To answer the question right off the bat - yes, it's certainly possible, but there hasn't been any photographic evidence that such a planet exists just yet.
On the other hand, there are other forms of evidence that scientists are using to base this theory off of, such as the fact that the outer planets in our solar system all have a very odd orbital track around the Sun that never really seems to be uniform, unlike the planets closer to the Earth.
The only thing scientists can think of that would be influencing such a strange gravitational track would be if something larger were out there, influencing the orbit around the Sun for planets like Neptune, Uranus, and even the dwarf planet Pluto.
Such a planet, dubbed planet nine, would be massive compared to the Earth - at least 10 times larger, and its distance from the Sun would be significant. Its existence is expected to be somewhere in the Kuiper Belt, or even beyond.
Because of its distance, the only way we might ever get any kind of images of such a spatial body is to send a spacecraft out there, which would take more than a decade at the rate that it took New Horizons to get to Pluto.