The TRAPPIST-1 system is the first transiting planet system
found orbiting an ultracool dwarf star1. At least seven planets
similar in radius to Earth were previously found to transit
this host star2. Subsequently, TRAPPIST-1 was observed as
part of the K2 mission and, with these new data, we report
the measurement of an 18.77 day orbital period for the outermost
transiting planet, TRAPPIST-1 h, which was previously
unconstrained. This value matches our theoretical expectations
based on Laplace relations3 and places TRAPPIST-1 h
as the seventh member of a complex chain, with three-body
resonances linking every member. We find that TRAPPIST-1 h
has a radius of 0.752 R⊕ and an equilibrium temperature of
173 K. We have also measured the rotational period of the star
to be 3.3 days and detected a number of flares consistent with
a low-activity, middle-aged, late M dwarf.