Pluto's dinky diameter wasn't the official reason it was demoted from the planetary club back in 2006, but symbolically, size was the last straw. When Caltech astronomer Mike Brown spotted the object we now call Eris back in 2005 and astronomers figured it to be larger than Pluto, the former ninth planet's fate was sealed. Now Pluto's reclassification as a "dwarf planet" and the subsequent public outcry is behind us, but new research suggests that the former planet's symbolic death knell—Eris' size advantage—was wrong. The argument has been rekindled by astronomers who just completed detailed viewings of Eris from observatories high in the Chilean Andes. According to Bruno Sicardy of the Paris Observatory, Eris must be no wider than 1,454 miles, while the accepted value for Pluto's diameter is 1,456.5 miles. And because of the uncertainty of measuring diameter of such a distant object, Kelly Beatty at Sky & Telescope says, Eris' official size could decease another 30 miles or more after astronomers analyze more of this data.
Images taken in December 2005 by Brown and others with the Hubble Space Telescope indicated a diameter of 1,500 miles (2,400 km), just 5% larger than Pluto's. But the true size remained uncertain because even Hubble's supersharp gaze is only barely able to resolve Eris's disk. (Remember: it's some 9 billion miles from the Sun, twice as far away as Pluto.) [Sky & Telescope]
