Helium is ubiquitous in the Universe. Large amounts were generated in the Big Bang1, and nearly every star begins its life by producing helium in its core through the nuclear fusion of hydrogen. The atmospheres of giant exoplanets are expected to have an abundance of helium2, because these planets formed from recycled gas and dust from a previous generation of stars. However, searches for helium in such atmospheres have been unsuccessful3. In a paper in Nature, Spake et al.4 report the discovery of helium atoms in the eroding atmosphere of the giant exoplanet WASP-107b. Their work opens a new chapter in the study of exoplanetary atmospheres.
WASP-107b is of comparable size to Jupiter, but has about one-eighth the mass. The exoplanet’s low mass relative to its substantial size makes it difficult for the planet to retain its atmosphere — especially in the presence of strong ultraviolet radiation from its host star. Although this star is smaller and cooler than the Sun, it is threaded with magnetic fields produced by the star. Contortions of these fields emit ultraviolet radiation that energizes the planet’s atmosphere.
Spake et al. observed WASP-107b using a camera on board the Hubble Space Telescope, and concluded that the planet’s atmosphere escapes to form a comet-like tail (Fig. 1). Astronomers have long known that giant planets can lose their atmospheres in this fashion5, so this aspect of Spake and colleagues’ work is not surprising. But the authors have added a key twist to the story. Until now, only hydrogen (the main component of giant planets) and a few elements with low abundances6 have been identified in eroding exoplanetary atmospheres.
Atoms in the gaseous tail of an exoplanet are most easily detected when they absorb stellar light during a transit — a passage of the planet in front of its host star. However, atoms in such a tenuous tail have a tendency to relax to their lowest-energy (ground) state. In this state, most atoms absorb mainly ultraviolet light, and measuring such absorption is difficult for two reasons.
First, Earth’s atmosphere is opaque to most ultraviolet light, which means that absorption measurements must be made from space. Currently, only Hubble has the capability for ultraviolet studies of exoplanetary atmospheres, and this telescope could reach the end of its mission lifetime in the next decade. Second, the pattern of how much ultraviolet stellar light is absorbed by transiting planets as a function of time or wavelength tends to be complex. Such complexity makes it difficult to interpret ultraviolet measurements of a transiting planet’s atmosphere.
Fortunately, helium atoms have a long-lived (metastable) state, in addition to the ground state. Metastable helium atoms absorb near-infrared stellar light, which has a wavelength only slightly beyond the limits of human vision. Measurements at this wavelength are much easier to interpret than those at ultraviolet wavelengths.
Spake and colleagues observed a transit of WASP-107b, and measured the amount of near-infrared stellar light that was transmitted through the planet’s eroding atmosphere as a function of wavelength. The authors identified a narrow absorption feature that they associated with metastable helium atoms (see Fig. 1 of the paper4). This signal is more than five times greater than any false signal that could be produced by stellar activity.
Detecting helium in the escaping atmospheres of other exoplanets will be difficult because the absorption signal is intrinsically weak, especially for planets smaller than WASP-107b. However, astronomers will eagerly rise to the challenge. The near-infrared signature of metastable helium is readily transmitted through Earth’s atmosphere, which means that eroding exoplanetary atmospheres could be probed using ground-based telescopes. The advent of a new generation of extremely large telescopes at ground-based observatories7 will allow astronomers to study the escaping atmospheres of planets as small as Neptune, which has a radius four times that of Earth.
Theorists have predicted that the atmospheres of Neptune-sized exoplanets could be rich in helium8, owing to differences in the rates at which hydrogen and helium are lost to space. Like other giant planets, these bodies are thought to start out with atmospheres of predominantly hydrogen, abundant helium and smaller amounts of elements heavier than helium. As their atmospheres escape, hydrogen is lost fastest, leading to a gradual relative enrichment in the helium content of the atmosphere.
Heavier elements such as carbon and oxygen would be slow to escape, and could in principle be present in exoplanetary atmospheres in concentrated amounts. These heavier elements are key to understanding both how planets form and how they acquire their atmospheres. For planetary astronomers, an escaping atmosphere that is rich in heavy elements is something of a cosmic treasure, providing ample scientific opportunities to study planetary formation and evolution. Spake and colleagues’ detection of helium in WASP-107b will enable astronomers to look for atmospheres that are rich in helium, and perhaps in heavier elements, thereby opening a new subfield of exoplanetary science.
Nature 557, 35-36 (2018)
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