On October 19th, 2017, the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) in Hawaii announced the first-ever detection of an interstellar asteroid, named 1I/2017 U1 (AKA 'Oumuamua). Interestingly enough, there has also been some speculation that based on its shape, 'Oumuamua might actually be an interstellar spacecraft (Breakthrough Listen even monitored it for signs of radio signals!). Astronomers from the Harvard Smithsonian Center for Astrophysics (CfA) analyzed the strange cigar shape of the object, and an unexpected boost in speed and shift in trajectory as it passed through the inner solar system last year. The researchers say the strange acceleration could the the result of solar radiation pushing a giant solar sail. The pair suggested that ‘Oumuamua could actually be a light sail, a spacecraft that relies on radiation pressure for power, that's been sent from an alien civilization looking for signs of life in our solar system. Its origin could be either natural (in the interstellar medium or proto-planetary disks) or artificial (as a probe sent for a reconnaissance mission into the inner region of the Solar System)." "One possibility is that �?Oumuamua is a lightsail, floating in interstellar space as a debris from an advanced technological equipment", the authors claim, and it might be unused debris that has been thrown out into space from a planetary system. On the other hand, as Loeb told Universe Today, 'Oumuamua could be an active piece of alien technology that came to explore our Solar System, the same way we hope to explore Alpha Centauri using Starshot and similar technologies:

"The alternative is to imagine that `Oumuamua was on a reconnaissance mission. The reason I contemplate the reconnaissance possibility is that the assumption that `Oumumua followed a random orbit requires the production of ~10^{15} such objects per star in our galaxy. This abundance is up to a hundred million times more than expected from the solar system, based on a calculation that we did back in 2009," Loeb wrote. "A surprisingly high overabundance, unless ‘Oumuamua is a targeted probe on a reconnaissance mission and not a member of a random population of objects." If they are right that the object is being propelled by radiation pressure from the sun, then it will be a breakthrough even if it is entirely natural.
Astronomers from Harvard University suggest that the interstellar object 'Oumuamua may be an extraterrestrial solar sail from an alien civilization, in an effort to help explain the object's "peculiar acceleration".