonio Peñas

Just after Christmas of 2004, a team from the Palomar Observatory led by Mike Brown discovered a tiny, Pluto-size body in images taken the previous spring. The astronomers to nickname the rock "Santa." About the same time Brown's team published their information online, a group of astronomers from the Sierra Nevada Observatory led by José Luis Ortiz Moreno announced their discovery of the body in images taken in March of 2003.

Originally designated as 2003 EL61, the object was classified as a Kuiper Belt Object until the International Astronomical Union reclassified it as the fifth dwarf planet in the solar system, following Ceres, Pluto, Eris and Makemake.

When it was reclassified, 2003 EL61 was renamed after the Hawaiian goddess of childbirth and fertility. Its two moons were named for daughters of the goddess, Hi'iaka and Namaka, who were said to have been born from the body of Haumea.

Haumea takes 285 Earth years to orbit the sun. At its closest, it only comes within 34 times the Earth-Sun distance, while at its farthest, it lies more than 51 times as far away. These distances, combined with the dwarf planet's tiny size, would generally make it difficult for scientists to accurately determine its mass and density. However, in 2005, the first of two moons was found orbiting the body, enabling scientists to determine the mass of the bodies.

Weighing in at 4,000 billion billlion (1021) kilograms, (9,000 billion billion pounds), Haumea is approximately a third as massive as Pluto. Haumea spins on its axis once every four hours, making it the fastest spinning known large object in the solar system.

The dwarf planet's rapid spin keeps it from attaining a spheroid shape, instead causing it to look more like a slightly flattened football spinning end over end, as though it had been kicked. Haumea is 1,960 kilometers (1,218 miles) across at its longest axis, but only about half as wide — 996 km (619 miles) — at its shortest.

The rapid spin of the dwarf planet allowed scientists to calculate its density, because different materials would stretch out differently. As a result, scientists think that Haumea is made up almost entirely of rock.

Observations of the planet, however, reveal a brightly gleaming surface. Scientists have concluded that, though most of the dwarf planet's interior is rocky, it is covered by a thin icy shell.

Haumea also appears to have a dark red spot on its surface that may contain more minerals and organic compounds than the ice around it.

Dwarf Planet Veiled in Water-Ice

The two moons of Haumea are far smaller than the dwarf planet. The largest, Hi'iaka, is only about 1 percent as massive as Haumea, while the smaller, fainter Namaka weighs only about a tenth as much. 

Hi'iaka takes 49 days to travel around Haumea in a nearly circular orbit. The inner satellite, Namaka, takes 18 days to travel around its parent body in an elliptical orbit. Both moons are thought to be composed of almost pure water-ice, which may have originally come from Haumea itself.

Although roughly a tenth of all known Kuiper Belt objects have a satellite, very few of them have multiples, making Haumea somewhat unusual.

Astronomers think that, in the early solar system, Haumea was much like Pluto, composed half of rock and half of water. Billions of years ago, a large object may have collided with the body, knocking most of the surface ice away and imparting a rapid spin to the dwarf planet. The spin, in turn, elongated the dwarf planet into its unusual shape.

The moons of Haumea may have once been part of its surface, forming much like Earth's moon did after a collision.

The discovery of a number of small, icy debris in orbits similar to Haumea's seemed to further substantiate the idea of an ancient collision.