Artist’s impression of the ten hot Jupiter exoplanets studied by David Sing and his colleagues. From top left to to lower left these planets are WASP-12b, WASP-6b, WASP-31b, WASP-39b, HD 189733b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458b. Credit: ESA/Hubble & NASA
Astronomers have used Hubble and Spitzer Space Telescope to study the atmospheres of 10 hot, Jupiter-sized exoplanets in detail, the largest number of such planets ever studied. The team was able to discover why some of these worlds seem to have less water than expected—a long-standing mystery.
To date, astronomers have discovered nearly 2000 planets orbiting other stars. Some are known as hot Jupiters, hot, gaseous planets with characteristics similar to those of Jupiter. They orbit very close to their stars, making their surface hot, and the planets tricky to study in detail without being overwhelmed by bright starlight. Due to this difficulty, Hubble has only explored a handful of hot Jupiters in the past, across a limited wavelength range. These initial studies have found several planets to hold less water than expected.
Using the power of both telescopes allowed them to study the planets, of various masses, sizes, and temperatures, across an unprecedented range of wavelengths. Says David Sing of University of Exeter: “We found the planetary atmospheres to be much more diverse than we expected.”
All of the planets have a favourable orbit that brings them between their parent star and Earth. As the exoplanet passes in front of its host star, as seen from Earth, some of this starlight travels through the planet’s outer atmosphere. “The atmosphere leaves its unique fingerprint on the starlight, which we can study when the light reaches us,” explains Hannah Wakeford, NASA Goddard Space Flight Center.
These fingerprints allowed the team to extract the signatures from various elements and molecules—including water—and to distinguish between cloudy and cloud-free exoplanets, a property that could explain the missing water mystery. Models revealed that, while cloud-free exoplanets showed strong signs of water, the atmospheres of those hot Jupiters with faint water signals also contained clouds and haze, both known to hide water from view. Mystery solved!
“The alternative to this is that planets form in an environment deprived of water—but this would require us to completely rethink our current theories of how planets are born,” explained Jonathan Fortney of the University of California, Santa Cruz, “Our results have ruled out the dry scenario”. Hubble’s successor, the James Webb Space Telescope , will open a new infrared window on the study of exoplanets and http://www.spacetelescope.org/news/heic1524/
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