The discovery of a massive icy world that ended Pluto's reign

Space · Published 23 October 2026 Swipe more like this →

In short: In 2005, astronomers identified a glacial world in the outer solar system that was 25 percent more massive than Pluto. This discovery, known as Eris, forced scientists to confront a difficult dilemma: if this new object was a planet, thousands of other Kuiper Belt fragments might also deserve the title.

For decades, the solar system was taught as a stable family of nine planets. This changed in 2005 when researchers discovered Eris, a glacial orb located in the outer fringes of our solar system. While slightly smaller in diameter than Pluto, Eris is approximately 25 percent more massive, a discovery that earned it the nickname the "Pluto killer". This finding presented a profound dilemma: if Eris was a planet, then thousands of other frozen fragments in the Kuipect Kuiper Belt might also qualify.,

To resolve this, the International Astronomical Union (IAU) convened its 26th General Assembly in Prague. At this meeting, the IAU moved away from using "planet" as a loose descriptor and established a formal, three-part scientific definition.

To be a planet, a celestial object must orbit the Sun, be large enough to be shaped into a sphere by its own gravity, and possess enough orbital dominance to clear the neighborhood of other debris and asteroids.,

Pluto failed the final criterion. While it is large enough to be spherical, it has not cleared the surrounding area of other objects, leading the IAU to demote it to a "dwarf planet" in 2006. This decision sparked significant public outcry, with many arguing the change broke with tradition rather than following scientific logic. Today, the debate continues as new discoveries, such as the complex system around the star CD-35 2722, continue to challenge the boundaries of planetary classification.

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