“To me it made no sense to pull one of even a few objects out of the swarm and call them something other than part of the swarm.” -Mike Brown, A.K.A. PlutoKiller
In our Solar System, the four rocky planets dominate the inner portion, while the four gas giants dominate the outer Solar System. But out beyond Neptune, thousands of icy, rocky worlds — including Pluto, the former ninth planet — make up a vast, wide ring known as the Kuiper Belt.
What began as a curious collection of a few icy worlds has since revealed itself to us as an incredibly busy place, where — since 1992 — literally hundreds of worlds have been discovered.
And while some of these objects (like Pluto) are big, round spheres, most of these objects don’t quite have enough gravity to pull themselves into a spherical shape.
As always, of course, there are some oddballs. Take a look at Haumea, which does have enough gravity to pull itself into a sphere, but is rotating so quickly that its shape has become distorted into an ellipsoid!
One of the ways we can learn a whole lot about a foreign world, even if it only takes up one pixel on a camera, is to watch how its light-curve changes over time.
Think about it in terms of our Moon.
No matter how far away you were from the Moon, so long as you could see it at all, you’d be able to measure a certain amount of light coming from it. And if the Moon rotated so that you could see different amounts of the near-and-far-sides over time, you would find that the amount of light you observed changed over time, in a periodic way, depending on which face you were looking at.
Well, what about Haumea, the irregular ellipsoid I showed you above? Let’s take a look at her light-curve and see what it looks like.
The curve shows us that not only is one side brighter than the other (why the left side is brighter than the right), but we can also see different amounts of the planet over time (why there are the big bright peaks and deep valleys)!
Moreover, we can conclude that the dimmer side is actually significantly redder, as there’s more red light than blue light emitted from that face. The same team constructed this video of what Haumea must look like from this data.
In fact, in 1989 some very highly-detailed radar imaging was done on a small asteroid, now known as 4769 Castalia. What we discovered, well, blew our minds.
What we found was a new class of asteroid, now known as a contact binary. A contact binary consists of two somewhat small objects, these are objects whose gravitational mass is insufficient to pull it into a sphere, that actually touch one another! Since this discovery, many more have been found, including the largest of all the Trojan Asteroids (the ones that orbit 60 degrees ahead of and behind Jupiter): 624 Hektor.
You may wonder, of course, whether such an artist’s rendition can be trusted, and rightly so. But we have not only discovered many of these contact binaries, we visited one of them! Say hello to the famous asteroid, Itokawa.
Thought to be a contact binary of two rubble-pile asteroids, Itokawa’s two halves have, under the combined gravitational pressure of these masses, begun to fuse together into its irregular, potato-like shape!
This is, although uncommon, not unheard of for asteroids. But we’ve never discovered a Kuiper Belt Object that does this before. We simply haven’t had the data or the resolution to do it.
All this week is the 2011 meeting of the European Planetary Society Congress, and earlier today, October 3rd, Pedro Lacerda released the above light curve of the distant Kuiper Belt Object, 2001QG298. (All subsequent images and videos credit to P. Lacerda.)
What shape would it take to cause such a bizarre light-curve? According to Lacerda:
Imagine that you glue two eggs together tip to tip – that’s approximately the shape of 2001QG298. It looks a bit like an hourglass.
The object is so distant that we cannot resolve its shape. But this brightness oscillation, called a lightcurve, reveals the strange shape of 2001QG298 as it spins round. The object appears faint at times because one lobe is hidden behind the other, so less area is reflecting sunlight. As the hidden component rotates back into view, we can see the full hour-glass shape. The reflecting area increases and the whole thing looks brighter.
In other words, it’s a contact binary whose halves are spinning in-and-out of our line-of-sight!
But, of course, you also noticed that the 2004 light curve is substantially different from the more recent one! What could be causing this?
Well remember, like all objects in our Solar System, KBO 2001QG298 is orbiting the Sun! As it does so, we get to see different sides of this object, which means differing amounts of overall light!
And because we know of so few Kuiper Belt Objects, percentage-wise, this configuration could be common among them. From the press release:
The most important consequence of this finding is that it suggests that this type of double KBO could be very common. When in 2004 Sheppard and Jewitt found 2001QG298 in a sample of 34 KBOs they realised that they were fortunate to spot its binary nature – if it had not happened to be edge-on at the time of their observations, they would not have seen the extreme lightcurve variation. They estimated that approximately 10% of all KBOs are contact binaries, assuming that their tilts are random.
However, Lacerda believes that the tilts of contact binaries may not actually be random and that objects similar to 2001QG298 could be even more common.
“It was a surprise to find that 2001QG298 is inclined by 90 degrees, but that’s not the first time we’ve seen this in a contact binary,” he speculates. “There is another famous doublet object, a large Trojan asteroid called 624 Hektor. That object is also tilted almost 90 degrees.”
If contact binaries tend to be highly tilted then the chance of spotting their characteristic variable lightcurve is even smaller – only about twice per orbit. The identification of one in a small sample implies that contact binaries may be even more abundant than Sheppard and Jewitt first thought. Lacerda estimates that as many as 25% of KBOs are contact binaries.
From these two sets of observations, Lacerda has been able to reconstruct just what this contact KBO looks like from our point of view, and has put together a video of it, as it spins about its center and travels around the Sun!