Ground-based transmission spectroscopy of the terrestrial exoplanets GJ 1132b and LHS 1140b

GJ 1132b and LHS 1140b, two terrestrial worlds transiting nearby mid-M dwarf  stars, offer  an  opportunity  for  comparative  planetology. GJ1132b is highly irradiated, orbits its host star on a 1.6-day period, and receives 19 times Earth’s insolation.  On the other hand, LHS 1140b is in the habitable-zone of its host star, orbits on a 25-day period, and receives only 0.4  times  Earth’s  insolation.   The  relatively  high  planet-to-star  radius  ratios for these two terrestrial exoplanets make them amenable to atmospheric characterization  via  the  transmission  spectroscopy  method.   With  ground-based telescopes we can test the cases of clear, low mean molecular weight atmospheres on these worlds.  We observed five transits of GJ 1132b with the LDSS3C multi-object spectrograph on Magellan Clay, and one transit of LHS 1140b (the only one observable from the ground in 2017) simultaneously with both the LDSS3C and IMACS multi-object spectrographs on Magellan Clay and Baade.  We developed custom Python pipelines to extract, calibrate, and simultaneously decorrelate multiple transits of each object.  We completed the analysis of the GJ 1132b transits and disfavor clear, low mean molecular weight atmospheres at >3 sigma confidence.  I will summarize these findings and present those for the LHS 1140b transit, and contextualize these results in the current state of the field.  These measurements are a basis for comparative planetology of terrestrial exoplanets and will inform future observations with HST and JWST.

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