Keck Observatory's NIRSPEC: Water vapour detected in the atmosphere of a hot Jupiter

Simulated data showing the method used for detecting water vapor features detected around the hot Jupiter tau Boo b. 

In this example, the planetary signal has been increased in strength relative by several orders of magnitude relative to the actual signal. 

The dotted lines show the blue- and red-shifts of the planetary and stellar lines in the data, respectively, due to the orbital motion of two bodies in the system. 

Credit: Alexandra Lockwood (CalTech), Background Image David Aguilar (CFA).

California Institute of Technology (Caltech) astronomers using data gathered at the W. M. Keck Observatory have developed a new technique for planetary scientists that could provide insight into how many water planets like Earth exist within our universe.

The results have been published on February 24th by Astrophysical Letters.

Alexandra Lockwood

Scientists have detected water vapour on other planets in the past, but these detections could only take place under very specific circumstances, according to graduate student Alexandra Lockwood, the first author of the study.

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

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

However, a significant portion of the population of extrasolar planets does not fit either of these criteria and there wasn't really a way to find information about the atmospheres of these planets.

Geoffrey Blake

Looking to resolve this problem, Lockwood and her advisor Geoffrey Blake, Caltech professor of cosmochemistry, planetary sciences and chemistry, were inspired by the recent detection of carbon monoxide in the extrasolar planet, 'Tau Boo b' and they wondered if they could detect water in a similar manner.

The method used to detect carbon monoxide 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 stellar features shift in wavelength. A large planet or a planet closer to its host star provides a larger shift.

An artistic impression of extrasolar planet, 'Tau Boo b' 

The team used the carbon monoxide study as a guide to expand the RV technique into the infrared to determine the orbit of extrasolar planet, 'Tau Boo b' around its star, and added further analysis of the light shifts via spectroscopy, an analysis of the light's spectrum.

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

Using data of extrasolar planet, 'Tau Boo b' collected with the Near Infrared Echelle Spectrograph (NIRSPEC) instrument at the W. M. Keck Observatory in Hawai'i, 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 vapour.

"The readout we get from Keck Observatory's NIRSPEC is like listening to an orchestra performance; you hear all of the music together, but if you listen carefully, you can pick out a trumpet or a violin or a cello, and you know that those instruments are present," Lockwood said.

"The instrument allows you to pick out different pieces; like this wavelength of light means that there is sodium, or this one means that there's water."

More information: "Near-IR Direct Detection of Water Vapour in Tau Boo b." Alexandra C. Lockwood, John A. Johnson, Chad F. Bender, John S. Carr, Travis Barman, Alexander J.W. Richert, Geoffrey A. Blake. arXiv:1402.0846 [astro-ph.EP].