Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1–1.7 micrometres). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted1,2,3,4,5. The low amplitude of water signatures could be explained by very low water abundances6,7,8, which may be a sign that water was depleted in the protoplanetary disk at the planet’s formation location9, but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes1,2,3,4, as found in some optical spectra3,4,10,11. Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3–5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures.
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This work is based on observations with the NASA/ESA HST, obtained at the Space Telescope Science Institute (STScI) operated by AURA, Inc. This work is also based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number 336792. D.K.S., F.P. and N.N. acknowledge support from STFC consolidated grant ST/J0016/1. Support for this work was provided by NASA through grants under the HST-GO-12473 programme from the STScI. A.L.E., P.A.W. and A.V.M. acknowledge support from CNES and the French Agence Nationale de la Recherche (ANR), under programme ANR-12-BS05-0012 ‘Exo-Atmos’. P.A.W. and H.W. acknowledge support from the UK Science and Technology Facilities Council (STFC). G.W.H. and M.H.W. acknowledge support from NASA, NSF, Tennessee State University and the State of Tennessee through its Centers of Excellence programme.
The authors declare no competing financial interests.
Extended data figures and tables
Black points show the altitude difference between the near-infrared (near-IR) and the mid-infrared (mid-IR) spectral features (Δ ZJ − LM) versus the difference between the blue-optical and mid-infrared (Δ ZUB − LM, see Table 1). Error bars represent the 1σ measurement uncertainties. Purple and grey lines show model trends for hazy and cloud atmospheres, respectively, with increasing Rayleigh scattering haze and grey cloud deck opacity corresponding to 10×, 100× and 1,000× solar. We also show clear-atmosphere models with sub-solar abundances of 0.1×, 0.01× and 0.001× solar (red line).
Black points show the altitude difference between the blue-optical and mid-infrared (mid-IR) spectral features (ΔZUB − LM) versus the amplitude of the 1.4-μm H2O absorption spectral feature (see Table 1). Error bars represent the 1σ measurement uncertainties. Purple and grey lines show model trends for hazy and cloud atmospheres, respectively, with increasing Rayleigh scattering haze and grey cloud deck opacity corresponding to 10× , 100× and 1,000× solar. We also show clear-atmosphere models with sub-solar abundances of 0.1× , 0.01× and 0.001× solar (red line).
Exoplanets with strong haze signatures have prominent optical slopes with ΔZUB − LM values above 3, while clear atmospheres have ΔZUB − LM indices near zero. The datapoint colours correspond to those in Fig. 1. The red solid line shows the linear regression between the two indices, with 1σ uncertainties (red dashed lines).
Model spectra17,39 assume a 1,200-K hot Jupiter with a surface gravity of 25 m s−2. Spectra in each panel are compared to a clear, solar-metallicity atmosphere (black line). a, Purple spectra have an added Rayleigh scattering haze corresponding to metallicities of 100× and 1,000× solar. b, Blue and grey spectra have an added grey cloud deck corresponding to 1× and 10× solar. c, Red and green spectra show clear atmospheres with sub-solar abundances of 0.01× and 0.001× solar.
Extended Data Figure 5 Brown dwarf and hot Jupiter pressure–temperature profiles and condensation curves.
Similar to Fig. 2, but alongside the ten hot-Jupiter pressure–temperature profiles we plot the profile of an 1,800-K brown dwarf (green line). The thicker portions of the lines indicate the pressures probed in transmission for the hot Jupiters (plotted in greyscale) and the visible photosphere for the brown dwarf (0.1–10 bar). While a shift in the pressure–temperature profile of a hot Jupiter to hotter and cooler temperatures could dramatically change which condensates may be found in the visible atmosphere, the same would not be true for much shallower brown dwarf pressure–temperature profiles.
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Sing, D., Fortney, J., Nikolov, N. et al. A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion. Nature 529, 59–62 (2016). https://doi.org/10.1038/nature16068
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