This site supplied an article about maxing out the VASIMR plasma rocket with nuclear fission or nuclear fusion reactors Note: nuclear fusion can have a superior option with the Bussard QED propulsion system (direct heating of electrons).
The new world is GJ 1214b, about 6.5 times as massive as the Earth, orbiting a small M-dwarf about a fifth the size of the Sun some forty light years from Earth.
But there’s more, a good deal more. At a distance of 1.3 million miles, the planet orbits its star every 38 hours, with an estimated temperature a little over 200 degrees Celsius. Because GJ 1214b transits the star, astronomers are able to measure its radius, which turns out to be 2.7 times that of Earth. The density derived from this suggests a composition of about three-fourths water and other ices and one-fourth rock
Astronomers say they have detected a planet just six and a half times as massive as Earth – at a distance so close its atmosphere could be studied, and with a density so low it’s almost certain to have abundant water.
The alien world known as GJ 1214b orbits a red dwarf star one-fifth the size of our own sun, 40 light-years away in the constellation Ophiuchus, the astronomers reported in Thursday’s issue of the journal Nature. News release from eurekalert