Eris, Pluto’s distant twin
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.
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