Eris is a dwarf planet located in the scattered disc, a region of the outer Solar System that overlaps with the Kuiper Belt. Eris was discovered in 2005 by a team of astronomers led by Mike Brown, and its discovery played a significant role in the reclassification of Pluto as a dwarf planet.

Eris is currently the most massive known dwarf planet in the Solar System, with a mass about 27% greater than Pluto. It has a diameter of approximately 2,326 kilometers (1,445 miles), making it slightly smaller than Pluto. Eris has a highly eccentric orbit that takes it as close as 37 astronomical units (AU) to the Sun and as far away as 97 AU. One full orbit around the Sun takes Eris about 558 Earth years.

Eris has one known moon, Dysnomia, which was discovered in 2005. Dysnomia is much smaller than Eris, with an estimated diameter of about 700 kilometers (435 miles). The moon orbits Eris once every 16 days.

The discovery of Eris sparked a debate among astronomers about the definition of a planet. Given that Eris was more massive than Pluto and shared many of its characteristics, the question arose as to whether Eris should also be considered a planet. In 2006, the International Astronomical Union (IAU) decided to create a new classification called “dwarf planet” to categorize objects like Pluto and Eris that did not meet all the criteria for being considered full-fledged planets. The primary distinction is that dwarf planets have not cleared their orbits of other debris, whereas planets have.

Since the reclassification, several other objects in the Kuiper Belt and beyond have been identified as dwarf planets, including Haumea, Makemake, and potentially many more yet to be discovered. The discovery of Eris and the subsequent reclassification of Pluto have expanded our understanding of the diverse range of objects that populate the outer regions of our Solar System.

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