Water in the atmosphere of WASP-39b

WASP-39b is a Saturn-mass exoplanet orbiting a sun-like star roughly 700 light years away in the direction of the constellation of Virgo. Although WASP-39b is the mass of Saturn, it is bigger than Jupiter, meaning it has a puffy extended atmosphere making it perfect for follow-up studies of its atmosphere. We used the Hubble Space Telescope to measure the light filtered through the puffy atmosphere of WASP-39b to measure the amount of water vapour that it contains. We want to measure the amount of water vapour as this is a good measure of the amount of the planets heavy elements under certain conditions, which may help us understand its formation.

Each molecule has a unique spectral fingerprint, and water’s fingerprint starts in hot giant planet spectra at around 0.9 microns. We used two different stepped-prisms in the Wide Field Camera 3 instrument on Hubble to look for these fingerprints at 0.95, 1.2 and 1.4 microns (see featured image). What is exciting about these measurements of WASP-39b’s atmosphere is that we actually found them.

One thing that could have stopped us from finding the water would have been the presence of thick clouds in the atmosphere that would block all the light from shining through. But without the presence of clouds, or at least only small amounts of thin clouds, we were able to measure a nice precise water absorption spectrum.

Here comes the critical bit. We then combined this with previous measurements of the same planet’s atmosphere from the optical, to the infrared. All of this data makes WASP-39b’s atmosphere one of the best studied exoplanet atmosphere we have.

Using all of this data together we were able to retrieve and constrain the amount of water vapor there is in the atmosphere of this giant planet. What we found was a little surprising. We expected to measure that it had a smilier amount of heavy elements as Saturn, as in our solar system the mass of the planet seems to dictate how much heavy elements there are in the atmosphere, but we found that WASP-39b contains much more. In fact WASP-39b has a heavy element abundance between two and four times that of Saturn, which is from 100 to 200 times that of the Suns heavy element abundance.

The large amount of heavy elements in the atmosphere could suggest that WASP-39b formed slightly differently and picked up lots of heavy material while it was forming. Future observations with the James Webb Space Telescope which is set to launch in spring 2019 will be able to tell us a lot more about other molecules in WASP-39b’s atmosphere, like carbon dioxide indicated in the feature image. But these observations currently stand alone in the Saturn-mass regime of exoplanet with well measured water abundances.

The exoplanet data currently cannot be adequately explained by a linear fit to the data.
This shows the mass of gas giant planets versus the heavy element abundance for their atmospheres. The Solar System data are based on measurements of methane (CH4), while the exoplanets are based on the measurement of water (H2O).


You can find the full scientific article on The Complete Transmission Spectrum of WASP-39b with a Precise Water Abundance on the open source arXiv


All data in the figures will soon be available on the Scientific Data page. Previously published data and models can be found by following the Transmission Spectral Library tab.

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