When the controversy surrounding Pluto ended it was not the end of the story however. Much uncertainty remained with regard to the size of Eris: was it really bigger than Pluto? How could the size of an object that was moving so far away from us, at 15 billion kilometers, that is to say three times further away from the Sun than Pluto, be determined? Even the most powerful telescopes in the world such as the Hubble Space Telescope are not able to resolve the angular diameter of Eris.

In the end, a very simple and indirect technique used by an international team which the researchers from Belgium joined, came to the aid of astronomers. This involved the occultation technique: in its path around the Sun, it can happen that an asteroid (or a TNO) passes in front of a star causing an apparent drop in the luminous flux of the star-and therefore its occultation-for a short period, determined by the size of the object, but also by its speed. As the latter parameter can be easily deduced from the orbit of the object, the measurement of the exact duration of an occultation is sufficient to gauge its size. “Occultation is a really brilliant technique; first of all, even if the object is very weak and very far away, what we look at is the star that will be occulted and which can, itself, be brighter. But above all, the resolution we can obtain on the observed asteroid can be very big and depends on the frame-rate we can obtain during the phenomenon.  Therefore we don’t need a big telescope even for an object as far away as Eris."

The greatest difficulty is predicting the phenomenon. The possible occultation of a star of the  Cetus constellation by Eris was predicted by Bruno Sicardy’s team (Observatory of Paris, CNRS, University Pierre and Marie Curie, University Paris-Diderot, France), thanks to precise and repeated observations of Eris and the field of stars crossed. Up to the day before the occultation, the predictions were continually adjusted by regular observations of the position of Eris and the star. Finally, the calculations predicted that the phenomenon had the greatest chance of being observed from Chile in South America during the night of 5 to 6 November 2010 around 02h20 UT. Twenty-six telescopes were mobilized around the world, six of which were technically redundant due to cloudy weather.