The flat and featureless transmission spectra of two intermediate-sized extrasolar planets, observed during the planets' passage across their host stars, shed light on the properties of their atmospheres. See Letters p.66 & p.69
Flat spectra do not typically excite astronomers, but there are times when a lack of spectral features tells you something interesting. Such is the case with observations of two separate sub-Jupiter-sized extrasolar planets made using the Hubble Space Telescope and reported by Knutson et al.1 and Kreidberg et al.2 on pages 66 and 69 of this issue, respectively.
The first planet, GJ 436b, has a mass and radius slightly greater than Neptune's. The second, GJ 1214b, is smaller, with a radius roughly 2.7 times that of Earth. Both exoplanets orbit very close to their host stars and are therefore quite warm by Earth standards. Owing to intensive observational scrutiny since their respective discoveries, these two planets have become the archetypes of the new 'Neptune-class' and 'super-Earth' categories of exoplanets.
Although the first extrasolar planets discovered — in the 1990s and 2000s — tended to be hot, massive, hydrogen-dominated worlds that most closely resemble Jupiter, astronomers have been methodically and efficiently chipping away at the harder-to-observe regime of smaller, denser and cooler Earth-like planets. Recent ground- and space-based surveys demonstrate3,4,5 that planets of sizes ranging between those of Earth and Neptune overwhelmingly dominate the observed exoplanet population. But what are these intermediate-sized exoplanets really like? Our Solar System does not provide sufficient clues about these planets, because we have only Earth and Venus at one end of the scale and cold Uranus and Neptune at the other to serve as examples. Do these mid-sized exoplanets have rocky surfaces, like Earth and Venus? Are they fluid planets with thick, deep atmospheres relatively rich in hydrogen and volatile elements, like Uranus and Neptune? Are they 'water worlds' with steam atmospheres overlying deep oceans? Are the atmospheres of these planets thick or thin, consisting predominantly of hydrogen, water, carbon dioxide or nitrogen, or something more exotic? Observations of GJ 436b and GJ 1214b provide important hints.
The orbital planes of both GJ 436b and GJ 1214b are almost exactly edge-on as seen from Earth, so that the planets periodically transit — pass directly in front of — their host stars, causing slight dips in the amount of light seen from the system. The depth of these transit dips allows the planetary radius to be determined, and their wavelength dependence provides information about atmospheric composition. During transits, the stellar light passes through the planet's atmosphere on its way to the observer. For a planet with an extended atmosphere, the apparent size of the planet can vary with observed wavelength because more stellar light is blocked at high altitudes at wavelengths for which atmospheric constituents have strong absorption bands. Conversely, more light passes through at low altitudes at wavelengths for which atmospheric constituents are less absorbing.
GJ 1214b is known to have a relatively flat (wavelength-independent) transmission spectrum, with weak absorption features at best6,7. Because the bulk density of the planet suggests that it must have a gaseous envelope, the two leading theories for explaining the flat transmission spectrum involve either widespread, high-altitude clouds or a hydrogen-poor atmosphere dominated by a high-molecular-weight constituent such as water or carbon dioxide (Fig. 1). In both cases, the stellar light at most wavelengths would be extinguished fairly abruptly within a small vertical region of the atmosphere.
In their study, Kreidberg and colleagues present near-infrared transmission spectra for GJ 1214b that finally allow one of the two competing theories to be ruled out for this super-Earth exoplanet. The extremely precise spectra, obtained from the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope, demonstrate that GJ 1214b's transmission spectrum is so flat and featureless in a wavelength region between about 1.1 and 1.6 micrometres that high-altitude clouds provide the only plausible explanation. The observations are precise enough that spectral features from a cloud-free atmosphere dominated by heavy molecules such as water, methane, carbon monoxide or carbon dioxide would have been detectable if such an atmosphere were present on GJ 1214b. Even an atmosphere composed of 99.9% spectrally neutral nitrogen with 0.1% water can be rejected on the basis of the lack of water-absorption features.
Meanwhile, new WFC3 observations of GJ 436b presented by Knutson and colleagues point to a similarly flat and featureless transmission spectrum between 1.1 and 1.6 μm for this Neptune-class planet. Given that one might expect the more massive GJ 436b to contain more hydrogen than GJ 1214b, the flat spectrum is, in this case, an even bigger surprise — a hydrogen-rich atmosphere would be vertically extensive, and expected trace species such as water and methane would have prominent deep absorption bands. However, unlike the situation for GJ 1214b, Knutson et al. demonstrate that a hydrogen-poor atmosphere (with or without clouds) and a hydrogen-rich atmosphere with high clouds are both statistically viable solutions to explain the observed flat transmission spectrum for GJ 436b. To distinguish between these scenarios, more precise moderate-resolution spectral observations at near-infrared wavelengths will be needed to unambiguously reveal any spectral features. Longer-wavelength eclipse observations8, acquired when the planet passes behind the star, could also help to discriminate between the two hypotheses.
Evidence is mounting that the hydrogen fraction within a planet is a strong function of planet size9, so it is not necessarily an 'either-or' situation for explaining the flat transmission spectra of GJ 436b and GJ 1214b: the atmospheres could be cloudy and have a large mean molecular weight. However, high-altitude clouds on these two exoplanets would not resemble the clouds we see in the Solar System. Possible candidates include potassium chloride or zinc sulphide 'dust' clouds. For the case of a relatively hydrogen-poor atmosphere, these two components would form clouds that are optically thick (opaque) enough at high altitudes on both planets that the transmitted stellar light would be abruptly blocked, leading to a flat transmission spectrum10. Alternatively, thick hazes such as those seen around Saturn's moon Titan could be produced from photochemical processing of atmospheric gases by ultraviolet stellar photons, although the lack of evidence for methane on either of these two planets2,8 suggests that any photochemical hazes present would be decidedly different from those on Titan.
Hydrogen-poor or not, dust-shrouded or not, super-Earth and Neptune-class planets collectively represent an intriguing and populous type of extrasolar planet whose exotic atmospheres may have no true analogues in the Solar System. The transmission spectra presented here — flat and featureless, and yet full of information — provide one piece of the puzzle needed to characterize such planets.
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