‘Oumuamua - the cigar-shaped body spotted last year that is the first known visitor from outside the Solar System - is a comet after all, reports a paper published online this week in Nature. The travelling body had been the object of some debate previously, first being classified as a comet, then an asteroid, and finally the first of a new class of ‘interstellar objects’.

First spotted by Hawaii’s Haleakala Observatory on 19 October 2017, ‘Oumuamua is an unusually elongate, about 800-metre-long, dark red body, of unknown origin, that tumbled along an unbound, hyperbolic path across the Solar System. Although the surface of ‘Oumuamua resembles the core of a comet, it did not appear to have the ‘coma’ of atmosphere and dust that forms as comets melt and release gases when passing near a star.

Marco Micheli and colleagues studied both ground- and space-based observations of the motion of ‘Oumuamua throughout our Solar System. They show that the arc along which the object travelled cannot be explained solely by the gravitational attraction of the Sun, planets and large asteroids. Instead, they find that part of the acceleration, which was predominantly directed away from the Sun, must be non-gravitational in nature. This motion is consistent with the behaviour of comets, which can be propelled by the gas released from them. From their modelling, the authors could also rule out other likely explanations for the non-gravitational motion, including pressure from solar radiation or magnetic interaction with the solar wind.