Water detected around 'hot Jupiter'

Researchers have applied a sophisticated Doppler technique to the infrared to detect water vapor in the atmosphere of a planet outside our solar system.

The planet, named tau Boo b, orbits the nearby star tau Bootis and belongs to a class of exotic planets called "hot Jupiters" that are not found in our solar system. A hot Jupiter is a massive extrasolar planet that orbits very close to its parent star.

Unlike our Jupiter, which is fairly cold and has an orbital period of about 12 years, tau Boo b orbits its star every 3.3 days and is heated to extreme temperatures by its proximity to the star.

The research team studied data collected at the W.M. Keck Observatory in Hawaii, using the Near Infrared Echelle Spectrograph instrument. Because a hot Jupiter is too close to its star to separate the planet's light from that of the star, the researchers adapted a Doppler technique previously used to detect low mass-ratio spectroscopic binary stars.

Application of this method to tau Boo b, however, posed a huge challenge, because the infrared radiation from the star is more than 10,000 times greater than that of the planet.

By comparing the molecular signature of water to the combined light spectrum of the planet and star, the scientists were able to measure the motion of the planet as it orbits the star and establish the presence of water vapor in the planet's atmosphere. The team also determined that the planet is six times more massive than Jupiter.

Dr. John Carr, a researcher in NRL's Remote Sensing Division and co-author on the paper, said that the detection of water vapor in tau Boo b is an exciting and important step in understanding the composition of these exotic planets.

He said that their result also demonstrates the power of this technique for measuring water and other molecules in the atmospheres of planets, giving us a new tool to study the nature and evolution of extrasolar planets.

The discovery has been published in The Astrophysical Journal Letters.