The dwarf planet Eris – its official name is Eris 136199 – was discovered in January 2005 by a group of astronomers at Palomar Observatory studying routine observations made in 2003. Later in 2005 the existence of Eris was confirmed and they also detected its moon, now called Dysnomia, whose existence allowed them to better estimate its mass. The observation of Eris’s movement allowed them to determine that its orbit is very eccentric with an orbital period of about 557 years that leads it to a distance from the Sun that goes from nearly 38 times to nearly 98 times that of the Earth from the Sun.
Initially, Eris was designated as the tenth planet of the solar system, because from an initial estimate its size appeared to be quite larger than Pluto. The observations of the outer solar system however brought the prospect of being able to discover other small planets so the very definition of planet was questioned.
In 2006 the International Astronomical Union (IAU) decided to create the category of dwarf planets, which led to the “demotion” of Pluto, which at that point was included in the new category. The uncertainty also affected the choice of the name of the newly discovered planet: initially it was informally called Xena (informally its moon was called Gabrielle, following the inspiration to the same TV show) and only in 2006 it was officially named Eris.
In November 2010 Eris passed in front of a star, a rare phenomenon called occultation. These are rare events that occur every few years and allow for a better observation of an object in space. Observatories with a suitable location to study Eris in those conditions were in Chile: the one at La Silla, which used the TRAPPIST telescope, and the one in San Pedro de Atacama, which used the Caisey Harlingten and the ASH2 telescopes.
The results of the observations of Eris made thanks to that occulation are now being published. The size of Eris was found to be lower than initially estimated and shows how this dwarf planet is almost a twin of Pluto with an estimated diameter of 2,326 km (1,445 miles). However Eris is about 27% more massive than Pluto, which means that it’s probably a rocky body covered by a relatively thin coat of ice.
According to estimates made with these observations, the temperature during the day on Eris should reach a maximum of about -238 degrees Celsius (-396 degrees Fahrenheit), but nights are even colder.
After the creation of the category of dwarf planets Ceres, the largest object in the asteroid belt, was included in it. Over the years other objects were discovered: some have been added among the dwarf planets and others are still under evaluation. Meanwhile, astronomers are still making new discoveries about Eris, the dwarf planet that caused a small revolution in astronomy.