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]
This month, however, the window of opportunity opened for astronomers. Eris passed directly in front of a star, which allowed them to more accurately gauge its size.
Water, water (or ice) everywhere—that’s the refrain this year. This week we covered the study declaring that the moon was home to perhaps 100 times more water than previously thought, and it was just two months ago that sky-watchers spotted the first frosty asteroid out in the Asteroid Belt. Now, in a study in Nature, a team of astronomers says they’ve found another icy surprise in our solar system: a bright shiny object way out in the Kuiper Belt.
The Kuiper Belt is that mess of objects orbiting the sun out beyond Neptune, but not as far as the Oort Cloud (once-proud Pluto is a Kuiper Belt object). There are plenty of icy bodies out there, including Pluto. But what doesn’t make sense about this one, KBO 55636, is how it stayed so pristine after a billion years of floating alone. MIT’s James Elliot, who led the study, says the object’s albedo, or reflectivity, is striking:
“That turned out to be very high, almost 90 percent… That’s consistent with it having a very highly reflective surface like water ice.” The finding was surprising because such old, distant bodies tend to have weathered, dull surfaces. “Objects orbiting that far out in space get generally darkened by accumulating dust… We don’t have an explanation for how it could stay so pristine” [Space.com].