Hot, Jupiter-sized extrasolar planets often have cloud or haze layers in their atmospheres, and this may prevent astronomers from detecting atmospheric water, says a team of astronomers led by NASA’s Jet Propulsion Laboratory (JPL).
Hot-Jupiter with hidden water. Image credit: NASA / JPL-Caltech.
Scientists have found many hot-Jupiters — massive gaseous planets that orbit very close to their host stars — with water in their atmospheres, but others appear to have none.
Aishwarya Iyera, a JPL intern and a student at California State University, and co-authors wanted to find out what the atmospheres of these giant worlds have in common.
The team looked at a collection of 19 hot-Jupiters previously observed by the NASA/ESA Hubble Space Telescope.
Hubble’s Wide Field Camera 3 had detected water vapor in the atmospheres of 10 of these planets, and no water on the other nine.
But that information was spread across more than a dozen studies. The methods of analyzing and interpretation varied because the studies were conducted separately. There had not been one overarching analysis of all these planets.
To compare the hot-Jupiters and look for patterns, Iyera and co-authors had to standardize the data.
The scientists combined the datasets for all 19 hot-Jupiters to create an average overall light spectrum for the group of planets.
They then compared these data to models of clear, cloud-free atmospheres and those with various cloud thicknesses.
They determined that, for almost every planet they studied, haze or clouds were blocking half of the atmosphere, on average.
The findings, published in the Astrophysical Journal (arXiv.org preprint), suggest that clouds or haze layers could be preventing a substantial amount of atmospheric water from being detected by space telescopes.
The clouds themselves are likely not made of water, as the planets in this sample are too hot for water-based clouds.
“Clouds or haze seem to be on almost every planet we studied. You have to be careful to take clouds or haze into account, or else you could underestimate the amount of water in an exoplanet’s atmosphere by a factor of two,” Iyer concluded.
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Aishwarya R. Iyer et al. 2016. A Characteristic Transmission Spectrum dominated by H2O applies to the majority of HST/WFC3 exoplanet observations. ApJ 823, 109; doi: 10.3847/0004-637X/823/2/109
