Extrasolar planets

Window on a watery world

The first definitive signs of water have been seen in the atmosphere of a Neptune-sized exoplanet, paving the way towards the search for water on smaller Earth-like planets. See Letter p.526

The molecules that make up the atmospheres of Neptune-sized exoplanets have remained elusive. Despite a protracted search for the atmospheric constituents of such planets using impressive instrumentation on world-class telescopes, observations have revealed only thick layers of clouds, haze or dust, which block astronomers' prying eyes and prevent a view of the world below1,2,3,4. On page 526 of this issue, Fraine et al.5 now report observations of the Neptune-sized planet HAT-P-11b using the Hubble and Spitzer space telescopes that reveal a world without thick clouds and provide a clear confirmation that water vapour, along with copious amounts of hydrogen, is present in the atmosphere of this planet.

It should not be surprising that clouds are prevalent in the atmospheres of many exoplanets. In our own Solar System, clouds are practically ubiquitous. From Earth's puffy water-vapour clouds, to the hydrocarbon haze of Saturn's largest moon, to Jupiter's stratified clouds of ammonia compounds and water, clouds dominate the skies of most large Solar System bodies. These clouds, although interesting in their own right, are not always an astronomer's friend. It is easy to see why. A layer of clouds acts as a thick blanket, obscuring the deeper regions of a planet's atmosphere (and the planetary surface, if one exists). Despite being our neighbouring planet, almost nothing was known of Venus's surface until the 1970s for this very reason. The planet is enshrouded in clouds of noxious sulphuric acid, which prevented a view of its surface until the Soviet Venera landers were finally able to physically penetrate the clouds and transmit pictures back to Earth6.

The same problem exists for the study of exoplanets, but sending a lander to these distant worlds is not an option. To determine the gaseous composition of an exoplanet atmosphere, astronomers rely on spectroscopic studies and the fact that different gases absorb light at specific wavelengths. If clouds obscure the deeper regions of a planet's atmosphere, then no absorption will be seen, and the composition of the atmosphere cannot be readily determined (Fig. 1). Already, four exoplanets of Neptune size or smaller, observed when passing in front of their host stars, have shown no absorption features1,2,3,4, and clouds are a likely culprit. But when Fraine et al. observed a fifth such planet, HAT-P-11b, they hit on something markedly different. The observations of HAT-P-11b reveal the crystal-clear signature of water-vapour absorption. From the strength of the absorption, the authors conclude that the planet's atmosphere has a composition not dissimilar to those of the giant planets of our Solar System — mostly hydrogen, with trace amounts of heavier atoms, including oxygen in the form of water vapour.

Figure 1: Concealing clouds.
figure1

a, An exoplanetary atmosphere with clouds blocks the transmission of starlight, producing a flat transmission spectrum with no features. b, A clear atmosphere with no clouds allows starlight to penetrate deeper into the atmosphere, where molecules such as water absorb light. The resulting transmission spectrum has absorption spectral features, which enable astronomers to infer the molecular composition of the atmosphere. This was the case for Fraine and colleagues' observations5 of exoplanet HAT-P-11b, which has water-absorption features.

The instrument that Fraine and colleagues used to unambiguously detect water vapour in the atmosphere of HAT-P-11b — the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope — was commissioned in 2009 during the final servicing of the telescope. The device produced stunningly high-precision measurements and is providing astronomers with a glimpse of what data might look like from next-generation space observatories. Searching for water-vapour absorption in the atmosphere of an exoplanet passing in front if its host star is akin to looking for a tiny insect passing in front of a bright coastal lighthouse lamp. To detect the minute signatures of molecular absorption, the WFC3 provides exceptional instrumental stability so as not to introduce spurious noise that could mask the signs of water vapour in an exoplanet's atmosphere. In 2018, NASA is due to launch the James Webb Space Telescope, which will carry instruments with sensitivity similar to that of the WFC3. The advantage of this new observatory is that the telescope itself will have a larger mirror than Hubble's, allowing it to detect even fainter signals than those detected by Fraine and colleagues. As a result, the telescope will be able to observe molecular absorption in the atmospheres of planets smaller than Neptune, but only if these planets do not have clouds.

The successful detection of water vapour in HAT-P-11b's atmosphere allows exoplanet researchers to breathe a sigh of relief. The finding that some of the smaller exoplanets do not have thick clouds bodes well for observations with the James Webb Space Telescope. However, there is still more work to be done in identifying suitable targets for these future observations. It has long been expected that water vapour should be present in the atmospheres of planets such as HAT-P-11b — those of Neptune size whose orbits carry them close to their host stars. But after multiple failed attempts to observe molecular absorption in the atmospheres of other similar planets, the surprise of Fraine and colleagues' results is that the atmosphere of HAT-P-11b is sufficiently devoid of clouds to allow the detection of water vapour.

Now that this planet has been shown to be free of clouds, despite their prevalence in the atmospheres of other similar planets, some questions arise. What fraction of Neptune-sized exoplanets has clouds? For the planets that do have clouds, what specific physical processes are responsible for their formation? Understanding the answers to these questions will allow astronomers to better identify planets unmarred by clouds. By first pinpointing and studying those planets that provide a clear window into their atmospheres, researchers will ultimately be able to extend the search for water and other molecules to smaller planets — perhaps even Earth-sized planets — with the James Webb Space Telescope and beyond.

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Correspondence to Eliza M. R. Kempton.

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Kempton, E. Window on a watery world. Nature 513, 493–494 (2014). https://doi.org/10.1038/513493a

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