  Planets with radii between that of the Earth and Neptune (hereafter referred
to as sub-Neptunes) are found in close-in orbits around more than half of all
Sun-like stars. Yet, their composition, formation, and evolution remain poorly
understood. The study of multi-planetary systems offers an opportunity to
investigate the outcomes of planet formation and evolution while controlling
for initial conditions and environment. Those in resonance (with their orbital
periods related by a ratio of small integers) are particularly valuable because
they imply a system architecture practically unchanged since its birth. Here,
we present the observations of six transiting planets around the bright nearby
star HD 110067. We find that the planets follow a chain of resonant orbits. A
dynamical study of the innermost planet triplet allowed the prediction and
later confirmation of the orbits of the rest of the planets in the system. The
six planets are found to be sub-Neptunes with radii ranging from 1.94 to 2.85
Re. Three of the planets have measured masses, yielding low bulk densities that
suggest the presence of large hydrogen-dominated atmospheres.
