Water vapour detected in Jupiter-like exo-planet's atmosphere

Researchers at Caltech and several other institutions have made the first detection of water in the atmosphere of the Jupiter-mass planet orbiting the nearby star Tau Bootis.

With further development and more sensitive instruments, a new technique to analyze the gaseous atmospheres of such extrasolar planets, could help researchers learn about how many planets with water-like Earth-exist within our galaxy.

Scientists have previously detected water vapor on a handful of other planets, but these detections could only take place under very specific circumstances, graduate student Alexandra Lockwood, the first author of the study, said.

"When a planet transits-or passes in orbit in front of its host star-we can use information from this event to detect water vapor and other atmospheric compounds," she said.

"Alternatively, if the planet is sufficiently far away from its host star, we can also learn about a planet's atmosphere by imaging it," she added.

However, significant portions of the population of extrasolar planets do not fit either of these criteria, and there was not really a way to find information about the atmospheres of these planets.

Looking to resolve this problem, Lockwood and her adviser Geoffrey Blake, professor of cosmochemistry and planetary sciences and professor of chemistry, applied a novel technique for finding water in a planetary atmosphere. Other researchers had used similar approaches previously to detect carbon monoxide in Tau Bootis b.

The method utilized the radial velocity (RV) technique-a technique commonly used in the visible region of the spectrum to which our eyes are sensitive-for discovering non-transiting exoplanets.

Using the Doppler effect, RV detection traditionally determines the motion of a star due to the gravitational pull of a companion planet; the star moves opposite that of the orbital motion of the planet, and the stellar features shift in wavelength. A large planet or a planet closer to its host star provides a larger shift.

Lockwood, Blake, and their colleagues expanded the RV technique into the infrared to determine the orbit of Tau Bootis b around its star, and added further analysis of the light shifts via spectroscopy-an analysis of the light's spectrum.

Since every compound emits a different wavelength of light, this unique light signature allows the researchers to analyze molecules that make up the planet's atmosphere.

Using data of Tau Bootis b from the Near Infrared Echelle Spectrograph (NIRSPEC) at the W. M. Keck Observatory in Hawaii, the researchers were able to compare the molecular signature of water to the light spectrum emitted by the planet, confirming that the atmosphere did indeed include water vapor.

The findings are published online in The Astrophysical Journal Letters.