Alien Exoplanet: Windy Fingerprint with Water

A NASA Spitzer Space Telescope press release announces that observations with NASA's Spitzer Space Telescope's infrared array cameras show water vapor in the atmosphere of a hot gaseous planet beyond our solar system, which was previously the first of 200 such exoplanets to have its weather analyzed and mapped.

Spitzer is in a orbit that follows the Earth around the Sun and slowly falls away from Earth's gravity a little more with each passing year. This way Spitzer drifts further into space and provides "eyes" to see outside our solar system.

As part of NASA's Astronomical Search for Origins Program to track our cosmic roots and how galaxies, stars ans planet develop and form, Spitzer is participating in a brand new field of science that investigates the climate on exoplanets, that is planets outside our solar system. NASA's Hubble Space Telescope is also participating in this new science by capturing data in visible light.

The planet for which atmospheric water has been found is exoplanet HD 189733b. This is the same hot, gaseous exoplanet--called a hot Jupiter--in NASA news on May 9 of this year. At that time, Spitzer's infrared images were used to map temperature variations over HD 189733b's entire surface.

To do this Spitzer captured images of the hot Jupiter, the nearest one to Earth, as it transited in front of and then behind its parent planet. By using these primary eclipses (in front of star) and secondary eclipses (behind star) analysis could be made of both faces of HD 189733b: the face fixed toward the star and the face fixed facing away from it. It is believed that hot Jupiters, with their rapid near orbits, are tidally locked to their suns in much the same way that our moon is tidally locked to Earth. Spitzer's infrared measurements, comprised of approximately a million data points, were assembled into pole-to-pole strips facilitating weather mapping of both faces of the entire planet.

This initial study of HD 189733b led Heather Knutson of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of the paper describing HD 189733b and co-author David Charbonneau, also of the Harvard-Smithsonian Center for Astrophysics to determine that the exoplanet is very likely whipped by winds roaring over the blazing surface at 6,000 miles per hour (9600 kilometers per hour). They believe winds to be spreading heat from its sun-facing--and therefore permanently lit--side to its dark side.

Part of the quest of Spitzer is to find water on other planets, this is part of its mission design. In initial infrared spectrometer studies of secondary eclipses of HD 189733b no indication of water was found: The spectrum, or "fingerprint," of the hot Jupiter's light came out as "dry."

At the same time, analysis of exoplanetary light via Spitzer's spectrometer was also released for exoplanet HD 209458b. Then, later, another team using NASA's Hubble Telescope visible-light images found some as yet undefined "hints" of water in HD 209458b's data.

Next, Giovanna Tinetti, a European Space Agency fellow at the Institute d'Astrophysique de Paris in France, and her team gathered data again from Spitzer in infrared light but this time while watching HD 189733b in primary orbit, which is when the planet eclipses its sun as seen from Earth's/Spitzer's perspective.

The technique Tinetti and her team employed was to view HD 189733b's parent star's light through Spitzer's infrared array camera in three infrared wavelengths as the starlight shone through the planet's atmosphere. Each wavelength reveals different characteristics of the atmosphere. Tinetti's team noticed that "for each wavelength a different amount of light was absorbed by the planet."

This variation in wavelength absorption produces a pattern. Each molecule has its own identifiable pattern. Therefore, the molecules producing the wavelength pattern in a previously unknown atmosphere can be identified by matching the pattern it produces with existing patterns of known molecules.

The pattern of HD 189733b's wavelength absorption variations matches the pattern created by water. Hence, water in the atmosphere of HD 189733b is identified. Tinetti said: "Water is the only molecule that can explain that [pattern] behavior. Observing primary eclipses in infrared light is the best way to search for this [water] molecule in exoplanets." The previous Spitzer analysis of HD 189733b coupled with other ground and space-based telescopes suggest that it might have dry clouds in its atmosphere, even though the water on HD 189733b is too hot to condense into wet clouds.

One point of greater significance pertaining to Tinetti's research is that, ultimately, astronomers "hope to use instruments like those on Spitzer to find water on rocky, habitable planets like Earth. As co-author of the Tinetti report, Sean Carey of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, said: "Finding water on this planet implies that other planets in the universe, possibly even rocky ones, could also have water."

HD 189733b flies around its star every 2.2 days in a near orbit. This hot Jupiter is in a subset of hot Jupiters that are closest to Earth and whose transits past their suns are visible from Earth making them readily available for study. Astronomers had predicted that hot Jupiters would contain water vapor in the atmospheres, but finding convincing data has been slippery. These new Spitzer data of the steaming exoplanet HD 189733b are most convincing in indicating that hot Jupiters are, in fact, "wet." Tinetti said, "We're thrilled to have identified clear signs of water on a planet that is trillions of miles away."

Note: The report on HD 189733b authored by Giovanna Tinetti and Sean Carey appears in the journal Nature. Other authors of the Nature paper include Alfred Vidal-Madjar, Jean-Phillippe Beaulieu, David Sing and Nicole Allard of the Institute d'Astrophysique de Paris: Mao-Chang Liang of Caltech and the Academia Sinica, Taiwan; Yuk Yung of Caltech; Robert J. Barber and Jonathan Tennyson of University College London in England; Ignasi Ribas of the Institut de Ciències de l'Espai, Spain; Gilda E. Ballester of the University of Arizona, Tucson; and Franck Selsis of the Ecole Normale Supérieure, France. JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, Pasadena. JPL is a division of Caltech. Spitzer's infrared array camera was built by NASA's Goddard Space Flight Center, Greenbelt, Md. The instrument's principal investigator is Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Newsroom, "NASA's Spitzer Finds Water Vapor on Hot, Alien Planet." Spitzer Space Telescope. URL: http://www.spitzer.caltech.edu/Media/releases/ssc2007-12/release.shtml

Newsroom, "NASA Finds Extremely Hot Planet, Makes First Exoplanet Weather Map.: NASA Spitzer Space Telescope. URL: http://www.spitzer.caltech.edu/Media/releases/ssc2007-09/release.shtml