A body other than a star or substellar object not gravitationally bound to a star. Its hyperbolic orbit would indicate an object not bound to the Sun. The first known ISO is 1I/'Oumuamua. ISOs are icy planetesimals that are expected to behave like the long-period comets of the solar system; volatile ices sublimate when the ISO approaches the Sun, developing a coma and a dust tail -- features that should make them bright and therefore easy to spot. The rocky ISOs, on the other hand, only reflect sunlight. As their albedo is expected to be extremely low they become dark (after eons of bombardment by high-energy cosmic rays), they would be extremely faint and hard to detect.