Persephone, also known as Planet 9 or Planet X, is 10 times the mass of Earth.
Due to it being so distant and dim it wasn't yet detected early in the 21st century. However, there's evidence of a signature of a large, lurking planet written into the peculiar orbits of far flung worlds. Called extreme Kuiper Belt Objects. Many space agencies are fairly convinced that it exists. It lurks deep in the outer area of Sol. Perspehone makes an exceptionally frigid and long period orbit around the sun every 20,000 years. It's about 450,000 LS from the sun.