Searching For Water Around Other Stars
July 5, 2013

Astronomers Find Evidence Of Water Molecules In An Exoplanet’s Atmosphere

John P. Millis, Ph.D. for - Your Universe Online

Astronomers search for planets using several different techniques; the most popular of which is to measure how a star "wobbles." As the surrounding planets orbit the star, their gravitational fields will cause the star to shift back and forth in response. From these wobbles researchers can derive the mass and distance of any planets present in the system.

Naturally, astronomers have been hoping for decades now to find an Earth-like planet - one that orbits at just the right distance for liquid water to exist, has just the right size, mass, and atmospheric composition to possibly support life. As yet, scientists have not found such a world.

But even if there is a candidate, there are significant challenges to confirming all of these details. One detail in particular, the composition of the atmosphere has proven challenging.

New research, however, is already proving valuable in filling this need. By taking the technique used to find planets in the first place, and isolating a different part of the data, researchers can now isolate the presence of different molecules in the atmospheres of these new worlds.

Pointing their telescopes at these stars, they observe the spectral lines shifting in response to the motions of the star. But lines generated by the lines of the planet will also shift, and do so much faster as their motions are considerably faster than that of the star.

The challenge is that the spectral lines are quite faint compared to the host star, making them difficult to identify. Additionally, ground based observatories have to deal with absorption effects from our own atmosphere, leading some to believe searching for certain molecules, such as water, would not work using this technique except with future space-based observatories.

Using the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) instrument mounted on the Very Large Telescope (VLT), the team studied the world HD 189733b, which orbits its host star every 2.2 days. Previous studies were able to isolate simple molecules, such as carbon monoxide, but surprisingly this effort was able to isolate the presence of water.

Project leader Jayne Birkby, professor from Leiden University, reports, "We knew our technique worked for simple molecules at shorter wavelengths, but in order to hunt for water, we had to go to longer wavelengths where the Earth's atmosphere really starts to obstruct the signals we are looking for, so we weren't sure we would find anything. Of course we were delighted when we saw the signal jump out at us. It means we can do much more with this technique. In the next decade our work will help astronomers refine their search for Earth-like planets - and even life - in orbit around other stars. It's incredibly exciting to think that in my lifetime we will reach a day when we can point up to a star and say with confidence that it has a world just like our own."

Birkby presented the results of his research at the Royal Astronomical Society's National Astronomy Meeting in St Andrews, Scotland.