One of the newly discovered dwarf worlds that got Pluto kicked out of the major planets club is sporting a distinct spot, scientists have discovered.

Icy plutoid Haumea has a red blotch that appears to be richer in minerals and organic materials than the surrounding surface. It could mark the location of an impact in relatively recent times.

Haumea, discovered by Mike Brown of Caltech in 2004, and named after the Hawaiian goddess of childbirth, is unusual because of its squashed ovoid shape. This is caused by its rapid spin – it makes one rotation in a little under four Earth hours.

The fast spin is also put down to an impact by another body such as a large asteroid around a billion years ago. The zone that Haumea inhabits, called the Kuiper Belt, is thought to be swarming with millions of small icy bodies. A NASA probe, New Horizons, is racing to visit the region out beyond Neptune.

Haumea seems to have captured two of them as moons – the plutoid was discovered by Brown’s team to have two tiny satellites, now called Hi’iaka and Namaka, in 2005.

The plutoid’s “great red spot” was discovered by measuring the dwarf planet’s change in brightness as it rotated. There was a clear light-curve which appeared different at different wavelengths, leading discoverer Dr Pedro Lacerda to deduce its colour.

Dr Lacerda, who is Newton Fellow at Queen’s University Belfast, will present his findings at the European Planetary Science Congress in Potsdam today.

He said: “Our very first measurements of Haumea told us there was a spot on the surface. The two brightness maxima and the two minima of the light-curve are not exactly equal, as would be expected from a uniform surface. This indicates the presence of a dark spot on the otherwise bright surface.

“But Haumea’s light curve has told us more and it was only when we got the infrared data that were we able to begin to understand what the spot might be.”

The spot’s colour may show that it is richer in minerals and organic compounds, or that it contains a higher fraction of crystalline ice. If it does mark the scar from an impact, then the material detected might be like that the impacting body was made of, possibly mixed with material from beneath Haumea’s icy surface.

Haumea is the fourth largest known plutoid, or Kuiper belt object, after Eris, Pluto and Makemake. It measures 1,250 miles (2,000 km) by 1,000 miles (1,600 km) by 625 miles (1,000 km). The way it spins tells us this is a rocky world beneath the ice.

Astronomers now plan to use the European Southern Observatory’s Very Large Telescope in Chile in 2010 to make detailed spectroscopic observations to try to identify the spot further and work out how it originated.

The plutoids are too distant to get detailed images of them. This is said to be the best picture of Pluto from Earth – it is said to be better than an image taken by the Hubble space telescope.

Picture: Part of a computer model showing how the red spot appears on Haumea. ( Credit: P Lacerda).

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