Presently the details are sketchy. A Neptune-ish size orb out past Neptune – semi-major axis about x20 Neptune, perihelion about 200 AU, and a period of roughly 10,000 years. The discovery paper is here: EVIDENCE FOR A DISTANT GIANT PLANET IN THE SOLAR SYSTEM
What would it be like? Odds are, if it’s one of Uranus or Neptune’s kin, then it’s not a ‘Super-Earth’. Instead it’ll be whatever concoction they are – several theoretical options are available, one of which is that they formed from mostly carbon monoxide ice. The CO then reacted with primordial H2 to make H2O and CH4 – the observed ‘ices’ in both. This could explain their depleted D/H ratios as compared to their supposed cometary building blocks. Some planet formation simulations do throw a fifth ‘Gas Giant’ into the Outer Dark, so it’s a live option.
Alternatively, it is a Super-Earth. If it was formed further out than the other Terrestrials, then it might’ve retained its primordial H2/He atmosphere. Too much of that and there’s no chance of liquid water, but if the surface pressure is under ~200 bar, then the hydrogen greenhouse effect will allow *liquid* water. An Ocean Planet is a real possibility. Perhaps the name ‘Poseidon’ should be considered. The ocean would be Stygian in its darkness, so maybe ‘Tartarus’ would be more apt.