  We present the discovery from TESS data of LTT 1445Ab. At a distance of 6.9
parsecs, it is the second nearest transiting exoplanet system found to date,
and the closest one known for which the primary is an M dwarf. The host stellar
system consists of three mid-to-late M dwarfs in a hierarchical configuration,
which are blended in one TESS pixel. We use data from MEarth and results from
the SPOC DV report to determine that the planet transits the primary star in
the system. The planet has a radius 1.38 R_Earth, an orbital period of 5.35882
days, and an equilibrium temperature of 433 K. With radial velocities from
HARPS, we place a three-sigma upper mass limit of 8.4 M_Earth on the candidate.
The planet provides one of the best opportunities to date for the spectroscopic
study of the atmosphere of a terrestrial world. The presence of stellar
companions of similar spectral type may facilitate such ground-based studies by
providing a calibration source to remove telluric variations. In addition, we
present a detailed characterization of the host stellar system. We use
high-resolution spectroscopy and imaging to rule out the presence of any other
close stellar or brown dwarf companions. Nineteen years of photometric
monitoring of A and BC indicates a moderate amount of variability, in agreement
with the observed low-level, short-term variability in the TESS light curve
data. We derive a preliminary astrometric orbit for the BC pair that reveals an
edge-on and eccentric configuration. The presence of a transiting planet in
this system raises the possibility that the entire system is co-planar, which
implies that the system may have formed from the early fragmentation of an
individual protostellar core.
