Sorry, E.T. fans: Interstellar visitor 'Oumuamua isn't an alien spacecraft. It's just passing gas.

An artist's depiction of the interstellar comet 'Oumuamua, as it warmed up in its approach to the sun and outgassed hydrogen (white mist), which slightly altered its orbit.

Since its surprise arrival in the solar system in 2017, the interstellar object 'Oumuamua has puzzled scientists. A duo of American astronomers now think they have solved one of the space rock's lingering mysteries. 

First thought to be an asteroid, later recast as a likely comet, and by some even considered a possible alien spaceship, the 650-foot-long (200 meters) 'Oumuamua zoomed through the central solar system in late 2017. During its brief visit, the rock approached Earth within 15 million miles (24 million kilometers), about 62 Earth-moon distances, and disappeared a few weeks after its discovery.