Which is bigger: Eris or Pluto? Eris... right? Not so fast.
In 2005, the discovery of dwarf planet Eris started a chain of events that rocked the astronomical community and plutonites alike. Pluto was found to be smaller than Eris, causing the "ninth planet" to be demoted to a dwarf planet and be renamed with a "minor planet designator." (Although "134340 Pluto" does have a certain ring to it.)
The nine planets of the solar system became eight, astronomers argued, Illinois wrote some nutty laws, McDonalds tried to set the record straight, schoolkids went wild... In short, it wasn't pretty.
But! Last weekend, an extremely rare and exciting event happened, adding yet another twist in the "Is Pluto a planet?" debate. Eris drifted silently through the outermost reaches of the solar system, blocking the light from a single star, an event known as an "occultation."
Although the event was predicted by astronomers, there was some uncertainty about who would get the best view -- this is, after all, the first time an occultation has been used to measure an object so distant (Eris is twice as far away from the sun as Pluto). All they knew was that the occultation would be visible from somewhere in South America.
Right on time, the dwarf planet Eris blocked the star from view and three telescopes in Chile witnessed the star's disappearance.
The starlight blinked out for 27 seconds at the La Silla Observatory, located at the southernmost edge of the Atacama Desert. 740 kilmeters (460 miles) north, two telescopes at the San Pedro de Atacama Celestial Explorations (SPACE) Observatory saw the dwarf planet block out starlight for 78 seconds.
Now, as astronomers know the orbital velocity of Eris, by using the background star as a marker, they could deduce the dwarf planet's diameter.
(The reason why the three telescope sites measured different occultation times is that Eris is spherical and therefore saw the starlight being blocked at different locations across Eris' disk.)
It turns out that Eris could be a lot smaller than previously thought. So small in fact that it is roughly the same size as Pluto! It could even be smaller.
WATCH VIDEO: Pluto Gets Demoted
"I have to say I find these new occultation measurements extremely exciting," Mike Brown, professor of planetary astronomy at Caltech and discoverer of Eris, told Discovery News.
"While everyone is more interested in the 'mine is bigger than yours' aspect, the real science is the shockingly large density of Eris."
From years of observing the motion of Eris' moon Dysnomia, a very accurate measure of the dwarf planet's mass has been arrived at. When it was thought Eris was larger than Pluto, it was logical to assume that both objects would have similar densities (and therefore similar compositions). However, these new measurements of Eris' size suggest it is smaller, and therefore more dense.
"No one would have predicted that the two largest objects in the Kuiper belt would be about the same size but have significantly different densities," Brown said. "You get big objects by accumulating smaller objects, so even if there is some variation in the smaller objects, by the time you get to the biggest ones you'd expect everything to be smoothed out. This is truly weird."
BIG PIC: Pluto: A Complex and Changing World
Occultation measurements are by their nature very accurate, and although more precision is needed before we get a definitive answer, Eris is certainly a lot smaller than it was thought to be.
"Almost certainly Eris has a radius smaller than 1,170 km [727 miles]," Bruno Sicardy, of the Paris Observatory, said in an email to Sky and Telescope Magazine. Pluto has a radius of approximately 1,172 kilometers (728 miles). Pluto therefore has an average density of 2.03 grams/cm3 and Eris has an average density of 2.5 grams/cm3.
These new observations have thrown up more questions than answers.
"Did [Pluto and Eris] come from very different places? Did they have very different histories?" Brown asks. "Something very different has been going on. And I don't know what."
Due to Brown's implication in Pluto's fall from planetary status -- a fact that is clear from the title of his upcoming book "How I Killed Pluto and Why It Had It Coming" -- he added:
"Also, on the mine-is-bigger-than-yours part, it may turn out that Pluto is ever-so-slightly bigger than Eris, but Eris is more massive than Pluto and the entire asteroid belt put together. So there."
So where does this leave Pluto? Or should I say 134340 Pluto?
Addendum: Watch a video of the the occultation (thanks to Nancy at the Universe Today!):
Image: Eris, as seen by the Hubble Space Telescope in 2007 (NASA)
Tags: Astronomy, Current Events, Pluto, Solar System, Space People
comments ( )