Scientists have detected water vapor in the atmosphere of the planet tau Boötis b, which orbits a star named tau Boötis, and their new technique could help us find more planets with water outside of our solar system.
This planet belongs to a class of relatively common extrasolar planets known as "hot Jupiters,” called such because they are massive exoplanets that orbit very close to its parent star. Unlike our Jupiter, which is pretty cold and takes 12 years to orbit around the Sun, tau Boötis b orbits its star every 3.3 days, and its proximity to the star heats it up to extreme temps. In those conditions, water exists as high temperature steam.
Before this, scientists have only detected water on a handful of planets outside of our solar system, and that’s because the main ways of doing so require very specific circumstances. "When a planet transits -- or passes in orbit in front of -- its host star, we can use information from this event to detect water vapor and other atmospheric compounds," study researcher Alexandra Lockwood from Caltech says in a press release. "Alternatively, if the planet is sufficiently far away from its host star, we can also learn about a planet's atmosphere by imaging it." Most extrasolar planets don’t fit these criteria.
So, Lockwood and collaborators from around the country developed a new technique to study the atmosphere of non-transiting exoplanets -- specifically hot Jupiters, which are too close to their star to separate the planet's light from that of the star. With tau Boötis, the infrared radiation from the star is more than 10,000 times greater than that of the planet.
They adapted a method called radial velocity technique, which uses the Doppler Effect to detect exoplanets. It’s traditionally used in the visible region of the spectrum, but the team expanded the technique into the infrared. Then they added further analysis of the light's spectrum; since every compound emits a different wavelength of light, researchers can use the unique “light signatures” to analyze molecules making up the planet's atmosphere.
Using data of tau Boötis b from the Near Infrared Echelle Spectrograph at the W. M. Keck Observatory in Hawaii, the researchers compared the molecular signature of water to the light spectrum emitted by the exoplanet, confirming that the exotic planet's atmosphere includes water vapor.
"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," Lockwood explains. "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."
In addition to studying atmospheric composition, the method allows researchers to analyze the mass of planets. That’s how they discovered that tau Boötis b is six times more massive than Jupiter. Hot damn.
The work was published in the Astrophysical Journal Letters earlier last week.
Image: David Aguilar, Harvard-Smithsonian Center for Astrophysics via NRL
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