Proxima Centauri b is not a transiting exoplanet

Jenkins, James S.; Harrington, Joseph; Challener, Ryan C.; Kurtovic, Nicolas T.; Ramirez, Ricardo; Pena, Jose; McIntyre, Kathleen J.; Himes, Michael D.; Rodriguez, Eloy; Anglada-Escude, Guillem; Dreizler, Stefan; Ofir, Aviv; Pena Rojas, Pablo A.; Ribas, Ig

VL / 487 - BP / 268 - EP / 274
We report Spitzer Space Telescope observations during predicted transits of the exoplanet Proxima Centauri b. As the nearest terrestrial habitable-zone planet we will ever discover, any potential transit of Proxima b would place strong constraints on its radius, bulk density, and atmosphere. Subsequent transmission spectroscopy and secondary-eclipse measurements could then probe the atmospheric chemistry, physical processes, and orbit, including a search for biosignatures. However, our photometric results rule out planetary transits at the 200 ppm level at 4.5 mu m, yielding a 3 sigma upper radius limit of 0.4 R-circle plus ( Earth radii). Previous claims of possible transits from optical ground- and space-based photometry were likely correlated noise in the data from Proxima Centauri's frequent flaring. Our study indicates dramatically reduced stellar activity at near-to-mid infrared wavelengths, compared to the optical. Proxima b is an ideal target for space-based infrared telescopes, if their instruments can be configured to handle Proxima's brightness.
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