Moonday: Haumea Help You?

(Yes, I know. It’s a terrible joke. I blame Phil Plait for making bad puns acceptable in astronomy blogging.)

Pluto’s five moons, with their orbits drawn in. Note how regular they are, lying in the same plane. (Courtesy of Astronomy Picture of the Day)

I mentioned in last week’s post on Pluto that I don’t think the little iceball is unique. Yes, it has five known moons, the most that are known of any world that isn’t a gas giant. However, since all of Pluto’s moons are regular satellites—they orbit in the same direction that Pluto rotates, and their orbits lie in the same plane—they seem to have a common origin. That origin is likely to have been a major impact early on in the Solar System’s history: some other body slammed into Pluto, breaking off the fragments that are now moons.

However, just as it’s too soon to say that Pluto has only five moons, I think it’s likely that other Kuiper Belt objects are similarly the centers of their own miniature solar systems. In particular, I think of the dwarf planet Haumea, which is a weird and wonderful object in its own right. For one thing, it’s the only object of significant size that isn’t even approximately spherical! All the planets, many moons, and at least one asteroid are oblate spheroids: spheres that have been slightly squashed, so they bulge a bit at the equator. Haumea, on the other hand, is ellipsoidal, meaning that whatever way you look at it, it appears like an ellipse.

Artist’s impression of Haumea and its known moons Hiʻiaka and Namaka. The moons are the correct relative size, but they are drawn much closer to Haumea than is true to life.

More than that, Haumea tumbles end over end, like a badly thrown American football. Its two moons, Hiʻiaka and Namaka, orbit in crazy paths: rather than neat, Keplerian ellipses, they follow very elongated, ever-changing orbits. (Haumea is a fertility goddess in Hawaiian mythology, while Hiʻiaka and Namaka are two of her daughters. I believe this is the only system where every body is named for a female in mythology. Correction: Eris and Dysnomia are also named for female characters in mythology.) The bizarre irregular orbits are possibly because of an encounter with another body, but as far as I know, no one has a perfect explanation yet. However, based on spectral analysis, Hiʻiaka and Namaka are made of the same stuff as Haumea, meaning they were likely broken off in an impact, just as Pluto’s moons were.

Whether Haumea has more moons than two, I suspect multi-moon systems may turn out to be the norm in the Kuiper Belt. I admit it’s mostly a hunch, but it seems plausible to me given how significant impacts were in the early days of the Solar System. Here are a few examples of phenomena that may have been caused by impacts:

  • Earth’s Moon was almost certainly torn off during a huge collision
  • Uranus’ huge axial tilt
  • Venus rotates opposite to the direction it orbits the Sun, the only planet to do so
  • Pluto’s large axial tilt and its moons

If impacts were commonplace in the Kuiper Belt, then we should expect the larger objects to have multiple satellites produced during collisions, and perhaps the smaller objects may be parts of swarms. I’ll be interested to see if my hunch is borne out as we learn more about the outer reaches of the Solar System.


14 responses to “Moonday: Haumea Help You?”

  1. With the lack of gravitational perturbations out there, it makes sense for Kuiper belt objects to have several smaller moons. After all, that far from the Sun, even a small object can have a fairly sizeable Hill sphere.

    Also, is it worth considering that Triton, with its retrograde orbit, is quite possibly a captured KBO. Perhaps an impact which was averted?

    1. I think Triton-as-captured-KBO is the conventional wisdom. How many objects collided with the gas giants during formation, we may never know. (I did write about Triton a while back, by the way. It’s a really interesting moon all around!)

      1. Likewise a couple of years back. IMHO it’s one of the most interesting objects in the solar system, with its geysers, weather systems and methane slushie terrain. :)

        Really hope someone sends a probe back there while we’re still around to appreciate it…

  2. No asteroids are spherical or oblate spheroids. By definition, if an object is in hydrostatic equilibrium, which means its gravity squeezes it into a round shape, it is a small planet. The term “asteroid” should be dropped regarding Ceres. Haumea may be football shaped, but it still is in hydrostatic equilibrium. Eventually, we should send robotic missions like New Horizons to visit all these worlds and possibly even landers or rovers to explore them. If they are anything like Pluto and Eris, they are not iceballs. Pluto is 70 percent rock, and being more massive, Eris is likely even more rocky.

    1. 70% rock by mass or by volume? Because rock is going to be much more dense than ice. So 70% by mass would mean little rock core with big ice covering, whereas volume would imply something (slightly) different. 30% ice is still a lot of ice. Earth is .02 % water.

    2. Torbjörn Larsson, OM Avatar
      Torbjörn Larsson, OM

      Your Pluto-is-a-planet crackpot agenda is showing; we all know it isn’t a planet any longer. Likewise Ceres _is_ an accepted member of the asteroid population, an Asteroid Belt member – and not a planet any longer. (It was, for 50 years or so.)

      Ceres, as likely many other large asteroids as Vesta, moons like Titan and Kuipers like Eris and Pluto, happen to be differentiated. Yes, Vesta is like Ceres as far as we now know. Except that it got dinged, twice in the same place, after differentiation. That may explain why it has less ice too.

      Ceres is as a matter of fact akin to Mars, a protoplanet that didn’t make it up to the runt class* of Earth and Venus even, but has a mineralogical evolution of wet differentiated objects with early volcanism and later impacts. It is first when you get up to the size where plate tectonics (Earth), or the lack of it (Venus), becomes influential that it starts to get interesting again.**
      * Assuming Kepler’s observation of a drop off in size distribution after Neptunes holds up, making superEarths the more common terrestrial.

      ** Size-wise after differentiation. Habitability (Enceladus, say) and climate (Titan, say) are interesting too.

  3. Awesome post Matthew! Even the therapist says so :)

    1. Therapist? Do you mean your cats?

  4. Torbjörn Larsson, OM Avatar
    Torbjörn Larsson, OM

    Because of P5 I googled up an oldish (IIRC) table of observed moons. There are quite a few asteroids that have 3 moons, most probably also impact scatter.

    To add to the list, Phobos & Deimos are on and off as impact remains that failed to coalesce as our Moon and likely Charon did. Certainly there is an elliptical impact scar near the equator that can only be predicted from a deorbited moon, as Phobos eventually will be, implying they were most likely originating as impact remains.

    We shouldn’t forget the newfangled theory that the backside of the Moon is the witness to another low velocity impact, of a 2nd moon. That too would have originated during the Earth-Moon impact event, but could have been trapped at the Sun-Earth L2 Lagrange point for a while.

    Such a coalescence could predict why Venus doesn’t have a surviving moon while having an unprecedented slow and retrograde rotation.

    Impacts, impacts everywhere.

  5. My views on what constitutes a “planet” are on record, but let me summarize again: I don’t really care whether Pluto is called a planet or not. What I do care is that we are thoughtful rather than emotional about it. It also seems to me that if we were starting from scratch, we wouldn’t have a single “planet” category. We’d probably divide bodies orbiting the Sun into smaller groups based on their obviously different formation histories and compositions. Vesta is far closer to Earth in history and composition than Pluto is, but it’s not spherical. Ceres…well, we need to wait until the Dawn probe gets there, but I imagine it will hold some surprises.

    “Iceball” is a term of endearment. I’ve been calling Pluto an iceball for years.

  6. If I may quote myself:
    “[I]f we break all our categories down far enough, we can begin to lump them in new and different ways. We have gotten it into our heads that “planet” is a sacrosanct notion, but in many ways that’s a carry over from pre-scientific times. Our categories serve us, not vice-versa: a one-size-fits-all definition for planet may be something we need to abandon.”

    1. Every field has lumpers and splitters, eh? We have this issue in human evolution studies as well. Same genus or different? Depends on how fine you want to distinguish your classification.

  7. “I believe this [Haumea + satellites] is the only system where every body is named for a female in mythology.” Eris and Dysnomia are both female, although perhaps by “system” you meant something with more than two objects.

    1. D’oh! You’re right, and I’m silly for missing that, especially since I’ve written about Eris several times. Correcting the post now, thank you!

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