In October 19, 2017, the Panoramic Inspection Telescope and the Rapid Response System-1 (Pan-STARRS-1) in Hawaii announced the first detection of an interstellar asteroid, called 1I / 2017 U1 Oumuamua.
In the months that followed, multiple follow-up observations were made that allowed astronomers to have a better idea of their size and shape, while also revealing that they had the characteristics of a comet and an asteroid. Interestingly, there have also been some speculations that, according to its shape, Oumuamua could be an interstellar spacecraft ( Breakthrough Listen even monitored it for signals from radio signals).
A new study by a pair of astronomers at the Smithsonian Center for Astrophysics at Harvard has taken it a step further, suggesting that Oumuamua may actually be a ‘light sail’ of extraterrestrial origin.
To summarize, Oumuamua was first seen by Pan-STARRS-1 40 days after it made its closest pass to the Sun (on September 9, 2017). At this point, it was about 0.25 AU from the Sun (a quarter of the distance between Earth and the Sun), and it was already coming out of the Solar System. At that time, the astronomers noticed that it seemed to have a high density (indicative of a rocky and metallic composition) and that it was spinning rapidly.
While it showed no signs of degassing as it passed close to our Sun (which would have indicated that it was a comet), a research team was able to obtain spectra indicating that Oumuamua was cooler than previously thought. Then, when it began to leave the Solar System, the Hubble Space Telescope took some final images of Oumuamua that revealed unexpected behavior.
After examining the images, another international research team discovered that Oumuamua had increased in speed, instead of slowing down as expected. The most likely explanation, they said, was that Oumuamua was discharging material from its surface due to solar heating (also known as degassing).
The release of this material, which is consistent with the behavior of a comet, would give Oumuamua the constant thrust it needed to achieve this speed increase. To this, Bialy and Loeb (authors of the new study) offer a counter-explanation. If Oumuamua was really a comet, why did not he experience degassing when he was closer to our Sun? In addition, they cite other research that showed that if degassing were responsible for the acceleration, it would also have caused a rapid evolution in the Oumuamua turn (which was not observed).
Source : http://aliensdocs.com