In Greek mythology, Eris is the goddess of warfare and strife — most known, perhaps, for inciting a conflict at the wedding of Peleus and Thetis, the parents of the Greek hero Achilles, that led to the start of the Trojan War. A fitting name for a planet that caused no small amount of strife about the “planet designation” — as massive as Pluto, either Eris was the 10th one, or Pluto was not a planet either.

On this day, July 29, in 2005,  Astronomers Mike Brown of Caltech, Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University) announced they had discovered a new planet — still a planet at the time — nearly 100 times further from the sun than Earth.

Eris’ discovery accelerated an already brewing debate within the astronomical society on what makes a planet. There was already widespread doubt that Pluto was a planet — one of Neptune’s moons was bigger — and subsequent measurements revealed that Eris, too, was 27% larger. Until the International Astronomical Union finished issued their conclusion, the planet was provisionally called Xena, and her moon Gabrielle.