Eris dwarfs Pluto.

Ex-planet Pluto just got a further demotion. Observations in 2005 had already revealed that it wasn't the kingpin of the outer solar system: A more remote denizen called Eris has a diameter 5 percent bigger than Pluto's.

That finding triggered last year's decree by the International Astronomical Union that Pluto should no longer be considered a planet. Instead, like Eris, it belongs to a newly defined class of objects called dwarf planets (SN: 9/2/06, p. 149).

Now, there's additional evidence that Pluto merits only second-class status. Datataken by two observatories show that Eris, previously nicknamed Xena, is nearly one-third more massive than Pluto.

Mike Brown and Emily Schaller of the California Institute of Technology in Pasadena were able to weigh Eris by tracking the orbit of its tiny moon, Dysmonia. Using images of the moon taken by the Hubble Space Telescope and the Keck Observatory on Hawaii's Mauna Kea, the researchers determined that Eris is 27 percent heavier than Pluto. The mass of Eris, along with its previously determined diameter, suggests that it contains both rock and ice and is similar in density to Pluto; Neptune's moon, Triton; and an outer-solar system object called 2003 EL61.

Brown and Schaller report their findings in the June 15 Science.--R.C.