Guest post from Chemistry World intern Daniel Johnson…

Our first family computer didn’t have much processing power. In fact it sometimes felt like the technological equivalent of a boxed monkey with an abacus. So when my brother installed SETI@home (it stands for Search for Extraterrestrial Intelligence) – a program designed to harness home PCs to crunch through radio telescope data – the result was a device incapable of supporting anything but a game of Minesweeper.

An artist’s impression of a hot Jupiter (at bottom right), a giant planet that orbits extremely close in to its host star. Credit: Leiden Observatory

Judging by recent developments, however, it turns out my curious and philanthropic sibling needn’t have bothered. Astronomers using the European Southern Observatory’s Very Large Telescope (VLT) have developed a new method which could allow us to analyse the make-up of exoplanets in greater detail than ever before. According to results presented at the Royal Astronomical Society National Astronomy Meeting on the 5^th July, the new technique will allow astronomers to ‘efficiently search for water on hundreds of worlds without the need for space-based telescopes’. The implications? Easier to look for planets that may harbour those green many-legged aliens, for a start. Furthermore, having improved the technique for water, the team, headed by Jayne Birkby of Leiden University, may now move on to other atmospheric molecules such as O₂, CO₂ and CH₄.

Looking at the method itself is a lesson in how old principles can be combined to make huge advances. The technique uses the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) instrument, mounted on the VLT. Just as important is the relationship between the Doppler shift of electromagnetic radiation and the velocity of a body (the same effect that makes the siren of a passing ambulance rise and fall in pitch).

The CRIRES spectrograph has been in service since 2006; astronomers having been using the Doppler shift to find exoplanets for years. So what’s new?

Well, previously astronomers measured the Doppler shift in a star’s spectrum caused by the planet’s gravitational field. As the star is much heavier, it barely moves, spinning in a small circle at only a few km/h. But Birkby and her team flipped this around, using the Doppler shift caused by the planet’s motion around the star (c. 400,000km/h). According to Birkby, hunting for water meant looking at longer wavelengths where ‘the Earth’s atmosphere starts to obstruct what we are looking for’.  Sited at high altitude in Chile’s Atacama Desert, the VLT is ideally placed to minimise atmospheric effects, but still the spectrum received a combination of signals: from the exoplanet, the star itself and some distortion owing to our atmosphere. However, the Doppler effect shifted the exoplanet’s contribution while the rest of the spectrum stayed the same. This is where the CRIRES comes in. Its extremely high resolution can distinguish individual water lines in the spectrum, allowing the overall pattern to be identified.

An analogy for the process is this: imagine you’re looking for a needle in a haystack (surely someone did it originally for the phrase to exist). Even more unfortunate for you is that Herr Doppler is making the needle move as you search. You must discount the useless hay (distortion from the Earth’s atmosphere and the spectrum of the star) and find the moving needle (the water signal). Your one flimsy advantage? You know what the needle looks like.

It’s easy, then, to understand the excitement when they actually found the signal. ‘Of course we were delighted when we saw the signal jump out at us,’ said Birkby, going on to convey her excitement that this technique could be used ‘to look for Earth-twin planets’.

The researchers can now move on to looking for more atmospheric molecules – such as methane, carbon dioxide and oxygen – gradually building up a picture of the atmospheres and histories of thousands of exoplanets. It raises the tantalising prospect of one day finding a planet with a similar atmosphere to our own, where life may have existed or even exists today. Our present understanding of the chemistry of alien atmospheres is extremely limited, but techniques like this will lead the way, allowing us to ask those initial questions and find out if Earth’s chemistry is unique or astronomically mundane. But for now, back to the Minesweeper.

                  

Source:
http://prospect.rsc.org/blogs/cw/2013/07/05/one-small-step-towards-understanding-the-chemistry-of-alien-atmospheres/