A few years ago, when 2% of astronomers decided Pluto should no longer a planet, the confusion was so great and the definition so arbitrary and so we now have five 'dwarf' planets in our solar system - Pluto, Ceres, Eris, Haumea and Makemake.  

Tiny Haumea moves beyond the orbit of Neptune, has the shape of a flattened rugby ball and is around 2,000 km long. It spins completely in less than four hours, at one of the fastest rotation speeds in the Solar System.    The precise orbits and sizes of Haumea are still not known (approximate scales of around 2,000 x 1,500 x 1,000 km) nor are those of its satellites. In reality, these are two very distant bright points of light, the data for which are obtained indirectly.  Haumea's existence was confirmed in 2005, when it was called 2003 EL61.

In Hawaiian mythology, Haumea is the goddess of fertility and birth, and Hi'iaka and Namaka are two of her daughters.     In space, Haumea is a 'dwarf' planet with two satellites, Hi'iaka and Namaka, and crystallized water that covers them makes them shine in the darkness of space.   75% of Haumea and 100% of Hi'iaka (which is around 400 km in diameter) are covered with crystallized water-ice and not, as would have been expected, with amorphous ice disorganized due to solar radiation.  A new study suggests that the planet is made up of a frozen outer layer and an internal section made up of between 88% and 97% rock (with a density of 3.5 g/cm3).

"Since solar radiation constantly destroys the crystalline structure of ice on the surface, energy sources are required to keep it organised. The two that we have taken into consideration are that able to generate radiogenic elements (potassium-40, thorium-232 and uranium-238) from the inside, and the tidal forces between Haumea and its satellites (as seen between the Earth and the Moon)", Benoit Carry, co-author of the study and a researcher at the ESAC Centre of the European Space Agency (ESA) in Madrid (Spain), tells SINC.

Haumea and satellites Hi’iaka and Namaka
Illustration of Haumea and its two satellites (Hi’iaka and Namaka).   Credit: SINC/José Antonio Peñas

The researcher also highlights other peculiarities of Haumea: "Its orbital plane is inclined at 28º with respect to the usual plane of planets in the Solar System, the orbits of its satellites are not on the same plane either – which is very unusual – and the entire system belongs to a single family within the frozen objects in the Kuiper Belt (at a distance of between 4.5 billion and more than 15 billion kilometres from the Sun)".

The two satellites could have been created by another object smashing into Haumea, which could also have originated the rapid rotation of the dwarf planet (3.9 hours) and have molded it into its rugby ball shape. Some numerical models say this could be caused by a fairly tangential impact.  Observations from the SINFONI instrument of the Very Large Telescope (VLT), the telescope of the European Southern Observatory (ESO) in Chile, were used in order to carry out the study, which has been published in the journal Astronomy&Astrophysics. ESO astronomer Christophe Dumas led this study from the observatory.

SINFONI is an integral field spectrometer that provides 'data cubes' in which two of the dimensions are spatial (like those of any flat image), while the third is spectral, meaning that each layer of the cube is an image taken with a different wave size.

In the case of the tiny Namaka (around 200 km in diameter), the signal at the time it was observed was so weak that it was impossible to obtain information about its surface, although new data on its orbit were gathered. Meanwhile, the models for the tidal forces of this strange system are also improving.

Another of the mysteries of Haumea is the presence of a dark, reddish spot, which contrasts with the whitish colour of the planet. "My interpretation of the infrared photometry is that this area could be a richer source of crystalline water-ice than the rest of the surface", Pedro Lacerda, co-discoverer of the spot and an astronomer at Queen's University in Belfast (United Kingdom), tells SINC. The researcher does not rule out the possibility of some kind of irradiated mineral or organic matter having caused this colouration.

Citation: Christophe Dumas, Benoit Carry, Daniel Hestroffer, Frederic Merlin, "High-contrast observations of (136108) Haumea. A crystalline water-ice multiple system",  Astronomy&Astrophysics 528: A105, abril de 2011. DOI: 10.1051/0004-6361/201015011.