In January 2005, the discovery of Eris obliged astronomers to review their definition of the word “planet”, resulting in the declassification of Pluto as a planet. In November 2010, Eris passed in front of a weak star in the Cetus (Whale) constellation. The observations of this occultation with the Belgian robotic telescope TRAPPIST has sent new shock waves through the scientific community. The results were published in Nature.

The discovery in January 2005 of the dwarf planet Eris – then named Xena by its discoverer Michael Brown – has upset the solar system and the scientific community in particular: should it be added on as the next planet after Pluto? In fact Eris has a bigger mass (25%) and seemed to be bigger than Pluto by some 500 km... at least this is what was thought at the time. It followed that if Pluto was a planet, Eris had an even greater claim to be one. Yet Pluto, like Eris, is much smaller than the other planets in the solar system. Even though it was thought, after its discovery in 1930, that Pluto was the size of terrestrial planets like Earth, it weighs only 0,24% of our planet and, with its diameter of 2300 km, is even smaller than the Moon (3400 km in diameter)! Moreover, since 1992 more than a thousand large frozen asteroids have been discovered in this remote region of the solar system. They are known as Trans-Neptunian Objects (TNO) because they are located beyond the orbit of Neptune in what astronomers call the Kuiper Belt.
An answer was needed urgently because the discovery of Eris opened up the possibility of the existence of other large objects of this size in the remote part of the solar system.

In August 2006 astronomers came to an agreement during a heated debate on the occasion of the General Assembly of the International Astronomical Union (IAU) which was held in Prague. They defined a “planet” as a body that is big enough to be in a spherical form (the force of gravity having beaten the internal cohesive forces) but also having cleared its orbit of other possible bodies. Due to the fact of not having cleared its orbit which it shares with other TNOs, Pluto has therefore been excluded from the category of “planet”, much to the despair of some Americans who did not hesitate to demonstrate in the streets. The astronomer Emmanuël Jehin, a FNRS Researcher Associate with the Astrophysics and image processing unit (ATI) of ULg. explains, “Pluto is the only planet in the solar system that was discovered by the Americans. It also gave its name to Mickey Mouse’s favourite dog, Pluto.”

But the die was cast. Following the recommendations of the IAU work group and a plenary session vote, our solar system now only has eight planets. Pluto lost its status as a “planet” and marked the beginning of the new category of “dwarf planets”. At the same time, Eris was promoted: from its status as a mere trans-Neptunian asteroid (TNO), it was also declared to be a dwarf planet just like Ceres, the biggest object in the main asteroid belt located between Mars and Jupiter. Since then, two other “dwarf planets” have joined them: Makemake and Haumea, two TNOs discovered in 2005 and 2003 respectively. And a dozen more are on the waiting list!

(1)" A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation " B. Sicardy , J. L. Ortiz , M. Assafin , E. Jehin , A. Maury , et al. Nature 478, 493-496 2011 doi:10.1038/nature10550