The christening of Eris, named after the Greek goddess of chaos and strife, was announced by the International Astronomical Union on Wednesday.
Weeks earlier, the professional astronomers' group had stripped Pluto of its planethood under new controversial guidelines.
Since its discovery last year, Eris ignited a debate about what constitutes a planet.
Astronomers were split over how to classify the object because there was no universal definition. Some argued it should be welcomed as the 10th planet since it was larger than Pluto, but others felt Pluto was not a full-fledged planet.
After much bickering, astronomers last month voted to shrink the solar system to eight planets, downgrading Pluto to a "dwarf planet," a category that also includes Eris and the asteroid Ceres.
Eris' discoverer, Michael Brown of the California Institute of Technology, said the name was an obvious choice, calling it "too perfect to resist."
In mythology, Eris caused a quarrel among goddesses that sparked the Trojan War.
In real life, Eris forced scientists to define a planet that eventually led to Pluto's getting the boot. Soon after Pluto was kicked out of the planet club, hundreds of scientists circulated a petition protesting the decision.
Eris had been without a formal name while astronomers grappled over its status. Brown nicknamed it "Xena" after the protagonist of the TV show "Xena: Warrior Princess," pending an official designation.
He admits the new name will take some getting used to.
"It's a little sad to see Xena go away," he said.
Eris' moon, nicknamed "Gabrielle" after Xena's traveling sidekick, also received a formal name: Dysnomia, the daughter of Eris, known as the spirit of lawlessness.
Eris, which measures about 70 miles wider than Pluto, is currently the farthest known object in the solar system at 9 billion miles away from the Sun.
[Another trans-Neptunian object, Sedna, is closer right now, but at the other end of its extremely elongated orbit, which takes more than 10,000 years to complete, it will be 90 billion miles from the Sun.]
Eris is also the third brightest object located in the Kuiper Belt, a disc of icy debris beyond the orbit of Neptune.