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Transmission spectroscopy is a powerful technique widely used to probeexoplanet terminators. Atmospheric retrievals of transmission spectra areenabling comparative studies of exoplanet atmospheres. However, the atmosphericproperties inferred by retrieval techniques display a significant anomaly: mostretrieved temperatures are far colder than expected. In some cases, retrievedtemperatures are ~1000 K colder than T_eq. Here, we provide an explanation forthis conundrum. We demonstrate that erroneously cold temperatures result when1D atmospheric models are applied to spectra of planets with differingmorning-evening terminator compositions. Despite providing an acceptable fit,1D retrieval techniques artificially tune atmospheric parameters away fromterminator-averaged properties. Retrieved temperature profiles are hundreds ofdegrees cooler and have weaker temperature gradients than reality. Retrievedabundances are mostly biased by > 1$\sigma$ and sometimes by > 3$\sigma$, withthe most extreme biases for ultra-hot Jupiters. When morning-eveningcompositional differences manifest for prominent opacity sources, H$_2$Oabundances retrieved by 1D models can be biased by over an order of magnitude.Finally, we demonstrate that these biases provide an explanation for the coldretrieved temperatures reported for WASP-17b and WASP-12b. To overcome biasesassociated with 1D atmospheric models, there is an urgent need to developmultidimensional retrieval techniques.

Why is it so Cold in Here?: Explaining the Cold Temperatures Retrieved from Transmission Spectra of Exoplanet Atmospheres