HST Transmission Spectra of the Hot Neptune HD 219666 b: Detection of Water and the Challenge of Constraining Both Water and Methane

Although Neptunian-sized (2–5 R⊕) planets appear to be extremely common in the Galaxy, many mysteries remain about their overall nature. To date, only 11 Neptunian-sized planets have had their atmospheres spectroscopically characterized, and…

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