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Haumea: The Strangest Icy Rock at the Edge of the Solar System

by Emily Schaller, Hubble Fellow

Orbiting the Sun at a distance 43 times that of Earth in a region of space called the Kuiper Belt is a dwarf planet named after the Hawaiian goddess of childbirth and fertility, Haumea. There are now so many dwarf planets known in the Kuiper Belt (Pluto was the first) that it is hard to keep track of them all. Haumea distinguishes itself by being one of the most interesting and strangest objects in the entire solar system.

Soon after its discovery in 2005, astronomers noticed the brightness of Haumea increased and decreased every two hours. This variation is due to Haumea's strange shape and fast rotation. Haumea is shaped like a football and rotates end over end every four hours. When the long end is facing us, Haumea is brighter, but when all we see is its short end, it is fainter. The odd shape is caused by its fast rotation--the rapid spinning causes it to stretch out into the football-like shape we observe.

Haumea
Dwarf planet Haumea is shaped like a football.

Haumea has two small moons orbiting it, Hi`iaka and Namaka, that are named for the children of the goddess. Their existence has enabled astronomers to "weigh" Haumea by watching how long the moons take to complete their orbits. If an object is very massive, its moons will whip around it very fast, but if it is light, they will orbit more slowly. It turns out that Haumea is very massive for its size. This means that unlike most large Kuiper Belt objects, which are made up of about half water ice and half rock, Haumea is almost purely rock. Interestingly, though, spectral observations of the light coming off of Haumea's surface have revealed that it is covered not with rock, but with pure water ice. Haumea is a large rock with a thin coating of water ice on its surface, like an M&M but without the chocolate.

In the outer solar system, most large objects are thought to have a rocky core surrounded by a water-ice mantle. Our observations of Haumea suggest that it probably experienced a large collision with an object about the same size early in its history. This collision stripped off most of its ice, left it spinning rapidly, and formed its two moons. Recently, small chunks of nearly pure water ice have been discovered in orbits very similar to that of Haumea. These chunks are likely additional pieces of its icy mantle.

In a stroke of astronomical good luck, Namaka, the smaller of the two moons, is circling Haumea in an orbit almost exactly edge-on as seen from Earth. Every nine days Namaka passes either directly in front of or directly behind Haumea, causing the entire system to become fainter by a very small amount. These "mutual events" last for about an hour and will provide a wealth of information about the Haumea system that would otherwise be totally unavailable to us.

orbits of Namaka and Hi`iaka

The orbits of Hi`iaka (outer satellite) and Namaka (inner satellite). Namaka's orbit is nearly edge on as viewed from Earth. Every nine days Namaka passes directly in front of or behind Haumea as seen from Earth. (Courtesy D. Ragozzine)

For example, the size and shape of Haumea is uncertain by several hundred miles. Haumea is so far away that it looks not much different from a point of light even through the largest telescopes. However, measuring the exact timing of the dip in brightness will allow us to determine Haumea's size very precisely (down to several tens of miles). The various paths that Namaka takes across Haumea over the course of several years will also allow us to precisely determine Haumea's odd shape.

I observed the first of these mutual events using the UH 2.2-meter (88-inch) telescope on January 31 and will observe several more over the coming months. Unfortunately, these events will not last forever--the alignment of Haumea and Namaka as seen from Earth will last for several years, but then will not occur again for another 140 years.