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Hunt For Ninth Planet Reveals Distant Solar System Objects (carnegiescience.edu)
schwit1 writes: Astronomers have discovered several new objects orbiting the Sun at extremely great distances beyond the orbit of Neptune. The most interesting new discovery is 2014 FE72: "2014 FE72 is the first distant Oort Cloud object found with an orbit entirely beyond Neptune," reports Carnegie Institution for Science. "It has an orbit that takes the object so far away from the Sun (some 3000 times farther than Earth) that it is likely being influenced by forces of gravity from beyond our Solar System such as other stars and the galactic tide. It is the first object observed at such a large distance." This research is being done as part of an effort to discover a very large planet, possibly as much as 15 times the mass of Earth, that the scientists have proposed that exists out there.
First off, there is no "theoretical maximum speed" to how fast a given propulsion mechanism can get you. You can get to 0,999c by shooting tennis balls out the back of a spacecraft with a slingshot - if you're willing to build a spacecraft comparable in size to the universe ;)
Secondly, nuclear pulse drives really are an antiquated idea, I don't know why people obsess over it. Their minimum sizes are way too large and they're inefficient, with low ISP compared to more modern ideas. Longshot, BTW, is technically NPP, although a more modern variety. Still inefficient and very heavy, and nowhere close to a technology that could be achieved in a reasonable timeframe from where we are today.
Of the many, many concepts now available, I'd personally go for fission fragment propulsion. It's so straightforward: get the most out of fission by having individual fission reactions propel your spacecraft directly. And from a design perspective, it's pretty straightforward particle physics / fission reactor design, just in an unusual (suspended) configuration - the suspension already demoed in the lab. But that's, again, just one of many possibilities.
"99 dead duelists of Dios on the wall. 99 dead duelists of Dios! Take one's ring, pass it around..."
Not in the Oort cloud, where all the volatiles are. We already have an example of weird low-temperature chemistry in Pluto.
well ... except that planetoids typically accrete around rocky objects after supernova, so without a previous star going nova, there's nothing to create the rocks to create the protoplanets to accrete planets. Consequently, the planet is full of gas clouds, and stars with significant planets pretty rare.