A super-massive planet has been discovered past the orbit of Pluto. This planet is apparently 10 times more massive than Earth, this should make it easier to see in a telescope if the astronomers know where to look. This has been speculated about since the 1930`s, there was a search for a planet that was disturbing the orbits of Neptune and Uranus. Pluto was discovered, but this planet is not massive enough to be doing this, and therefore, there was a dark body past Pluto that is causing this disturbance. If the planet has 10 times the gravity of Earth, it would not be a nice experience to land upon the surface as the astronauts would not be able to walk around. 10 times gravity would drain all of the blood out of the brain and you would pass out.
But the potential to explore an alien world would be worth it, even if we had to send robots down to the surface in stead of humans. Exploring the moons of Jupiter would have to be done with robots as there is too much radiation to support humans. In Science Fiction stories where this moon was colonized the humans had to live underground to survive the radiation from the Jovian magnetosphere. Of course a planet past the orbit of Pluto would be far too cold for any life to exist. Something like Earth would be preferable. Easier to grow food for a colony. Unless a space station could support a civilization, as in the movie Elysium, a giant wheel shaped space station would be perfect. The rotation of the space station provides gravity and walls along the edges of the wheel hold in the atmosphere.
Just like the space station in 2001 a Space Odyssey. Although that one was enclosed, it still spun to provide gravity. A ringworld like the one in Halo would be a similar idea, but impossible due to the amount of material required and the limitations of material science to construct a ringworld using material that can withstand the immense strain on the structure. The ringworld in the Larry Niven novel series was even larger, but constructed using an impossible material. It was 600 million miles (1,000 gigameters) in circumference), encircling a sunlike star. It rotates, providing artificial gravity that is 99.2% as strong as Earth’s gravity through the action of centrifugal force. It was a million miles wide as well. This means that there would be a massive amount of room for any civilization. I would be awed to see a ringworld in person.
There is more information about the ringworld here: http://news.larryniven.net/concordance/content.asp?page=Ringworld%20Appendix#Scale.