NASA’s Kepler space telescope has uncovered six new planets huddled around a sunlike star—odd worlds that astronomers have dubbed mini-Neptunes, scientists announced Wednesday.

Five of the new planets are closer to their parent star than Mercury is to the sun. The sixth world lies farther out, within a region that would fit inside the orbit of Venus.

“This is the most closely packed known planetary system,” said study co-author Jonathan Fortney, an astronomer with the Lick Observatory at the University of California, Santa Cruz.