Free Astronomy Magazine January-February 2016

SPACE CHRONICLES ing ten such planets in a bid to understand their atmo- spheres. Only three of these planetary atmospheres had previously been studied in de- tail; this new sample forms the largest ever spectroscopic catalogue of exoplanet atmo- spheres. The team used multi- ple observations from both the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope. Using the power of both telescopes allowed the team to study the planets, which are of various masses, sizes, and temperatures, across an unpre- T his video shows an art- ist’s impression of the ten hot Jupiter exoplan- ets 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. [ESA/Hubble & NASA] cedented range of wavelengths (the observations spanned from the ul- traviolet (0.3 μ m) to the mid-infra- red (4.5 μ m). “I’m really excited to finally ‘see’ this wide group of planets together, as this is the first time we’ve had sufficient wave- length coverage to be able to compare multiple features from one planet to another,” says David Sing of the Univer- sity of Exeter, UK, lead author of the new study. “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 exopla- net passes in front of its host star, as seen from Earth, some of this star- light travels through the planet’s outer atmosphere. “The atmosphere leaves its unique finger- print on the star- light, which we can T his image shows an artist’s im- pression of the ten hot Jupiter exoplanets studied by David Sing and his colleagues. The images are to scale with each other. HAT-P- 12b, the smallest of them, is ap- proximately the size of Jupiter, while WASP-17b, the largest plan- et in the sample, is almost twice the size. The planets are also de- picted with a variety of different cloud properties. There is almost no information about the colours of the planets available, with the exception of HD 189733b, which became known as the blue planet. The hottest planets within the sample are portrayed with a glowing night side. This effect is strongest on WASP-12b, the hot- test exoplanet in the sample, but also visible on WASP-19b and WASP-17b. It is also known that several of the planets exhibit strong Rayleigh scattering. This effect causes the blue hue of the daytime sky and the reddening of the Sun at sunset on Earth. It is also visible as a blue edge on the planets WASP-6b, HD 189733b, HAT-P-12b, and HD 209458b. The wind patterns shown on these ten planets, which resemble the visible structures on Jupiter, are based on theoretical models. [ESA/Hubble & NASA] study when the light reaches us,” ex- plains co-author Hannah Wakeford, now at NASA Goddard Space Flight Center, USA. These fingerprints allowed the team to extract the signatures from var- ious elements and molecules — in- cluding water — and to distinguish between cloudy and cloud-free exo- planets, a property that could ex- plain the missing water mystery. The team’s models revealed that, while apparently cloud-free exo- planets showed strong signs of wa- ter, the atmospheres of those hot Ju- piters with faint water signals also contained clouds and haze — both of which are known to hide water from view. Mystery solved! “The alternative to this is that plan- ets form in an environment deprived of water — but this would require us to completely rethink our current theories of how planets are born,” explained co-author Jonathan Fort- ney of the University of California, Santa Cruz, USA. “Our results have ruled out the dry scenario, and strongly suggest that it’s simply clouds hiding the water from prying eyes.” The study of exoplanetary atmo- spheres is currently in its infancy, with only a handful of observations taken so far. Hubble’s successor, the James Webb Space Telescope, will open a new infrared window on the study of exoplanets and their atmospheres. n