2020A&A...640A..29D

Large-scale exoplanet search surveys have shown evidence that atmospheric escape is a ubiquitous process that shapes the evolution and demographics of planets. However, we lack a detailed understanding of this process because very few exoplanets that have been discovered to date could be probed for signatures of atmospheric escape. Recently, the metastable helium triplet at 1.083µm has been shown to be a viable window for the presence of He-rich escaping envelopes around short-period exoplanets. Our objective is to use, for the first time, the Phoenix spectrograph to search for helium in the upper atmosphere of the inflated hot Jupiter WASP-127 b. We observed one transit and reduced the data manually since no pipeline is available. We did not find a significant in-transit absorption signal indicative of the presence of helium around WASP-127 b, and we set a 90% confidence upper limit for excess absorption at 0.87% in a 0.75Å passband covering the He triplet. Given the large scale height of this planet, the lack of a detectable feature is likely due to unfavorable photoionization conditions for populating the metastable HeI triplet. This conclusion is supported by the inferred low coronal and chromospheric activity of the host star and the old age of the system, which result in a relatively mild high-energy environment around the planet.