Surprise! ′Oumuamua (oh-MOO-ah-MOO-ah), the first known interstellar object to travel through our solar system, appears to be a comet, not an asteroid. Using observations from NASA’s Hubble Space Telescope and ground-based observatories, an international team of scientists have confirmed that there was something affecting its motion other than the gravitational force of the sun and planets.
After analyzing its trajectory through the solar system the team found that a surprise uptick in its speed was consistent with a comet. Yes, some of us would prefer to believe that an alien crew on the 1,000-foot-long object just put the pedal to the metal to get away from earthlings prying eyes. Interestingly, the research team who wrote up the paper for Nature, took this possibility into account. But the fact that the smooth and continuous change in speed isn’t typical for thrusters and that the object is tumbling on three axes spoke against it being an artificial object. The truth however, while more mundane, is equally fascinating.
Comets are affected by the gravity of planets and the sun just like asteroids are, but they also possess a wild card: blasts of dusty, gaseous material called jets. Sunlight heats a comet’s surface, vaporizing ices from cliffs, boulder fields and even beneath the surface. Material shot into space gives a comet a kick, affecting its motion and orbit. Astronomers have kept on eye on ‘Oumuamua ever since it was discovered on October 2017 with the Panstarrs-1 telescope on Mt. Haleakala in Maui. Never once did they detect the telltale signs of outgassing — a soft, blurry appearance or tail.
So they assumed they were looking at an asteroid. But after accounting for the tug of the sun and planets, that extra speed had to come from somewhere, with the most likely source ejected comet dust. The team estimates that ‘Oumuamua may have produced a very small amount of dust particles, enough to give it a push but not enough for our telescopes to detect. The object is currently over a billion miles (1.6 billion km) from the sun or nearly as far as Saturn and receding at around 70,000 mph. Four years from now, it will pass Neptune’s orbit as it makes its way back into interstellar space from whence it came.
Karen Meech, an astronomer at the University of Hawaii’s Institute of Astronomy and co-author of the study, speculated that small dust grains, present on the surface of most comets, eroded away during ′Oumuamua’s long journey through interstellar space. We know that both our solar system and other star systems routinely eject comets into space due to gravitational interactions with the sun and planets (or exoplanets), so there should be more interstellar comets out there. We only need to find them!