The Shapes of Kuiper Belt Objects

Can we see the shapes of Kuiper Belt objects?

Kuiper Belt objects (KBOs) are too far for us to be able to resolve their shapes — they appear point-like, just like stars do. If their shapes cannot be measured directly then we must infer them from other things we can measure. Here I will describe how to learn about KBO shapes using lightcurves.

As described here, most lightcurves are produced by objects that are not spherical. So, if an object produces a lightcurve, we already know something about it's shape: it is not spherical*. The less spherical, or more elongated an object is, the larger will be the lightcurve range of variation. About 35% of the KBOs searched for lightcurves show significant brightness variation. This fact alone can be used to rule out the possibility that all KBOs are spherical.

Now we just have to extend this idea to shapes other than a sphere. Suppose all KBOs were shapes like an egg. And supposed all eggs have exactly the same shape. Then we would immediately know what lightcurve range to expect, and it would be the same for all KBOs. This gets a bit more complicated when we consider the possibility that the spin axis of the KBO points at us. In that case, even if the KBO is elongated it will still produce no brightness variations.

* [For simplicity, I am considering only objects with uniform surfaces.]