The past few years, much public attention has been brought to the outskirts of our solar system by the demotion of Pluto. Love it or hate it, suddenly the Kuiper Belt is getting a lot more attention than it used to. Sometimes, people even know the names of some of the objects in it. But that's only the bigguns like Quaoar and Sedna. There's many others out there, and one that gets little attention is Haumea.
Haumea is only 1/3 the mass of Pluto, but is notable because of its unusual shape: It's elongated. Additionally, it has a very fast rotation period (only 4 hours). Lastly, it has 2 moons and at least 6 other, smaller objects associated with it due to to similar surface properties and orbits. As such, the leading theory is that Haumea was struck by a large object which shattered it, causing it to be misshapen and knocking bits off.
But a recent paper disagrees. It notes that the velocity dispersion for the other objects associated with the main body (not the moons) is too low for what we would expect to see in a collision.
Instead, it proposes a new hypothesis: A collision long ago may have given Haumea its unusual shape and fast spin, but the ejected material coalesced into a loosely bound moon which slowly drifts away. That moon then undergoes another collision which created the collisional family.
Although it sounds like it fits this particular piece of evidence better than the single collisional model, I'm still not completely convinced. The reason is that, in today's Kuiper Belt, the time scale for collision is somewhere on the order of 1012 years. Even if you want to rewind this and say it all happened earlier in the history of the solar system when chances could be 100 times more likely, that still leaves the chance for the initial collision at 1010 years. (The authors use another estimation here and somehow come up with a timescale of ~8 x 106 years here.) To invoke a second collision would be even more unlikely since the new target (the satellite) would be a much smaller target than Haumea before the initial impact. Even using the author's calculation of 8 x 106 years for the initial collision, multiplying that by a second collision puts the timescale over 1013 years! Keep in mind the age of the solar system is just 4.5 x 109 years.
So while the hypothesis may seem to favor a double collision, the math says it's not likely.
Schlichting, H., & Sari, R. (2009). THE CREATION OF HAUMEA'S COLLISIONAL FAMILY The Astrophysical Journal, 700 (2), 1242-1246 DOI: 10.1088/0004-637X/700/2/1242