NASA | ESA | STScI | HubbleSite | 2017 Jun 05
[img3="This diagram compares Hubble Space Telescope observations of two "hot Jupiter"-class planets orbiting very closely to different sunlike stars. Astronomers measured how light from each parent star is filtered through each planet's atmosphere. Hubble near-infrared observations were used to measure the spectral fingerprint caused by the presence of water vapor in the atmosphere. HAT-P-38 b did have a water signature indicated by the absorption-feature peak in the spectrum. This is interpreted as indicating the upper atmosphere is free of clouds or hazes. By contrast, a very similar hot Jupiter, WASP-67 b, has a flat spectrum that lacks any water-absorption feature. This suggests that most of the planet's atmosphere is masked by high-altitude clouds.Atmospheres of Two Hot Jupiters: Cloudy and Clear
Illustration Credit: NASA, ESA, and Z. Levy (STScI)
Science Credit: NASA, ESA, and G. Bruno (STScI)"]https://cdn.spacetelescope.org/archives ... o1722b.jpg[/img3][hr][/hr]
Astronomers once thought that the family of planets that orbit our sun were typical of what would eventually be found around other stars: a grouping of small rocky planets like Earth huddled close to their parent star, and an outer family of monstrous gaseous planets like Jupiter and Saturn.
But ever since the discovery of the first planet around another star (or exoplanet) the universe looks a bit more complicated — if not downright capricious. There is an entire class of exoplanets called "hot Jupiters." They formed like Jupiter did, in the frigid outer reaches of their planetary system, but then changed Zip code! They migrated inward to be so close to their star that temperatures are well over 1,000 degrees Fahrenheit.
Astronomers would like to understand the weather on these hot Jupiters and must tease out atmospheric conditions by analyzing how starlight filters through a planet's atmosphere. If the spectral fingerprint of water can be found, then astronomers conclude the planet must have relatively clear skies that lets them see deep into the atmosphere. If the spectrum doesn't have any such telltale fingerprints, then the planet is bland-looking with a high cloud deck.
Knowing the atmospheres on these distant worlds yields clues to how they formed and evolved around their parent star. In a unique experiment, astronomers aimed the Hubble Space Telescope at two "cousin" hot Jupiters that are similar in several respects. However, the researchers were surprised to learn that one planet is very cloudy, and the other has clear skies.
