The TRAPPIST telescope measures the size of the celestial body which has dethroned Pluto

A first was achieved on the night of last November 6 by the University of Liege's astronomers, by means of their TRAPPIST telescope in Chile. The Ličge researchers have become the first to observe a stellar occultation by the dwarf planet Eris. This planet occulted a 17 magnitude star for a period of 30 seconds.

This unlikely observation, combined with that also carried out by the San Pedro Observatory, also in the Atacama desert (Chile), will enable the size of Eris to be measured to within a few dozen kilometres or so. Up until now its size was only known within a range of some hundreds of kilometres. The observation of this occultation is the longest ever to have been carried out: a distance of 14 billion kilometres (three times as far as Pluto).

This discovery is not without casting doubts on the planetary order. In effect it had generally been agreed that Eris was broader than Pluto, which in 2006 had led Pluto to be stripped of its planetary status. 'However,' explains Emmanuël Jehin, one of the scientists in charge of TRAPPIST and who carried out this unique observation, 'our observations give Eris a smaller diameter than that of Pluto, which thus replaces the former as the largest of the small planets! In fact Pluto and Eris are as good as twins, with a pretty similar surface composition.'

The observation has maybe also allowed Eris' atmosphere to be detected. 'If that is confirmed, the presence of an atmosphere so far from the Sun will need to be explained,' adds Emmanuël Jehin.

To refresh your memory, the TRAPPIST telescope was installed in 2010 by the University of Ličge on the La Silla mountain, in Chile's Atacama desert, where the European Southern Observatory (ESO) already existed. With a diameter of 60cm and completely automated, it should allow astronomers to gain a better understanding of the formation and evolution of planetary systems and comets.

The observation of this stellar occultation by Eris comes hardly a few days after the debut of TRAPPIST operations. It is a first major success for this instrument, which is now continuing its observations of the Hartley 2 comet and extrasolar transits.

Provided by University of Liege