A 1.55 R$_{\oplus}$ habitable-zone planet hosted by TOI-715, an M4 star near the ecliptic South Pole

A new generation of observatories is enabling detailed study of exoplanetary
atmospheres and the diversity of alien climates, allowing us to seek evidence
for extraterrestrial biological and geological processes. Now is therefore the
time to identify the most unique planets to be characterised with these
instruments. In this context, we report on the discovery and validation of
TOI-715 b, a $R_{\rm b}=1.55\pm 0.06\rm R_{\oplus}$ planet orbiting its nearby
($42$ pc) M4 host (TOI-715/TIC 271971130) with a period $P_{\rm b} =
19.288004_{-0.000024}^{+0.000027}$ days. TOI-715 b was first identified by TESS
and validated using ground-based photometry, high-resolution imaging and
statistical validation. The planet's orbital period combined with the stellar
effective temperature $T_{\rm eff}=3075\pm75~\rm K$ give this planet an
instellation $S_{\rm b} = 0.67_{-0.20}^{+0.15}~\rm S_\oplus$, placing it within
the most conservative definitions of the habitable zone for rocky planets.
TOI-715 b's radius falls exactly between two measured locations of the M-dwarf
radius valley; characterising its mass and composition will help understand the
true nature of the radius valley for low-mass stars. We demonstrate TOI-715 b
is amenable for characterisation using precise radial velocities and
transmission spectroscopy. Additionally, we reveal a second candidate planet in
the system, TIC 271971130.02, with a potential orbital period of $P_{02} =
25.60712_{-0.00036}^{+0.00031}$ days and a radius of $R_{02} =
1.066\pm0.092\,\rm R_{\oplus}$, just inside the outer boundary of the habitable
zone, and near a 4:3 orbital period commensurability. Should this second planet
be confirmed, it would represent the smallest habitable zone planet discovered
by TESS to date.