Helium in the eroding atmosphere of an exoplanet

Abstract: Helium is the second-most abundant element in the Universe after hydrogen and is one of the main constituents of gas-giant planets in our Solar System. Early theoretical models predicted helium to be among the most readily detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres. Searches for helium, however, have hitherto been unsuccessful. Here we report observations of helium on an exoplanet, at a confidence level of 4.5 standard deviations. We measured the near- infrared transmission spectrum of the warm gas giant WASP-107b and identified the narrow absorption feature of excited metastable helium at 10,833 angstroms. The amplitude of the feature, in transit depth, is 0.049 +/- 0.011 per cent in a bandpass of 98 angstroms, which is more than five times greater than what could be caused by nominal stellar chromospheric activity. This large absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 10^10 to 3 x 10^11 grams per second (0.1-4 per cent of its total mass per billion years), and may have a comet-like tail of gas shaped by radiation pressure.

• The paper presents the first 4.5σ detection of excited helium in WASP-107b’s extended, eroding atmosphere.
• It employs HST's WFC3 near-infrared transmission spectroscopy to isolate a helium absorption feature at 10,833 Å with a transit depth increase of 0.049±0.011%.
• The findings suggest significant atmospheric mass loss, providing a novel method to study escape phenomena compared to traditional Lyman-alpha observations.

Detection of Helium in Eroding Exoplanet Atmospheres

The paper presents a landmark achievement in the field of exoplanet atmospheric studies by reporting the first detection of helium in an exoplanetary atmosphere, specifically that of WASP-107b, utilizing near-infrared transmission spectroscopy obtained through the Hubble Space Telescope (HST). By achieving a confidence level of 4.5σ, this publication not only confirms the presence of helium but also discusses the implications of its detection in understanding atmospheric escape phenomena.

WASP-107b is characterized as a low-density gas giant orbiting a K6-type star every 5.7 days at a distance of 0.055 astronomical units (AU). The exoplanet has a radius comparable to Jupiter's but a significantly lower mass. This configuration contributes to an extended, possibly eroding atmosphere, now confirmed to contain excited, metastable helium.

Methodology and Findings

The researchers utilized HST's Wide Field Camera 3 (WFC3) and the G102 grism to capture transmission spectra across the 8,000–11,000 Å wavelength range. The data reduction process involved corrections for instrumental effects, prominent among which is the electron trapping in the WFC3 detector. The narrowed helium absorption feature was pinpointed at 10,833 Å, presenting an absorption signal amplitude significantly higher than could be attributed solely to stellar activity.

The quantitative results indicate the detection of helium with a transit depth increase of approximately 0.049±0.011%—a value markedly above stellar activity levels. This suggests a large scale atmospheric loss rate in the range of 10⁸ to 3×10¹¹ g/s, potentially leading to a comet-like tail formed by stellar radiation pressure.

Implications and Future Research

This study has substantial implications for the theoretical and practical understanding of exoplanetary sciences. The detection method of the helium absorption triplet at 10,833 Å offers an alternative approach that complements existing ultraviolet detections of hydrogen-driven escape, traditionally observed via Lyman-alpha lines. It provides a means to examine the extended atmospheres of exoplanets using infrared wavelengths, which is feasible with both existing ground-based infrared spectrographs and future space telescopes like the James Webb Space Telescope (JWST).

WASP-107b's atmospheric erosion provides empirical evidence for theories of atmospheric mass loss and composition alteration, particularly relevant to the observed paucity of super-Earth and sub-Neptune exoplanets at certain radii likely sculpted by such processes. Continued observations focusing on infrared spectral features, particularly the helium triplet, have the potential to elucidate the underlying dynamics and escape mechanisms of planetary atmospheres.

In closing, this paper exemplifies a significant advancement in exoplanetary atmospheric studies, providing novel insights into planetary formation and evolution. The successful detection of helium opens new avenues for the exploration of atmospheric escape phenomena, essential for retracing exoplanetary evolution and understanding the distribution of diverse planetary types observed today.