The Kuiper Belt's Spinning Hourglass
Spinning hourflass object may be the first of many to be discovered in the Kuiper Belt
Kuiper Belt Objects (KBOs) orbit the Sun beyond Neptune and are the best preserved leftovers of the formation of the planets. Studying these objects can thereby provide clues about the processes involved in planetary formation. Scientists believe that the Kuiper Belt may also contain objects that periodically enter the inner Solar System, potentially causing large impacts with planetary bodies like Earth. 2001QG298 is a remarkable KBO made up from two components that orbit each other very closely, possibly touching.
"Imagine that you glue two eggs together tip to tip – that's approximately the shape of 2001QG298. It looks a bit like an hourglass," says Lacerda.
The strange shape of 2001QG298 was uncovered by Dr Scott Sheppard and Prof David Jewitt in 2004. They noticed that 2001QG298's apparent brightness periodically tripled every 7 hours or so.
"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," explains Lacerda.
"It was impossible to tell from the original observations if the rotation and the orbital plane of 2001QG298 were aligned or perpendicular i.e. whether the object spun round horizontally like the blades on a helicopter or rotated vertically like the propeller of an aeroplane. The changes to the lightcurve variation show that it must be approximately vertical. As 2001QG298 moves round in its orbit of the Sun, our viewing geometry of the 'propeller' is gradually opening out from edge-on to seeing the whirling 'blades' full-face. This means that more of the reflecting surface is becoming visible at all times so the variation in the object's brightness gradually disappears," says Lacerda.
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.
"If contact binaries do tend to be very inclined that may be telling us something about how these objects formed," concludes Lacerda.