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Video: Stellar Occultation by Eris

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.

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.

Read the entire story on Mike Brown’s Planets.

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.”


Nancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.

Comments on this entry are closed.

  • Torbjorn Larsson OM November 11, 2010, 10:59 PM

    Mike Brown on the topic of that the observed Kuiper belt objects vary immensely in density, including a promising hypothesis that apparently doesn’t quite meet all the requirements.

    It’s a new conundrum in planetary system formation! (As if we needed more.)

    the biggest dwarf planet (yes, I know, an oxymoron)

    Either you are “plunning” us (“planet funning” us) which isn’t clear from the context, or this is a mistake.

    Planets and dwarf planets comes from different populations (objects migrating to gravitational stable configurations respectively belt objects) and have observably different characteristics (cleared neighborhood for stability respectively not).

    As an analogy (always a risky business, mind) it isn’t like “the worlds biggest dwarf”, it is like “the worlds biggest Efé” (a well known population of central African pygmy people). It isn’t an oxymoron, it is even quantifiable if you measure all of the population. (I know, “a tall” order!)

  • mastercope November 11, 2010, 11:18 PM

    Make no mistake about it. It is The Trumpet’s they are gathering all the stones in the Kuiper Belt to make the Foot. That also explains the ripples in Saturn’s Rings. They be skimming fuel. Soon Eris will be lost cause it is on the way. ;-(

  • Mister T November 12, 2010, 3:43 AM

    That’s no dwarf planet!!

    Turn the ship around !!


  • laurele November 12, 2010, 12:11 PM

    Pluto and Eris are both planets and Kuiper Belt Objects. One does not preclude the other. They are planets because they are large enough to be rounded by their own gravity. They are Kuiper Belt Objects because they are located in the Kuiper Belt. Ceres too is a small planet because it is large enough for its gravity to pull it into a spherical shape. The IAU misappropriated the term “dwarf planet,” which was first coined by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto, to indicate a third class of planets which are large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for “dwarf planets” to be classed as not planets at all. The IAU did not “have” to do anything other than allow Eris’s discoverer to name it while holding off on any additional classification until more information is discovered about remote planets in this solar system and all planets in other solar systems.

    Significantly, there are quite a few exoplanet systems in which multiple planets orbit the host star in various different planes. Some have orbits far more eccentric than Pluto’s, yet they are giant planets the size of Jupiter or larger. According to the IAU definition, none of these objects are planets!

    Saying there are more differences between Pluto and the eight closer planets to the Sun depends on what aspects one considers. Earth actually has far more in common with Pluto than with Jupiter. Both have surfaces on which we can place rovers and landers. Both have a large moon formed by giant impact; both are geologically differentiated into core, mantle, and crust, and both have nitrogen in their atmospheres. Other than orbiting the Sun, what do Earth and Jupiter have in common?

    It is premature to pronounce declarations that these faraway objects are definitively not like the other planets or that one is larger than the other. We just do not have enough data at this point to do more than make educated estimates. What we really need to do is send robotic missions like New Horizons to Eris as well as Haumea and Makemake. Yes, that will take time and money, but it is a far better investment than the black holes the endless wars in Iraq and Afghanistan have become.

    Also, memorization is not important. It is much more important to teach the characteristics of each category of planet than to ask kids to memorize a bunch of names. We don’t ask them to memorize the names of rivers or mountains on Earth, so why do so with planets, and why allow a need for convenient memorization to determine how we classify them?

  • SteveZodiac November 12, 2010, 8:09 AM

    The apostrophes are running wild on this page, although I’ll accept the glottal stop one.

  • Torbjorn Larsson OM November 12, 2010, 9:02 AM

    including a promising hypothesis that apparently doesn’t quite meet all the requirements.

    Um, no, strike that. Mike Brown, updated in “Dwarf planets are crazy”:

    “They are all thoroughly different. How could this possibly be?

    No answer is immediately obvious, but it is immediately obvious that one or more of the assumptions of the standard scenario are going to have to be discarded. Earlier this summer I had constructed a new hypothesis that did an adequate (though, frustratingly not great) job of explaining some of the crazy variability in the Kuiper belt as being due to a random series of giant collisions which knocked the ice off of some objects, leaving just the rocky cores. I gave a couple of talks on the hypothesis, and even wrote the first draft of a scientific paper describing the details. But I fear now that the draft is going to have to go to the recycle bin. Even in my hypothesis once things grow to a certain size they should be more or less the same. Eris and Pluto are just too big to be different.”

    “So what happened instead? Did they form in different places? In different solar systems? Did Eris spend time close to the sun? None of these hypotheses is immediately appealing, but somewhere in there there must be a kernel of what really happened. Pluto and Eris and all of the rest of the dwarf planets must have a widely divergent set of histories of formation or evolution or interaction or all of the above.”

  • Torbjorn Larsson OM November 12, 2010, 12:55 PM

    Pluto and Eris are both planets and Kuiper Belt Objects. One does not preclude the other.

    That’s like saying that they are cats and dogs simultaneously, see my previous comment. And that is presumably why astronomers now define planets differently than you do.

  • HeadAroundU November 12, 2010, 12:59 PM

    It isn’t an oxymoron. There can be the smallest or the biggest Dwarf planet.

    An oxymoron would be “the biggest smallest planet”, without ‘smallest planet’ being a serious category in astronomy. Or, “Giant dwarf planet”, as a serious category.

  • Peter November 12, 2010, 2:33 PM

    Okay, let me get this straight, Eris is 23 hundred odd kms in diameter but its shadow can blot out a star??? I didn’t even know you could cast a shadow on a star!
    “The arrow shows where the star is, and you can see clearly how it disappears (from Eris’ shadow), and then emerges again.”
    Shouldn’t that be, (from behind Eris)
    And I thought the point was it was SMALLER than we thought!

  • HeadAroundU November 12, 2010, 5:18 PM

    Fancy Nancy, another oxymoron or maybe not? :D

  • ILOVETHESTAR November 12, 2010, 8:18 PM

    I can understand saying Pluto and Eris should be Planets because they are round, however, small lightweight moons around Saturn are also round because they are mostly ice and rocky rubble. In the future, I have a feeling there is a body out there with an albedo of .01 and the size of Mars far out in the Kuiper belt- in fact,I believe in the future,perhaps in 20 or so years, a dozen or so Mars size Kuiper belt objects will be out there. IMHO, I think for now, keep the number of planets at 8.

  • Manu November 13, 2010, 4:26 AM

    Science in the making. This is so exciting! :D

  • Torbjorn Larsson OM November 13, 2010, 7:40 AM

    @ Peter:

    I’m not sure if you are pointing out the funny ambiguity in the situation, but there it is: you can either put yourself in the shoes of an Earthly observer or someone else. Nancy is describing the shadow as seen by an observer observing the Earthly observer.

    Confusing!? Not at all… ;-) Read the article of Brown Nancy links to, there the shadow and its size is described.

    [But roughly: since the star is so far away the rays are efficiently parallel as seen by us, so Eris casts a ~ 2300 km spherical shadow on a ~ 12 000 km sphere (Earth). You “just” have to be observing from the right area of Earth.]