Extreme and Extremely Tilted KBO

Tilted and oddly shaped Kuiper belt object may be the first of many to be discovered

Belfast, UK, October 2011 – The oddly shaped Kuiper belt object 2001QG298 is surprisingly tilted with respect to its orbit around the sun. The most important implication of this discovery is that it suggests that contact binaries such as 2001QG298 are the most common type of binary in the Kuiper belt. I will present this finding at the Joint Meeting of the European Planetary Science Congress and the Division for Planetary Sciences in Nantes, France, on 3 Oct 2011.

Model of KBO 2001 QG298.
Model of KBO 2001QG298.

The Kuiper belt objects (KBOs) orbit the sun beyond Neptune and are the best preserved leftovers of the formation of the planets. The awkwardly named 2001QG298 is a remarkable Kuiper belt object made up of two components that orbit each other very closely, possibly touching (Figure 1). Imagine that you glue two eggs together tip to tip. That’s approximately the shape of 2001QG298.

Image sequence to illustrate the rotation of 2001 QG298.
Sequence illustrating the rotation and brightness change of 2001QG298.

The strange nature of 2001QG298 was uncovered by Scott Sheppard and David Jewitt in 2004. They noticed that 2001QG298’s apparent brightness periodically tripled every 7 hours or so. This brightness oscillation, called a lightcurve, is caused by the object’s elongated, binary shape as it rotates (Figure 2). The object is so distant that we cannot resolve its shape. But because it is spinning, when one component hides behind the other the object appears faint because less area is reflecting sunlight. Then as the hidden component rotates back into view the reflecting area increases and the whole thing looks brighter.

As it turns out, the rotation of 2001QG298 is almost perpendicular to its orbit around the sun. I found this by re-measuring the object’s lightcurve late last year and noticing that it had changed from what Sheppard and Jewitt had reported (Figure 3).

Change in the lightcurve of 2001 QG98.
Change in the lightcurve of 2001QG98.

If the rotation and the orbital plane were aligned then 2001QG298 would always appear the same to a person on earth as it spins and moves in its orbit around the sun (Figure 3, left). But because the object is tilted, as it moves around the sun there are times when we see the object’s equator and other times when we face one of the object’s poles (Figure 3, right). In 2004 the object was seen nearly equator-on, but when I re-observed 2001QG298 last year, it had moved in its orbit and was now visible slightly at an angle. The change in observing geometry caused the lightcurve variation to become visibly shallower (Figure 3).

Observing geometry of 2001 QG298.
Observing geometry of 2001QG298.

The most important consequence of this finding is that it suggests that these type of double KBOs are very common. When in 2004 Sheppard and Jewitt found 2001QG298 in a sample of 34 KBOs they realised that the discovery of these objects by chance was unlikely – if the object did not happen to be equator-on when they looked at it they would not have seen the extreme lightcurve variation. They used this reasoning to de-bias the apparent abundance of 1/34 and found that approximately 10% of all KBOs are contact binaries assuming that their tilts are random. But maybe the tilts of contact binaries are not random. 2001QG298 is surprisingly inclined by 90 degrees but that’s not the first time we see this. 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 tilted then the chance of finding them equator-on is even smaller, only about twice per orbit. That implies that contact binaries may be even more abundant than Sheppard and Jewitt first thought – as many as 25% of KBOs may be contact binaries. If contact binaries do tend to be very inclined that may be telling us something about how these objects formed.

This video shows an artistic rendering of the strange contact-binary Kuiper belt object 2001QG298 as seen from earth between 2003 and 2020. It shows how the observing angle and the lightcurve change as the object moves in its orbit along the sun. The black line on the surface of 2001QG298 marks its equator. 2001QG298 is a contact binary which was first identified between 2003 and 2004 by Scott Sheppard and David Jewitt. Its extreme shape was inferred from the large brightness oscilations (larger than 1.1 magnitudes, equivalent to a brightness variation of 3 to 1) produced as the object spins. The lightcurve (brightness variation with rotation) observed in 2003/2004 is shown in red. In 2010 the object was re-observed by Pedro Lacerda who found that the variation had decreased. The 2010 lightcurve is shown as white circles. The two sets of data imply that 2001QG298 is tilted by almost 90 degrees with respect to the plane of its orbit, as shown by the model on the left. The lightcurve produced by the model is shown as a yellow line overplotted on the red and white data points. The model lightcurve varies with time as the object moves along its orbit around the sun. The black line on the surface of 2001QG298 marks the equator and the orbital plane is considered to be horizontal.

A press release on this discovery can be found here. Contact me for alternative formats of the figures.

The scientific paper can be found here.