Previous estimates by ground-based telescopes suggested the object known as 2003 UB313 was 30 percent bigger than Pluto.
But the latest measurement by the Hubble Space Telescope has a smaller margin of error and is probably a more accurate estimate, said lead researcher Michael Brown of the California Institute of Technology.
According to Hubble, UB313's diameter measures 1,490 miles, give or take 60 miles. Pluto is about 1,422 miles across.
Brown previously reported that UB313 could be up to 2,175 miles in diameter based on its brightness. He said he was surprised by Hubble's findings, which will be published in an upcoming issue of Astrophysical Journal.
The discovery of UB313, which Brown nicknamed Xena, after the syndicated-television warrior princess, reinvigorated the debate about what is considered a planet.
Some astronomers have questioned whether Pluto should keep its planetary status, while others say UB313 should be the 10th planet because it is bigger than Pluto.
(HT Fox news here.)
The website for 2003 UB313 is here.