The phrase "unseen planet" refers to a planet that we cannot see or observe. It implies that this planet is unknown or hidden from our sight due to distance, darkness, or other factors. Full definition
Those stellar motions can reveal the presence of unseen planets whose gravitational influence produces periodic patterns of motion in the host star's position. (scientificamerican.com)
Such asymmetry would most likely be the result of dynamical sculpting by one or more unseen planets in the system. (arxiv.org)
The best explanation for the observations is that a suspected unseen planet, about one to 20 times the mass of Jupiter and in an orbit within the secondary disk, is using gravity to sweep up material from the primary disk. (hubblesite.org)