It's official. Water has been detected in the atmosphere of a planet orbiting the star Tau Boötis. 

This planet, Tau Boötis b, is roughly the size of Jupiter, and orbits close to its home star. Astronomers refer to this class of planets as "hot Jupiters." This planet was found in 1996, making it one of the first planets ever discovered outside the solar system. It lies 51 light years (or 300,000,000,000,000 miles) away from Earth. 

New techniques allow astronomers to accurately measure levels of water vapor in the atmospheres of distant worlds. Using these new tools, researchers hope to determine the number of planets in the Milky Way that could harbor large oceans. Liquid water is usually considered a necessary ingredient for life, even on alien worlds.

Astronomers often use a prism or other device to break light from a distant object into different wavelengths of light, like a rainbow. By looking at bright and dark lines in the spectrum, astronomers are able to determine the chemical makeup of the object they are studying. 

"The information we get from the spectrograph 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. With the telescope, you see all of the light together, but the spectrograph 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," Alexandra Lockwood, a Caltech graduate student who participated in the study, wrote in a statement.

Spectroscopy is an old technique, and water has been detected on alien worlds in the past. In previous discoveries, however, the planet had to be between its companion star and the Earth. The orbit of Tau Boötis does not allow for that type of observation. 

Lockwood and her team used the Near Infrared Echelle Spectrograph (NIRSPEC) at the Keck Observatory in Hawaii to measure the nearly-hidden signal from the distant planet. A similar technique was previously used to detect carbon monoxide around the planet. That poisonous gas, produced by automobile engines on Earth, is believed to be the second most common gas on hot Jupiters, after hydrogen. 

So far, the new techniques can only be used on hot Jupiters. But soon, astronomers may be able to look for water on smaller worlds that do not orbit on paths allowing analysis using older methods. These could include superearths, planets slightly larger than our own world, which could be home to alien life.

The first planet orbiting another star was discovered in 1992. Since that time, well over a thousand alien worlds have been found, with hundreds of other candidate findings awaiting confirmation. 

Details of the observations were published in the Astrophysical Journal Letters.

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