This observation raises questions: how could a rocky planet (even a dwarf planet) form so far away? Did Eris form nearer the Sun, before being ejected into an very eccentric orbit? “It’s possible”, replies our astronomer. “We know that shake-ups have reorganised the solar system since its formation, giving rise to migrations of planets which led to the ejection of many bodies. Three billion years ago, Jupiter and Saturn reached the 1-2 resonance: Jupiter circled the Sun exactly two times while Saturn circled it only once, creating a terrible gravitational instability in the region of giant planets. Billions of comets were then ejected within the confines of the solar system. Today they form Oort’s cloud. But it is also possible, that Eris was bigger in the past and was enclosed in a much thicker layer of ice. This could have been shattered during a collision with another object, leaving the most inner part of the planet uncovered.”

The brightness of Eris is also very surprising: its reflective power is now estimated at 96% and greatly exceeds the albedo of fresh snow which is 80%. Eris is a veritable mirror! “This hyper-brilliant surface is a surprise”, explains Emmanuël Jehin, “because Eris is the same age as the solar system, that is to say nearly 4.5 billion years. Its surface has been bombarded for billions of years by cosmic rays and micrometeorites. Most asteroids have a very dark surface, strongly modified by the ageing of rocks permanently exposed to space. Why not Eris? The answer to this mystery should be sought in the absence of an atmosphere. Our observations show that Eris has no atmosphere in contrast with Pluto, although they are of very similar size and composition.” The spetroscopic measurements with large telescopes show that in fact the surface of Eris is, just like that of Pluto, covered with a fine layer of frozen Nitrogen and Methane. But we mustn’t forget that Pluto is three times nearer the Sun allowing for evaporation of ice from the surface. The atmosphere of Eris must have frozen and spread along the surface like frost when it moved away from the Sun, thanks to its very eccentric orbit. This ice uniformly covered all the physical features with a very reflective material. But when Eris comes back to its perihelion in a few hundred years time, its distance from the Sun will be 38 UA, which is similar to that of Pluto currently, and an atmosphere should form again. Eris therefore gives us an image of what Pluto will be when it reaches its aphelion at almost 50 UA from the Sun in more than a century.”


The full potential of the technique of stellar occultation is revealed here. All this information concerning size, density and albedo of a TNO, but also indirectly, concerning the history of the solar system has been obtained, thanks to a simple curve of light taken at a strategic moment, and obtained with a very modestly-sized telescope but which was available in the right place and at the right time. This occultation will remain exceptional because Eris is now heading towards a field which has a lower density of stars: the next occultation of a star by Eris is predicted for August 2013, but it will only be visible from the Pacific, which makes the observation more difficult. But occultations by other TNOs are approaching and theories about the formation of the solar system will have to wait.