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