S04E08: The 21 Second Excitation

We preempt tonight’s science to bring to you breaking news about last week’s science.  Astronomers just announced Eris may be smaller than Pluto.


Pluto and Eris are dwarf planets that orbit the Sun in the Kuiper Belt.

Eris is a dwarf planet discovered only within the last ten years.  Along with Pluto, it orbits the Sun in the Kuiper Belt, at over 30 times the Earth’s distance to the Sun.    But Eris’s discovery was a day of reckoning.  Eris was larger than Pluto.  How embarrassing.  How could Pluto be a planet, so the public argument went, if an even larger body was not?   It was a simple and persuasive argument.  Certainly easier to explain than the full reason for demoting Pluto: that some planet-like objects like Pluto are different than the eight solar planets since they don’t clear their orbits.  Eris was larger than Pluto–what could be more direct.

Up until now, the size of Eris was estimated by a number of techniques.  One way was from its mass.  Its mass is measured from the time it takes for its moon, Dysnomia, to complete its orbit. The larger the mass of the central body, the faster its moons will orbit it.  (Mathematically, the duration of the orbit of a small body around a large, central one goes inversely with the square root of the mass of the central body.)   For example,  if  Earth were four times as massive, our own moon, Luna, would orbit us in just 14 days.  That is, the gravitational  force on the Moon from the Earth would be four times as strong and the only way the Moon could travel in a circle around us at its current distance  would be to increase its speed by a factor of two.  We’d have 24 months to remember.   Then, assuming we counted months properly, it would be Christmas in Vigintiquattuorber.


Eris is orbited by its moon Dysnomia. The period of orbit tells us Eris's mass, but not its size.

So anyone can compare the mass of Pluto to the mass of Eris and determine which is physically larger right?  Wrong.  That’s only if you know the two objects have the same density.

To measure the physical size of an object you need to do something geometrical.  A few techniques: extracting the cross section from its brightness and even direct imaging gave some results, but up until now with significant experimental uncertainty.  The experimental uncertainties were always such that Eris could have been smaller than Pluto, but it just looked like that would be unlikely.

A special astronomical event changed all this.  One of the most useful moments in astronomy is when an object passes in front of  a star.  This event is known as a occultation, and can be thought of as a kind of eclipse.   Occultation and eclipse are not exactly the same thing.  In an “occultation” the nearer body completely covers the farther one–in this case Eris passing in front of a star in the constellation Cetus.  Distant stars appear point-like to us.   An “eclipse” can be an occultation but also when a body passes into the shadow of another is a completely different kind of event also called “eclipse”.  Sometimes the nearer body does not completely obscure the body behind it, in what is called a “transit”.  Too many things to remember?  Just tell your friends it is a “syzygy”.

Watch the occultation of a star by an asteroid (0:56)

Stars occupy such a small angle on the sky that predicting these events are hard, but astronomers are up to the task.  Eris is so small that its shadow is much smaller than the size of the Earth.  Astronomers had to predict which spots on Earth had the best chance and a few found it. When Eris passed in front of a dim star in the constellation  Cetus, astronomers measured exactly how long the star was blotted out.   The larger Eris is, the longer the star disappears from the sky.  A telescope in Chile found it dim for 76 seconds.   A few other measurements at other telescopes yielded Eris’s diameter.  And to everyone’s surprise, Eris was smaller than its mass suggested.  (Actually this was within the range of experimental uncertainties of the previous measurements.)   It is a physically smaller, more compact object than Pluto.

For a wonderful account of this observation made by one of the discoverers of Eris, see Mike Brown’s blog about the event, which is where I found most of my information.

Dr. Tyson should come back.  And he should bring Dr. Brown with him.  I suspect Sheldon needs to have a word with them.

7 Responses to “S04E08: The 21 Second Excitation”

  1. feldfrei Says:

    Thanks for bringing the news about Eris. Interestingly, the estimates for Pluto’s size were monotonically decreasing since the first discovery in 1930. Originally, Astronomers were searching for “Planet X” responsible for the small remaining distortions of Uranus’ orbit. Today, it is known that the reason for that was an error in determining Neptune’s mass.

    Anyway, in the 1950s Pluto’s mass and size was estimated to be similar to that of our home planet Earth. Since then it was continuously corrected down – including the discovery of Pluto’s moon Charon in 1978 which provides much more precise mass information. Pluto’s diameter was determined in occultation with Charon. It’s just fascinating how our view of the Universe changes 🙂

  2. Zig zag zug Says:

    Cool! I’m pro-planet Pluto camp, all the way! After all, if you start calling someone an asteroid… they might start acting like one.

    I think a functional definition of planet is better than “clears debris in orbit and is spherical”

    I’d go with “Can it sustain an atmosphere for more than 100 years if enough heat was there? Yes/No.” (or not there, in the case of Mercury)

    So, people getting off the Enterprise, or astronomers looking at its spectra, have something you can immediately point to and say “Planet ho Captain! By Davy’s Jones locker, she’s a garden world cap’n!”

    A dwarf planet would be something that requires work to have an atmosphere. It requires either umbrellas or a space heater and make it a homey place to live. There is an inherent ambition to this definition I enjoy.

    Any interaction we have with an extraterrestrial body will be greatly influenced by atmosphere, from observation, to landing, to inhabitation, and why shouldn’t the most interactive part be the generator of an objects classification?

    A definition we can quibble over this much isn’t fun. Otherwise:
    “Hay guys, a Dyson sphere falls under the definition of planet!”
    “Nuh-uh, it doesn’t!”
    “It soooo does.”

  3. Arturo Quirantes Says:

    So Eris seems to be a bit more massive than Pluto, buy perhaps is not larger. No big deal, methinks. Now a planet is defined depending not on size alone, but on whether or not it clears irs own orbit, so Eris is more planet-like than Pluto. Hmm, meybe we have to re-classify both as “Bodies Formerly Known as Planets -or Not- Which Might Be Planets After All”

    If only Dr Tyson had come up on TBBT a week later! Just a week! I can only dream of the possibilities:

    Sheldon: I don’t like you because you killed Pluto
    Tyson: Well, how about Eris? It’s more massive, so why not upgrade it to a planet?
    Sheldon: Pluto is larger, so upgrade it back, too
    Tyson: It’s still not large enough. Plus, Eris has a large moon, so it almost qualify as a planet
    Sheldon: So does Pluto! What do you think, Raj? You worked on Transneptunian
    Raj: And just why do you think I left that field?
    Tyson: OMG, now I’ll have to change that exhibition back. I don’t wanna get any more 3rd-grade hate mail
    Sheldon: Well, if you had paid more attention to Mike Brown’s latest news on the subject, instead of wasting your time at Leno’s or Stewarts’, you’d save us and the world the humiliation of ….

    Clash of titans!

  4. kingthorin Says:

    I’m a bit confused by this part:
    “When Eris passed in front of a dim star in the constellation Cetus, astronomers measured exactly how long the star was blotted out. The larger Eris is, the longer the star disappears from the sky. A telescope in Chile found it dim for 76 seconds. A few other measurements at other telescopes yielded Eris’s diameter. ”

    In order for us to make any conclusion based on this information wouldn’t we either:
    1) Need to know exactly how far away the star is and how big it is….. (uh we don’t know how big Eris is and it’s “close”).
    2) Need to perform the exact same timing and compare vs. Pluto…

    • David Saltzberg Says:

      What you need to know is that the angular size of the star being occulted is much less than the angular size of Eris. Because the stars are so far away, that is true. (By my reckoning, the angular size of Eris is 1e-7 radians which is larger than that of the closest decent sized star, Alpha Centauri,2e-8. Since the star used here for the occultation is much farther way, and presumably not too much larger the approximation holds.)

  5. Ted Seeber Says:

    I still want them to name a trans-neptunian object “Rupert” (after the trans-neptunian 10th planet in “Mostly Harmless”, the fifth book of the Hitchhiker’s trilogy)

  6. Tradução: “S04E08: The 21-Second Excitation (A Animação dos 21 Segundos)” « The Big Blog Theory (em Português!) Says:

    […] feita por Hitomi a partir de texto extraído de The Big Blog Theory, de autoria de David Saltzberg, originalmente publicado em 11 de Novembro de […]

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