Have you ever heard of the exoplanet, Tau Boötis b? Well, it was discovered back in 1996 and is one of the closest exoplanets to us. Tau Boötis b is about 51-light-years away and is considered to be a “hot Jupiter” because it is a gas giant orbiting close to its parent star. Now, with the advances in techniques used to scan planetary atmospheres, something else has been discovered about Tau Boötis b: the fact that it has water vapor.
In order for scientists to find this water vapor, they begin by scanning the atmosphere of a planet’s faint glow. What are they looking at? All the different wavelengths of light: a spectra that reveals individual chemical identities.
“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,” says Alexandra Lockwood, a graduate student at Caltech. “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.”
Typically, scientists use a spectrographic analysis to analyze a planet that passes in front of their parent star, a technique previously used on Tau Boötis b to find carbon monoxide (the second most abundant gas found in gas giants’ atmospheres), but Tau Boötis b doesn’t pass in front of its parent star from our vantage point in the universe. However, Lockwood and colleagues were able to filter out the dim light emitted by the planet using the Near Infrared Echelle Spectrograph (NIRSPEC) at the Keck Observatory in Hawaii.
The results: the atmosphere reflected the unique molecular signature of water.
Unfortunately, this technique can only be used for Jupiter-like planets. “While the current state of the technique cannot detect Earth-like planets around stars like the sun, with Keck it should soon be possible to study the atmospheres of the so-called ‘super-Earth’ planets being discovered around nearby low-mass stars, many of which do not transit,” said Geoffrey Blake, a Caltech professor. “Future telescopes such as the James Webb Space Telescope and the Thirty Meter Telescope (TMT) will enable us to examine much cooler planets that are more distant from their host stars and where liquid water is more likely to exist.”