Astronomers using NASA/ESA Hubble Space Telescope have detected water vapor in the atmosphere of a Saturn-mass exoplanet known as WASP-39b. By combining new Hubble data with older data from other telescopes in space and on the ground, they have produced the most complete spectrum of an exoplanet’s atmosphere possible with present-day technology.
This artist’s concept illustrates the hot-Saturn exoplanet WASP-39b. Image credit: NASA / ESA / G. Bacon, STScI.
The hot-Saturn exoplanet WASP-39b is a highly inflated transiting gas giant with a radius of about 1.27 Jupiter radii and a mass of just 0.28 Jupiter masses.
It orbits WASP-39 — a late G-type dwarf star located about 700 light-years from Earth in the constellation Virgo — once every 4 days.
Although it is called a ‘hot Saturn,’ WASP-39b is not known to have rings. Instead, it has a puffy atmosphere that is free of high-altitude clouds, allowing astronomers to peer down into its depths.
“We need to look outward to help us understand our own Solar System,” said Dr. Hannah Wakeford, from the University of Exeter and the Space Telescope Science Institute.
“But exoplanets are showing us that planet formation is more complicated and more confusing than we thought it was. And that’s fantastic!”
Dr. Wakeford and colleagues combined the capabilities of Hubble with those of other ground- and space-based telescopes for a detailed study of WASP-39b.
“By dissecting starlight filtering through the planet’s atmosphere, we found clear evidence for atmospheric water vapor,” they said.
“In fact, WASP-39b has three times as much water as Saturn does.”
Comprehensive spectrum of the hot-Saturn exoplanet WASP-39b. Image credit: NASA / ESA / G. Bacon & A. Feild, STScI / H. Wakeford, STScI & Unviersity of Exeter.
Although the astronomers had predicted they would see water vapor, they were surprised by the amount that they found.
This surprise allowed to infer the presence of large amount of heavier elements in the atmosphere.
This in turn suggests that WASP-39 was bombarded by a lot of icy material which gathered in its atmosphere. This kind of bombardment would only be possible if this planet formed much further away from its host star than it is right now.
“The analysis of the atmospheric composition and the current position of the planet indicate that WASP-39b most likely underwent an interesting inward migration, making an epic journey across its planetary system,” Dr. Wakeford and co-authors said.
Having made its incredible inward journey WASP-39b is now 8 times closer to its parent star than Mercury is to the Sun. The planet is also tidally locked, meaning it always shows the same side to its star.
The team measured the temperature of WASP-39b to be a scorching 1,430 degrees Fahrenheit (776.7 degrees Celsius).
Although only one side of the planet faces its parent star, powerful winds transport heat from the bright side around the planet, keeping the dark side almost as hot.
“Hopefully this diversity we see in exoplanets will help us figure out all the different ways a planet can form and evolve,” said co-author Dr. David Sing, from the University of Exeter.
The findings are published in the Astronomical Journal (arXiv.org preprint).
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H.R. Wakeford et al. 2018. The Complete Transmission Spectrum of WASP-39b with a Precise Water Constraint. AJ 155, 29; doi: 10.3847/1538-3881/aa9e4e
