Keck data helps scientists find water vapor on distant planet


Scientists using W.M. Keck Observatory have applied a relatively new technique to the search for the presence of water on distant planets.

“We’re still at a point where we believe water is necessary for life,” Caltech graduate student Alexandra Lockwood said Tuesday, discussing the discovery she wrote about in this week’s Astrophysical Journal Letters. “Water vapor is one molecule that can be chemically stable in warm temperatures.”

So finding evidence of water is a big deal, particularly in exoplanets, those planets outside our solar system, scientists said.

The technique Lockwood used, called the radial velocity technique, was combined with spectroscopy, an analysis of the light’s spectrum. Every molecule emits a different wavelength of light, creating a “unique light signature” that researchers can analyze. Lockwood used data of extrasolar planet tau Boo b collected by Keck’s Near Infrared Echelle Spectrograph (NIRSPEC) instrument.

“We were able to detect a variety of organic molecules,” Lockwood said. “The technique has been used once before to find carbon monoxide. It opens up a whole new set of planets we can find out about.”

Lockwood likened the research to listening to music.

“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.”

The technique, right now, is limited to “hot Jupiter” planets, gas giants, specifically ones that orbit near to their host star.

Still, that’s more planets than could previously be studied, Observatory Director Taft Armandroff said.

“There was a very limited set of exoplanets we could look for water on,” Armandroff said. “We’re just seeing so much creativity and interest in exoplanets right now. It’s an exciting time to be an astronomer.”

Lockwood said she hopes to eventually use the technique on any type of planet.

Lockwood also used the method to analyze the mass of planets.

“They’re actually two separate findings, but they’re both interesting,” she said in a news release. “When you’re doing calculations to look for the atmospheric signature — which tells you that there’s water present — you also determine the 3D movement of the star and the planet in the system. And with this 3D movement information, if you also know the mass of the star, you can determine the mass of the planet.”

This method can provide a “true mass” of a planet, rather than an indicative mass, which scientists said was more of an estimate of its minimum mass. Getting the true mass will help scientists as they study planet formation and the evolution of individual planets’ planetary system, scientists said.