In 2017, a bizarre cigar-shaped object dubbed 'Oumuamua zipped through our solar system. It was the first time astronomers had seen an interstellar object pass through our solar system — meaning something that originated in another star system. The properties of 'Oumuamua, at least those which we could observe, were way out of the norm for what most asteroids and comets look like: it was unusually reflective; very oddly shaped (with one long axis and one thin one); and perhaps most bizarrely, it unnaturally accelerated as it left.
The researchers did calculations to see if 'Oumuamua was big enough to store enough entrapped hydrogen, and if that could account for its peculiar acceleration.
