HET/HPF observations of Helium in warm, hot, and ultra-hot Jupiters

Abstract

The near-infrared helium triplet line is a powerful tool for studying atmospheric escape processes of close-in exoplanets, especially irradiated gas giants. Line profile fitting provides direct insight into the mechanisms driving atmospheric mass loss of close-in, Jupiter-sized planets. We present high-resolution transmission spectroscopy results for the helium triplet line of sixteen gas giants ($R_{\rm p} > 0.5 R_{\rm Jup}$). These observations are part of an extensive helium survey conducted using the Habitable Zone Planet Finder spectrograph on the 10m Hobby-Eberly Telescope. For the first time, we provide constraints on the helium line for HAT-P-12b, HAT-P-17b, HD118203b, TrES-1b, and WASP-156b. Additionally, we are able to confirm previous robust or tentative detections for HD189733b, HD209458b, WASP-52b, WASP-69b, and WASP-76b, and non-detections for HAT-P-3b, WASP-11b, WASP-80b, WASP-127b, and WASP-177b. We do not confirm the previous helium narrow-band detection in HAT-P-26b using high-resolution observations. To identify trends within the population of warm, hot, and ultra-hot Jupiters, we combined our results with available helium studies from the literature. As predicted by theory, we find that warm Jupiters with helium detections orbit K-type stars. However, the helium detections in hot Jupiters are found in low-density planets orbiting F-type stars. There are no helium detections in ultra-hot Jupiters. We compiled a list of 46 irradiated gas giants, but more helium studies are needed to increase the sample and improve our understanding of atmospheric mass loss through helium observations.

Figures (14)

• Figure 1: Radius-density diagram for planets with $R_{\rm p}$$>$ 0.5 R$_{\rm Jup}$. The planets observed by our HET/HPF Helium survey are labeled and color-coded according to their equilibrium temperature. The grey points represent all known planets with radius and density determined with precision better than 25% and 33%, respectively (data from NASA Exoplanet Archive as of 1 August 2025).
• Figure 2: Residual maps and transmission spectra around the He i triplet lines for HAT-P-3 b, HAT-P-12 b, HAT-P-17 b, and HAT-P-26 b. Left panels: Residual maps in the stellar rest frame as a function of wavelength and orbital phase. Relative absorption is color-coded. The dashed and dotted green horizontal lines indicate the different contacts during the transit. The dashed cyan tilted lines indicate the predicted trace of the planetary signal. The regions affected by strong OH residuals and telluric H$_2$O absorption are marked. Right panels: Planet transmission spectra (TS) in the planet rest frame. We show the original data in light gray and the data binned by 0.2 Å in black. When an absorption signal is fitted, a red line and shaded region show the best fit model with its $1\sigma$ uncertainty. The dotted cyan vertical lines indicate the He i triplet line position.
• Figure 3: Same as Figure \ref{['Fig: TS plot 1']} for HD118203 b, HD189733 b, HD209458 b, TrES-1 b.
• Figure 4: Same as Figure \ref{['Fig: TS plot 1']} for WASP-11 b (HAT-P-10 b), WASP-52 b, WASP-69 b, WASP-76 b.
• Figure 5: Same as Figure \ref{['Fig: TS plot 1']} for WASP-80 b, WASP-127 b, WASP-156 b, and WASP-177 b.