On November 6, 2010, the dwarf planet Eris occulted a faint 16 magnitude star and this was the first time astronomers were able to witness an occultation by Eris. Additionally, at 96.6 Astronomical Units away, it was the most distant object for which this kind of occultation — where one astronomical object passes in front of another — had been seen. Why was this dim, distant event important? It has helped refine the size of what is (was?) thought to be the biggest dwarf planet (yes, I know, an oxymoron) we know of.
“Most of the ways we have of measuring the sizes of objects in the outer solar system are fraught with difficulties,” wrote astronomer and discoverer of Eris, Mike Brown, on his website ‘Mike Brown’s Planets.’ “But, precisely timed occultations like these have the potential to provide incredibly precise answers.”
The size of Eris has been a point of contention for many, since its discovery was the tipping point that “demoted” Pluto. Plus, it’s size hasn’t’ been precisely, unequivocally known: Brown’s data said Eris was 2,400 km in diameter, plus or minus about 100 km; radio astronomy measurements put it at about 3,000 km. (Pluto is about 2370 km).
Now, after three separate telescopes in Chile were able to view the occultation, initial estimates from the data say Eris is somewhere between 2,320 – 2,350 km in diameter. Which would mean Eris is actually smaller than Pluto, albeit, they are extremely close in size.
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In measurements of mass, however, Eris still comes out the heavyweight, which implies that Eris contains more rock than ice than Pluto does. “Eris and Pluto are very different bodies,”Brown wrote. “Though Eris is substantially more massive, they are essentially the same size. Eris must be made almost entirely of rock with a little coating of frost – which we see – on the outside. How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside? As of today I have absolutely no idea…. Something is going on in the outer solar system, and I don’t know what.”
The video is from observations with a 40 cm. robotic telescope from The Andalusian Astrophysics Institute (Instituto de Astrofísica de Andalucía, Spain), located at San Pedro de Atacama in Chile. The arrow shows where the star is, and you can see clearly how it disappears (from Eris’ shadow), and then emerges again.
Addendum: Mike Brown added another post to his blog last night (probably about the same time I post this) in which he talks more about how “Dwarf Planets are Crazy.”