Tumbling motion of 1I/'Oumuamua reveals body's violent past

Abstract

Models of the Solar System evolution show that almost all the primitive material leftover from the formation of the planets was ejected to the interstellar space as a result of dynamical instabilities. Accordingly, minor bodies should also be ejected from other planetary systems and should be abundant in the interstellar space. The number density of such objects, and prospects for their detection as they penetrate through the Solar System, were speculated about for decades, recently rising high hopes with the Pan-STARRS and LSST surveys. These expectations materialized on 18 October 2017 with the Pan-STARRS's discovery of 1I/'Oumuamua. Here we report homogeneous photometric observations of this body from Gemini North, which densely cover a total of 8 hr over two nights. A combined ultra-deep image of 1I/'Oumuamua shows no signs of cometary activity, confirming the results from earlier, less sensitive searches. Our data also show an enormous range of brightness variations > 2.5 mag, larger than ever observed in the population of Solar System objects, suggesting a very elongated shape of the body. But most significantly, the light curve does not repeat exactly from one rotation cycle to another and its double-peaked periodicity of 7.5483 $\pm$ 0.0073 hr from our data is inconsistent with earlier determinations. These are clear signs of a tumbling motion, a remarkable characteristic of 1I/'Oumuamua's rotation, consistent with a catastrophic collision in the distant past. This first example of an impacted minor body of exosolar origin indicates that collisional evolution of minor body populations in other planetary systems is not uncommon.