A Mirage or an Oasis? Water Vapor in the Atmosphere of the Warm Neptune TOI-674 b

Abstract

We report observations of the recently discovered super-Neptune TOI-674 b (5.25 Earth radii, 23.6 Earth mass) with the Hubble Space Telescope's Wide Field Camera 3 instrument. TOI-674 b is deep into the Neptune desert, an observed paucity of Neptune-size exoplanets at short orbital periods. Planets in the desert are thought to have complex evolutionary histories due to photoevaporative mass loss or orbital migration, making identifying the constituents of their atmospheres critical to understanding their origins. We obtained near-infrared transmission spectroscopy of the planet's atmosphere with the G141 grism, which we detrended and fit. After extracting the transmission spectrum from the data, we used the petitRADTRANS atmospheric spectral synthesis code to perform retrievals on the planet's atmosphere to identify which absorbers are present. These results show evidence for increased absorption at 1.4 μm due to water vapor at 2.1σ (Bayes factor = 3.2). With these results, TOI-674 b joins the exclusive club of exoplanets with featured transmission spectra. TOI-674 b is a strong candidate for further study to refine the water abundance, which is poorly constrained by our data. We also incorporated new TESS short-cadence optical photometry, as well as Spitzer/IRAC data, and re-fit the transit parameters for the planet. We find the planet to have the following transit parameters: R_p/R_∗ = 0.1135±0.0006, T₀ = 2458544.523792 ± 0.000452 BJD, and P = 1.977198 ± 0.00007 d. These measurements refine the planet radius estimate and improve the orbital ephemerides for future transit spectroscopy observations of this highly intriguing warm Neptune.