Two teams of scientists using the Hubble Space Telescope to study the atmospheres of planets orbiting distant stars have found water on five of them, proving that our Solar System is not the only place where an abundance of the life-giving liquid can be found. The exoplanets in question are WASP-17b, HD209458b, WASP-12b, WASP-19b, and XO-1b, and are all “Hot Jupiters”, meaning that they are very large gas giants orbiting extremely close to their parent stars. The first two planets on that list showed the strongest water signature, with the remaining three showing more moderate amounts. “We’re very confident that we see a water signature for multiple planets,” said Avi Mandell, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and lead author of an Astrophysical Journal paper, published today, describing the findings for WASP-12b, WASP-17b and WASP-19b. “This work really opens the door for comparing how much water is present in atmospheres on different kinds of exoplanets, for example hotter versus cooler ones.”
The study relied on a spectroscopic technique in which the colours making up the light of a star are compared with the colours while the planet is transiting across its surface. During the transit, some of the star’s light shines through the planet’s atmosphere, with the molecules of different gasses selectively filtering out specific wavelengths of light. Water vapour absorbs infrared light, so by comparing spectra and noting how far the water-specific colours in the infrared spectrum are reduced, astronomers can calculate how much water was in the air.
What surprised the astronomers most of all, however, was not the presence of water — they anticipated that — but how much less was found than expected. “These studies, combined with other Hubble observations, are showing us that there are a surprisingly large number of systems for which the signal of water is either attenuated or completely absent,” said Heather Knutson of the California Institute of Technology. “This suggests that cloudy or hazy atmospheres may in fact be rather common for hot Jupiters.” Clouds or smoggy haze dim the light passing through an exoplanet’s atmosphere, in the same way that fog or mist dims light on earth and seems to wash out all colours. If these worlds’ atmospheres are indeed hazy, that would explain the attenuated signal picked up by Hubble.
A transit is similar to an eclipse, and occurs whenever an object passes directly between a star and an observer. If the star is nearby, like the Sun, then we can watch the silhouette of the object as it moves across the disk. But stars are so distant that we cannot see a disc, so all we can detect is the light from the star dipping slightly for the duration of the transit. The careful observation of stars, looking for these dips in brightness, has been used since the 1990’s to detect explanets, which are almost impossible to image directly and usually need to be found with these indirect methods. It is only recently that technology has advanced to the point where spectroscopic study of the atmosphers of alien worlds has become possible through the transit method.
The work leading to this discovery can be read in two papers: The first, looking at planets WASP-12 b, WASP-17 b, andWASP-19 b (lead author Avi Mandell), was published in the Astrophysical Journal in December. The second looks at planets HD 209458b and XO-1b and is by a rather larger team, led by L. Drake Deming of the University of Maryland in College Park, and was also published in the Astrophysical Journal, in September last year.