"The inferred low rock fraction of the 2002 UX25 system makes the formation of rock rich larger objects difficult to explain in any standard coagulation scenario," Brown writes.
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It is thought that KBOs formed in a similar way to asteroids and planets. Over the evolution of our solar system, small bits of rocky and icy debris coalesced, eventually forming planetesimals that then gathered more and more debris as their gravitational oomph grew. In this scenario, one would expect the density of minor planetary bodies to increase with increasing mass; the gravitational pressure of progressively larger bodies would cause more compression, thus increasing the density.
However, the very low densities of smaller KBOs are hard to explain without assuming that the bodies have a high degree of porosity. Porosity is a known factor in the formation of asteroids throughout the solar system - gaps throughout the structure of rocky bodies less than 350 kilometers in diameter are thought to lower the overall density. Asteroids over 350 kilometers become so massive that porosity decreases; the gravitational compression pulls the material closer together, reducing porosity and increasing density.
According to Brown, this porosity transition should occur in KBOs larger than 350 kilometers wide. But as 2002 UX25 shows, this transition hasn't happened up to a size of 650 kilometers. This factor creates a problem. If larger KBOs over 1,000 kilometers (620 miles) formed through the coalescence of smaller KBOs (like 2002 UX25), it isn't possible that large rock-rich KBOs could have such high densities.
In the case of an object the size of Eris, for example, with a measured density of 2.5 g/cm3, even with the gravitational compression exerted by the 2,326 kilometer-wide dwarf planet, the low density, high porosity material from an objects like 2002 UX25 cannot be compressed to such a high degree. Such an object "would still have a density close to 1 g/cm3 rather than the 2.5 g/cm3 density of Eris," writes Brown. On this evidence alone, large KBOs cannot form through agglomeration of many small KBOs like 2002 UX25.
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So what's going on in the Kuiper belt? Brown offers a few explanations.
Perhaps there is some observational bias in the measurements of KBO density, or perhaps 2002 UX25′s density is not representative of mid-sized KBOs - it could be the ‘black sheep' of the Kuiper flock. Could it be that the highest density, large KBOs formed through conventional agglomeration processes, only to have their densities beefed-up by energetic collisions early in the solar system's history? Dwarf planet Haumea shows evidence for a massive collision in its past, which smashed the majority of its icy mantle away, leaving a rocky core behind - this had the effect of increasing the overall density of the object.
"None of these alternatives appears likely," concludes Brown. "We are left in the uncomfortable state of having no satisfying mechanism to explain the formation of the icy dwarf planets. While objects up to the size of 2002 UX25 can easily be formed through standard coagulation scenarios, the rock rich larger bodies may require a formation mechanism separate from the rest of the Kuiper belt."
Brown often refers to the Kuiper belt as a "war zone" or the "blood spatter" of the solar system; the outermost region of shattered rocky and icy bodies preserved for aeons, unchanging evidence of the violent formative years of our star. Is 2002 UX25 just a forensic oddity? Or does it challenge planetary formation theories, proving the Kuiper belt is even stranger than we imagined?
It sounds like a lot more mid-sized KBOs need their densities probed until we have an answer.
Preprint: "The density of mid-sized Kuiper belt object 2002 UX25 and the formation of the dwarf planets," Michael Brown, 2013. arXiv:1311.0553 [astro-ph.EP]
via Nature News
Image credit: NASA