The search for life in the universe

Guest post by Markus Hammonds

The question of whether or not we are alone in the universe is debatably one of the deepest philosophical questions that modern science is trying to answer. The idea of life on other worlds is, by now, deeply ingrained in our collective consciousness. Whether you know of the idea from H.G. Wells or Carl Sagan, it’s hard to find anyone who hasn’t at least encountered the idea of aliens. Whether or not we admit it, the very question of something as unknown as alien life is somehow captivating. As a child, the idea always fascinated me. I was an avid reader of stories by people like Isaac Asimov and Larry Niven, and a keen watcher of shows on TV like Lost in Space and Star Trek. That the uncountable millions of stars in the night sky might possess planets of their own, perhaps with life unlike anything around us here on Earth, fuelled many summer nights of stargazing.

Excitingly, the human race is, perhaps, finally on the brink of being able to answer the question of whether life exists elsewhere in the galaxy. While the same has been said before by people far greater than I, it is my firm belief that in the very near future, astronomers will finally have firm evidence of extraterrestrial life. I won’t put an estimate on how many years it may take. Such estimates have been made before and seldom have any true meaning. But I do believe it will be soon, and I certainly expect it to be within our lifetimes.

Kepler’s Staring Match

Kepler-11 is a sun-like star around which six planets orbit. Image: NASA/Tim Pyle

Planets orbiting other stars have been discovered en masse in recent years. As of May 15th 2012, there were 770 such exoplanets confirmed to exist. Scores more of them are being investigated by astronomers even as you’re reading this. NASA’s Kepler space observatory has, since its launch in 2009, been staring unblinkingly at a small patch of the sky. Even though the patch of sky it’s been gazing at for the past three years is only about 12° in angular size (about the size you fist would appear to be at arms length), there are 145,000 stars in its field of vision, which are being watched constantly. The stars in question are referred to by astronomers as ‘Main Sequence’ stars, and they’re typical adult stars like the Sun. A prime hunting ground for planets, and prospectively for life. The reason why Kepler never moves from watching the same part of the sky is that it’s looking for planetary transits – planets passing between their parent star and us here on Earth (exactly the way Venus is set to pass in front of the Sun early next month).

Kepler started out with no idea where to look. The chances of planetary transits are very small, and Kepler’s detector array is exceptionally sensitive to be able to pick out the infinitesimal change in the stars brightness as tiny planets pass in front of them. The task has been likened to trying to spot a moth in front of a floodlight at a football pitch, but nonetheless in the first 16 months of its operation, Kepler detected thousands of light fluctuations of exactly the kind it was searching for. To date there are still 2,321 unconfirmed detections of planets around 1,790 host stars. In fact it’s expected that there are, on average, 3 planets for every two stars in the galaxy. And that isn’t counting the trillions of rogue planets believed to exist, drifting through the icy vastness of interstellar space.

The first Earth-size planets found around a sun-like star compared to planets in our own solar system. Image: NASA/Ames/JPL-Caltech

The reason why these planets are unconfirmed is that there are lots of other things which could cause similar drops in a star’s brightness. So Kepler continues its quiet staring match with the stars, until it spots the same drop in brightness three times. Only then can the astronomers picking through Kepler’s data be absolutely certain that they’ve genuinely found a planet around that star.

Most recently Kepler has been the first telescope to detect Earth-sized worlds in other star systems. A star slightly smaller than the Sun, dubbed Kepler-20, around 950 light years away from us in the constellation of Lyra is host to a remarkable system of planets. It was announced in late December last year that Kepler-20 has at least 5 planets in tow, three of which are Neptune-like giants and two of which are Earth-sized planets. These two worlds, Kepler-20e and Kepler-20f are likely too hot to host any life as we know it, with surface temperatures expected to be 720°C and 427°C. Nonetheless, with more and more systems being discovered with several planets, such as the star HD 10180 with at least 7 and possibly nine worlds revolving around it, there’s still plenty of searching to do.

Detecting Chemical Disequilibrium

A question which may arise at this point is how exactly we might determine whether any exoplanets support life. To do this, we need to use a slightly different kind of telescope observation. Spectroscopy is the science of separating light into its constituent wavelengths and drawing conclusions based on what light we see. Different chemicals absorb and emit light at specific wavelengths, characteristic of each atom or molecule. Oxygen and methane, amongst others, show up as obvious lines in the spectrum of our planet, but these two molecules should not normally exist together. Normally, they’d react over time and form water and carbon dioxide. The only reason that both exist in Earth’s atmosphere is because both are being replenished by living things on Earth. The technical term for this is ‘chemical disequilibrium’, meaning that two chemicals in Earth’s atmosphere are out of balance. With the powerful telescopes being constructed on Earth right now, we should be able to see if any similar disequilibrium exists in the atmospheres of distant worlds.

Molecules have already been detected in the atmospheres of exoplanets. A team of researchers led by Giovanna Tinetti at UCL, for instance, have discovered water, methane, and carbon dioxide in the atmosphere of a planet known as HD 189733b. While this planet itself is a ‘hot jupiter’ (too massive and close to its parent star for any kind of life we know of), the fact that these molecules can be detected means that the next generation of telescopes should be easily able to peer into the atmospheres of other far away worlds. The hopes of astrobiologists right now are largely pinned on NASA’s James Webb Space Telescope (JWST) which, once finished and launched into space, will be powerful enough to make detailed spectroscopic observations of exoplanet atmospheres. Dr Tinetti and others are also doing everything they can to get ESA to agree to construct another telescope, and this one would be specialised for the task of analysing exoplanets. The Exoplanet Characterisation Observatory, or EChO for short, would be a small space telescope whose primary purpose would be to make follow-up observations of planets detected by telescopes like Kepler. EChO has been selected by ESA’s Cosmic Vision roadmap for further studies and is currently competing against three other space mission proposals. If EChO is selected, it may give us one of our best hopes of detecting life around another star.

Potential of 12 Billion Stars

In the meantime, it’s a big galaxy out there, and we’re still doing little more than scratching around in our back yard. Most exoplanets detected are within about 300 light years of us here on Earth. However, the Milky Way Galaxy in which we live is at least 100,000 light years in diameter. Based on the observations made so far, of the 30 billion sun-like stars in our galaxy, 20% are expected to be host to Jupiter-sized planets, and 40% are expected to have smaller planets circling them, including Earth-sized planets. In other words, there should be roughly 12 billion stars in our galaxy which may be home to planets like Earth. While our search has barely begun, I must say I like those odds.

Markus Hammonds (@InvaderXan) is a final year PhD student in Molecular Astrophysics who blogs at SupernovaCondensate.net. He spends his life looking at very small things on very large scales, and trying to better understand the chemistry of interstellar space.

Other posts in this series:

2 thoughts on “The search for life in the universe

Comments are closed.