  Upcoming NASA astrophysics missions such as the James Webb Space Telescope
will search for signs of life on planets transiting nearby stars. Doing so will
require co-adding dozens of transmission spectra to build up sufficient signal
to noise while simultaneously accounting for challenging systematic effects
such as surface/weather variability, atmospheric refraction, and stellar
activity. To determine the magnitude and impacts of both stellar and planet
variability on measured transmission spectra, we must assess the feasibility of
stacking multiple transmission spectra of exo-Earths around their host stars.
Using our own solar system, we can determine if current methodologies are
sufficient to detect signs of life in Earth's atmosphere and measure the
abundance of habitability indicators, such as H2O and CO2, and biosignature
pairs, such as O2 and CH4. We assess the impact on transmission spectra of
Earth transiting across the Sun from solar and planetary variability and
identify remaining unknowns for understanding exoplanet transmission spectra.
We conclude that a satellite observing Earth transits across the Sun from
beyond L2 is necessary to address these long-standing concerns about the
reliability of co-adding planet spectra at UV, optical, and infrared
wavelengths from multiple transits in the face of relatively large
astrophysical systematics.
