The International Astronomical Union said Makemake, named after a Polynesian god, is a small orb that is among the largest objects in the outer solar system, Space.com reported.
But the fourth dwarf planet so far is smaller and dimmer than Pluto, which was reclassified as a plutoid in June, astronomers said.
Astronomers discovered Makemake in 2005 and believe frozen methane covers its surface. The plutoid is bright enough to be seen with a high-end hobbyist's telescope, the IAU in Paris said.
"The orbit is not particularly strange but the object itself is big," said astronomer Mike Brown of the California Institute of Technology in Pasadena, who led the team that discovered Makemake. "Probably about two-thirds the size of Pluto."
Pluto, Makemake and a third object -- Eris -- are classified as plutoids, as well as dwarf planets. The solar system's largest asteroid, Ceres, is a dwarf planet, but a plutoid because its orbit -- in an asteroid belt between Mars and Jupiter -- is smaller than that Neptune's trip around the sun.