Astronomy can be a cosmic detective story, and our enduring fascination with the solar system's mysterious hinterland of small icy objects in the Kuiper belt is no exception.
In fact, as wonderfully detailed by Mike Brown, professor of planetary astronomy at the California Institute of Technology (Caltech), his detective work focuses on the growing population of dwarf planet discoveries in the Kuiper belt, revealing the frigid region used to be a pretty violent place.
Brown took a retrospective look at his research during the W. M. Keck Observatory 20th Anniversary Science Meeting at The Fairmont Orchid, Hawaii, the Big Island, on Thursday. Brown is famously known as the "Pluto Killer" - he discovered the dwarf planet Eris in 2005 that, ultimately, led to Pluto being reclassified by the International Astronomical Union (IAU) in 2006. The fascinating story is revealed in his best-selling book, "How I Killed Pluto and Why It Had It Coming."
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Although each Kuiper belt object its own unique story of discovery, Brown discussed the detection and investigation of the oddball dwarf planet (136108) Haumea - a world approximately a third of the mass of Pluto. Originally discovered by Brown's Caltech team using the Palomar Observatory in 2004, key observations were carried out with the Keck Observatory telescopes atop Mauna Kea, Hawaii. "We named the object Haumea in honor of the work done at Keck," said Brown. In Hawaiian mythology, Haumea is the goddess of fertility and childbirth.
It was Keck's spectroscopic observations that picked-up an exciting lead that ultimately led to a detailed postmortem of a massive Kuiper belt smash-up that theorists never thought possible.
The initial detection of Haumea quickly turned into confusion when analysis of the object revealed it had a very fast rotational period. In fact, it was spinning too fast. By watching it brighten and dim, the dwarf planet was found to be spinning once every 2 hours - a rate that should rip it apart under centrifugal stress.
There could only be one answer for this "impossible" Kuiper belt object (KBO) - Haumea wasn't a spherical body, it had to be elongated. From our perspective, Brown's team was watching the irregularly-shaped object rotate nearly edge-on. Haumea's oblong shape made it look as if it was changing brightness, when in fact one moment you'd see the body's "end" (darker) and then it's "side" (brighter) rotate into view.
This meant Haumea actually had a 4 hour rotational period. That was still remarkably fast, faster than any other known body of that size in the solar system, but at least it explained why Haumea didn't rip apart.
Through high-resolution spectroscopy by the Keck II telescope, the fingerprint of pure water ice was detected on Haumea's surface. "Haumea has the most pure water-ice surface than anything measured in the outer solar system," said Brown.
This discovery created another conundrum - density calculations of Haumea suggested it was the average consistency of rock and yet it looked like a block of pure water ice that should have a far lower density. Only adding to the odd nature of the object, Brown's team realized that Haumea was "rocky on the inside, icy on the outside - kinda like an M&M that you really would not want to eat! A very, very strange object," he said.
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Only 3 weeks after Haumea was discovered, a small moon was spotted in Haumean orbit. Named Hiʻiaka - the daughter of Haumea in Hawaiian mythology - this tiny satellite was yet another conundrum. At an estimated one percent the mass of Haumea, it was highly unlikely to have been a captured object from the Kuiper belt. And, through Keck's analysis of the moon's spectrum, it was found to have a large quantity of pure water ice on its surface in a similar manner to Haumea. It was another extremely rare KBO.
Many Kuiper belt satellite objects are of comparable size to the primary, meaning that binary KBOs are fairly common whereas Haumea's tiny satellite probably wasn't captured through gravity. "If you wanted to somehow come up with a capture mechanism for this satellite, you'd have to first capture - which is impossible - and then you'd have to accidentally capture the most unusual other Kuiper belt object you've ever seen," Brown remarked.
What could have created such an object? A massive collision with another large body could have started it spinning at a fast rate, causing the ice - blasted from the body during collision - to settle back on its surface. That would be the most likely explanation of Haumea's elongated shape and icy crust. That would make Hiʻiaka the remnant of such a collision.
Theorists, however, did not agree. At the time, they calculated such a collision had a probability of 1 in one billion of occurring, said Brown.
And then another tiny, icy moon was discovered, named Namaka, another daughter of the mythological Haumea. The likelihood of the random gravitational capture on one unusual KBO seemed slim, but the discovery of another only strengthened Brown's argument that it had to be caused by a "huge massive impact" and not some unlikely gravitational capture mechanism.
Over time, other KBO's spectra were analyzed and a group of objects appeared to also have the pure water ice signature of Haumea and her moons, but they also had very similar orbits in the Kuiper belt. They also had a strange 12:7 resonance with Neptune (for every 12 Neptune orbits around the sun, this peculiar group would orbit 7 times). By Brown's team's reckoning, all these objects had the same origin - they were the remnants of (you guessed it) a "huge massive impact" and the fast-spinning Haumea was at the center of the mess.
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So why the discrepancy between theory and what we're seeing in the Kuiper belt?
"The easy answer is that the theory is wrong," Brown told Discovery News. "When you observe something and it's pretty clear it's telling you that something's happened and theory says that it cannot happen, chances are that the theory is just wrong."
And all this detective work around Haumea - which appears to have been smacked really hard in its past, causing it to spin, elongate, have a thin crust of pure water ice, small moons, accompanied by other spectroscopically-similar KBOs - only bolsters the idea that the Kuiper belt was once a very violent place to be a dwarf planet.
"We now know that Pluto had a massive impact; Haumea had a massive impact; we think Eris had a massive impact; Quaoar had a massive impact ... these things can't happen now, so it's not an active ‘war zone.' It had to have been in the past, when there were more objects, when things were flying around at high speeds," Brown concluded.
As fascinating as it is to look back, there's a very immediate concern for the NASA New Horizons probe currently racing it's way toward the Kuiper belt - scheduled for a Pluto close approach in 2015. Since it's launch in 2006, astronomers have confirmed four more moons around Pluto in addition to Charon, Pluto's largest satellite. This has led to the uncomfortable realization that the newly demoted Pluto may have been the scene of a pretty cataclysmic impact that generated not only small moons, but also a debris field that ground based telescopes cannot see.
Work by astronomers like Brown, and the understanding that far from the Kuiper belt was far being a quiescent place in the past, is critical for understanding how much hazardous debris could be in Pluto's (and, indeed other KBOs) potentially rough neighborhood.
Image credit: NASA