  JWST secondary eclipse observations of Trappist-1b seemingly disfavor
atmospheres >~1 bar since heat redistribution is expected to yield dayside
emission temperature below the ~500 K observed. Given the similar densities of
Trappist-1 planets, and the theoretical potential for atmospheric erosion
around late M-dwarfs, this observation might be assumed to imply substantial
atmospheres are also unlikely for the outer planets. However, the processes
governing atmosphere erosion and replenishment are fundamentally different for
inner and outer planets. Here, an atmosphere-interior evolution model is used
to show that an airless Trappist-1b (and c) only weakly constrains stellar
evolution, and that the odds of outer planets e and f retaining substantial
atmospheres remain largely unchanged. This is true even if the initial volatile
inventories of planets in the Trappist-1 system are highly correlated. The
reason for this result is that b and c sit unambiguously interior to the
runaway greenhouse limit, and so have potentially experienced ~8 Gyr of
XUV-driven hydrodynamic escape; complete atmospheric erosion in this
environment only weakly constrains stellar evolution and escape
parameterizations. In contrast, e and f reside within the habitable zone, and
likely experienced a comparatively short steam atmosphere during Trappist-1's
pre-main sequence, and consequently complete atmospheric erosion remains
unlikely across a broad swath of parameter space (e and f retain atmospheres in
~98% of model runs). Naturally, it is still possible that all Trappist-1
planets formed volatile-poor and are all airless today. But the airlessness of
b (and c) does not require this, and as such, JWST transit spectroscopy of e
and f remains the best near-term opportunity to characterize the atmospheres of
habitable zone terrestrial planets.

Earth and Planetary Astrophysics