The answer would be a bit tricky, considering the universe which vast—and we've only explored tiny fraction of it. However, the exoplanet ROXs 42 Bb is probably one of the candidates for the title of "biggest planet" based on what we currently know.ABOUT ROXs 42 BbAbout 500 light-years separate Earth and this gas giant. It is thought to have a radius that is roughly 1.12 times that of Jupiter. Jupiter is already roughly eleven times wider than Earth, to give you an idea! It is thought to be about nine times as massive as Jupiter.Even though ROXs 42 Bb is a good contender to be the largest planet that we currently know of, there may be even bigger surprises in the universe!