Non-detection of He I in the atmosphere of GJ1214b with Keck/NIRSPEC, at a time of minimal telluric contamination

TL;DR

This study used Keck/NIRSPEC to observe helium in GJ 1214b's atmosphere during minimal telluric contamination but found no helium absorption, contrasting previous tentative detections and highlighting the challenges of atmospheric characterization.

Contribution

The paper provides a high-quality non-detection of helium in GJ 1214b using Keck/NIRSPEC during optimal conditions, clarifying previous conflicting results and emphasizing the importance of telluric contamination considerations.

Findings

No helium absorption detected at 10830 Angstroms.

Upper limit on excess absorption size is 1.22% (95% confidence).

Discrepancy with previous CARMENES detection remains unexplained.

Abstract

Observations of helium in exoplanet atmospheres may reveal the presence of large gaseous envelopes, and indicate ongoing atmospheric escape. Orell-Miquel et al. (2022) used CARMENES to measure a tentative detection of helium for the sub-Neptune GJ 1214b, with a peak excess absorption reaching over 2% in transit depth at 10830 Angstroms. However, several non-detections of helium had previously been reported for GJ 1214b. One explanation for the discrepancy was contamination of the planetary signal by overlapping telluric absorption- and emission lines. We used Keck/NIRSPEC to observe another transit of GJ 1214b at 10830 Angstroms, at a time of minimal contamination by telluric lines, and did not observe planetary helium absorption. Accounting for correlated noise in our measurement, we place an upper limit on the excess absorption size of 1.22% (95% confidence). We find that the discrepancy between the CARMENES and NIRSPEC observations is unlikely to be caused by using different instruments or stellar activity. It is currently unclear whether the difference is due to correlated noise in the observations, or variability in the planetary atmosphere.