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. Data taken by two observatories show that Eris, previously nicknamed Xena, is nearly one-third more massive than Pluto.
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