Exoplanetary Vapor

Water’s awfully important to life here on Earth. Without it, we wouldn’t be here. So, we take a great interest in finding water at other planets. Mars, for example, is a huge focus of attention. It’s clear that there’s water in its polar caps and water vapor in its atmosphere (not much though, compared to Earth). And, there are flow features on the surface, plus evidence that something has flowed very recently. It’s likely water, but there are no seas or lakes like there here on Earth.

When it comes to finding water on planets around other stars, it’s a tough search. We can just barely make out those planets, and seeing their surfaces is probably impossible from Earth-based instruments. So, how can you tell if a planet has any water? Researchers at the California Institute of Technology, along with scientists at Pennsylvania State University, the Naval Research Laboratory, the University of Arizona, and the Harvard-Smithsonian Center for Astrophysics have figured out a way to analyze the gaseous atmospheres of exoplanets. They did it by using a method called the radial velocity technique. That’s where you measure the motion of a star due to the gravitational pull of a companion planet. As the planet orbits the star, it affects the stellar motion. You can find this motion by studying the spectrum of the star—that is, analyze the wavelengths of light it radiates. A big planet provides a large shift in the spectrum; a small one provides a small shift.

The light coming from the star system they studied, call Tau Boötis, included infrared light radiated by the hot Jupiter called Tau Boötis b. It glows in infrared because it’s a warm body, a hot one, in fact. But, it’s still cooler than its star. So, its light can be separated out.

In the spectrum of the planet, researchers saw the fingerprints of many compounds (gases) that make up the planet’s atmosphere. Among those fingerprints were those of water vapor.

Now, this technique worked pretty well with a super-Jupiter, and with further refinement, it can and will be applied to the search for water at super-Earth planets. And, when the James Webb Space Telescope comes on line (in a few years), the chances for those detections will go up quite a bit. Finding cooler planets with water will tell us that Earth-like planets with water are not as rare in the galaxy as once thought.

If you want to read the paper this work is based on, check out Near-IR Detection of Water Vapor in Tau Boo b.