Abstract
Investigating the atmospheres of rocky exoplanets is key to performing
comparative planetology between such worlds and the terrestrial planets that
reside in the inner Solar System. Terrestrial exoplanet atmospheres exhibit
weak signals and attempting to detect them pushes at the boundaries of what is
possible for current instrumentation. We focus on the habitable zone
terrestrial exoplanet LHS 1140b. Given its 25-day orbital period and 2-hour
transit duration, capturing transits of LHS 1140b is challenging. We observed
two transits of this object, approximately one year apart, which yielded four
data sets thanks to our simultaneous use of the IMACS and LDSS3C multi-object
spectrographs mounted on the twin Magellan telescopes at Las Campanas
Observatory. We present a jointly fit white light curve, as well as jointly fit
20 nm wavelength-binned light curves from which we construct a transmission
spectrum. Our median uncertainty in Rp^2/Rs^2 across all wavelength bins is 140
ppm, and we achieve an average precision of 1.28x the photon noise. Our
precision on Rp^2/Rs^2 is a factor of two larger than the feature amplitudes of
a clear, hydrogen-dominated atmosphere, meaning that we are not able to test
realistic models of LHS 1140b's atmosphere. The techniques and caveats
presented here are applicable to the growing sample of terrestrial worlds in
the TESS era, as well as to the upcoming generation of ground-based giant
segmented mirror telescopes (GSMTs).
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