First spotted just over a year ago by astronomers in Hawaii, ‘Oumuamua, whose name translates to “messenger from afar arriving first,” immediately captured scientists’ imaginations because it was the first known interstellar object to pass through the solar system. The mystery around the discovery quickly grew.

For starters, Loeb said, its reflection of sunlight suggested that its shape was much more elongated or flattened than any known asteroid or comet, and its motion indicated that it originated from the so-called “local standard of rest” obtained by averaging the random motions of all nearby stars. Fewer than one in 500 local stars moves as slowly in that frame, Loeb said.

What’s more, if ‘Oumuamua originated from a population of similar objects on random trajectories, its discovery would require the production of a thousand trillion such objects per star in the Milky Way — far more than the theoretical calculations Loeb and colleagues performed.

By far the most intriguing observation, Loeb said, suggested that ‘Oumuamua was not just accelerating, but deviating from its expected trajectory.

“The deviations aren’t large, only about one-tenth of a percent, but nevertheless that is significant,” Loeb said. “The question is what is making it deviate. We have seen deviations of this magnitude in comets when they experience outgassing as ice is warmed by the sun, but there is no cometary tail around ‘Oumuamua, and we don’t see its spin changing as we would with outgassing … so it occurred to me that this may be due to radiation pressure from the sun.”

The notion of using a solar sail to move across vast distances isn’t all that unusual, Loeb said.

In 2010, a Japanese spacecraft dubbed IKAROS demonstrated the first successful use of the technology. Loeb is the chair of the advisory board for the Breakthrough Starshot project, which aims to launch a solar-sail-driven probe to the nearest star system.

“It’s possible that, once a civilization reaches a certain maturity technologically, that this is a very common technological solution,” Loeb said. “A rocket is limited to a certain speed … because you are carrying the fuel with you. But if you are using light to push yourself, you’re not limited in the same way.”

Today, ‘Oumuamua is too far away to allow definitive answers on the questions raised by Loeb and other astronomers, and moving too fast to be chased down by probes launched from Earth. Loeb hopes that the next such opportunity will end with more answers.

“My hope is that the next time around, when there is an object like this, people will be intrigued, and … we will do everything we can to figure out what it is,” he continued. “It may well be that it’s a natural object, and if it is, I would argue that it has so many peculiar features that we should understand where it comes from, and we will learn something new from it. But until then, all possibilities should be on the table.”