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Spot makes strange dwarf planet even stranger

Astronomers have detected what appears to be an organic-rich red spot on a dwarf planet already known for its unusual shape and fast spin.
Image: Haumea's red spot
A series of images shows how Haumea's red spot changes position as the dwarf planet rotates.P. Lacerda
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A dwarf planet in our solar system, called Haumea, is known for its unusual shape and fast spin. Now astronomers have discovered another distinguishing feature: a dark red spot which appears to be richer in minerals and organic compounds than the surrounding icy surface.

Haumea, discovered in 2004, orbits the sun beyond Neptune, in a region known as the Kuiper Belt. It is classified as a dwarf planet — a celestial body that is big enough to have been rounded by its own gravity, but which has not cleared its orbital region of similar objects. The International Astronomical Union has designated four other bodies as dwarf planets: Ceres, Pluto, Eris and Makemake. Haumea is the fourth-largest dwarf planet.

Haumea is also the fastest-spinning large object in the solar system — one day on Haumea is equal to about 3.9 hours on Earth. This rapid rotation distorts Haumea, elongating it into a football-like shape.

Most of what we know about this object was determined from studying variations in its brightness, called a "light curve." And it is through examination of this light curve that scientists have found the dark spot.

"The two brightness maxima and the two minima of the light curve are not exactly equal, as would be expected from a uniform surface," said Pedro Lacerda of Queen's University Belfast. "This indicates the presence of a dark spot on the otherwise bright surface."

Additionally, the light curve is not exactly the same shape in all wavelengths. Small but persistent differences indicate that the dark spot is slightly redder in visible light and slightly bluer at infrared wavelengths.

"Our very first measurements of Haumea already told us there was a spot on the surface, but it was only when we got the infrared data that we were able to begin to understand what the spot might be," Lacerda said.

While the origin of the spot is unknown, possible interpretations of these measurements are that the spot is richer in minerals and organic compounds, or that it contains a higher fraction of crystalline ice. If the spot is a scar of a recent impact, then the spot material might resemble the composition of the impactor, perhaps mixed with material from the inner layers of Haumea.

Lacerda will present the discovery Wednesday at the European Planetary Science Congress in Potsdam, Germany.

New observations of this spot are planned for early 2010 using the ESO Very Large Telescope. "Now we will get detailed spectroscopy of the spot to hopefully identify its chemical composition and solve the puzzle of its origin" Lacerda concluded.