Last year astronomers and star-gazers alike were abuzz with excitement over the mysterious cigar-shaped interstellar space rock that zipped through our solar system. It was the first observed interstellar object to enter our solar system, which made its all the more fascinating. The object, which was first spotted through a telescope in Hawaii back in October 2017, and was named "Oumuamua". The name translates as "scout" or "messenger arriving first from afar".
Until recently, scientists were undecided on how to classify the object. Opinions were split between declaring it a comet or an asteroid. The object lacked a tail or coma, which are common indications of a comet. However, scientists also noted that the possibility of Oumuamua being an asteroid was also low. To add further difficulty to the classification process, scientists had relatively little time within which to study the object. Now, eight months later, we finally have some satisfying answers.
The answers came from a research team led by Italian astronomer, Marco Micheli, whose work was published in this week's volume of Nature. The team had studied the trajectory of the object and its speed as it passed through our solar system to best determine the object's nature. They believe that Oumuamua's speed was the result of not just gravity, but the release of gases.
Gaseous shedding like this is a trademark of comets, but settling on this classification wasn't easy. Though the researchers found what they believed to be evidence of gaseous carbon monoxide, carbon dioxide, and water, these gases didn't initially appear to have a great deal of influence on the object's speed or trajectory. It was only when examined further that the effect of the out-gassing became clear. According to Paul Chodas, manager of NASA's Center for Near-Earth Object Studies at Jet Propulsion Laboratory in Pasadena, California, "We could actually see the change in position caused by the outgassing."
While it seemed like strong evidence to support the comet theory, all other theories still needed to be disproven. According to Micheli, it was unlikely that the object would have been an asteroid, as our current data suggests that instellar bodies from the outskirts of other systems are more commonly comets. Identifying Oumuamua as an asteroid would have raised far more questions about our solar system and interstellar bodies than it would have answered.
With that in mind, classifying Oumuamua as a comet seems to be the neatest explanation. Though it's certainly an unusual comet, with its lack of dust tail, the researchers on the study say this can be theorized upon too. One suggestion is that Oumuamua bore large dust grains that were unaffected by its flight path, or that its simply beyond anything within our current scope of understanding.
We can now safely say that Oumuamua is a comet, but many mysteries remain. We still don't know where it came from, and due to the fact that its motion isn't entirely gravitational, determining its source is all the more difficult. The elusive Oumuamua will continue to puzzle scientists for years to come.