Read more: “Instant Expert: Astrobiology“
For the first time we have the technology that will allow us to look for life in our solar system and beyond. But with more than 20 planets and large moons around the sun and 200 billion stars in the Milky Way we need to narrow down the search. Powerful telescopes, clever astronomical techniques and field trips to some of Earth’s most extreme and remote locations are helping us to identify the most promising places to start.
Looking for Earth-like planets
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Finding planets that resemble Earth is a good start but that is not the only consideration. Though our galaxy, the Milky Way, contains several tens of billions of stars like our sun they are not in the majority. Over 70 per cent of its stars are less than half the mass of the sun and less than 1/10th as bright. The smallest are the diminutive red objects called M-dwarf stars that can be 1000 times fainter than the sun but which models predict can live for trillions of years. Most of our closest exoplanetary neighbours may orbit stars like this.
They may, however, be challenging environments for life. Planets orbiting such dim stars could sustain liquid water only if they orbited very close to their parent. At that distance, gravitational tides might “lock” a planet’s rotation, so that it has permanent day and night sides. What’s more, M-dwarf stars are prone to emitting massive solar flares that would irradiate the planet’s surface and erode any nascent atmosphere.
Planet size also matters. Solid planets up to about 10 times the mass of Earth – often called Super-Earths – may have a range of compositions, including a greater proportion of water and other volatile substances than Earth. Though surface gravity will be a little higher, many could share with Earth some of the characteristics favourable to life, including atmospheres and geological activity. Super-Earths are easier to detect and study than smaller planets, making them good targets for observational techniques that might ultimately be used to study worlds more directly resembling our own.
First contact?
Some scientists claim that we have already observed the signs of alien life in our solar system. Most others dispute this, however, leaving the question mired in controversy.
In 1976, NASA’s twin Viking landers arrived on Mars. Each carried four experiments designed to look for signs of life. They sniffed for telltale chemicals, applied nutrients and heated samples of the Martian soil. Only one experiment showed a strong reaction between the soil and a nutrient mixture. Yet there was no sign of organic compounds in the soil, and it was generally concluded that the reaction was a purely chemical one, rather than one involving any life form.
Now we are not so sure. Recent studies of soil from the Atacama desert in Chile – which resembles that on Mars – suggest that organic compounds could have been present in the Viking samples, leaving the door open for the presence of life. Only further Mars missions can settle the question.
Evidence of a different kind emerged in 1996, when NASA scientists claimed to have found microscopic traces of fossil life inside a meteorite from the Allan hills in Antarctica. The meteorite was a chunk of Mars that had been blasted away some 3 to 4 billion years ago and had wandered through space before crashing onto Earth about 10,000 years ago. Like the Viking findings, these results remain highly controversial.
The Martian atmosphere has also provided tantalising indications of a seasonally varying release of methane gas. Most of Earth’s methane is produced by Archaea, so could Mars also have living methanogens? Studying the proportion of the isotopes of the carbon and hydrogen atoms making up the methane molecules might give us the answer, since living creatures have a preference for lighter isotopes.
Other intriguing results appeared in 2010, when observations of Saturn’s largest moon, Titan, indicated that hydrogen gas is flowing downwards to the moon’s surface and disappearing. Some areas on Titan also appear to be deficient in the hydrocarbon acetylene. Comparison with the behaviour of hydrocarbon-eating microbes on Earth suggest that some kind of life might just be responsible.
Sending instruments to these worlds will be key to solving these mysteries. NASA’s latest robotic Mars explorer, Curiosity, should arrive in 2012 and will sniff for carbon compounds and other signs of life as it roams the Martian surface. In 2020 NASA and the European Space Agency are hoping to visit Europa and Ganymede, and plans are being discussed for further trips to Titan that could include a probe floating across one of its methane seas.
It is even conceivable that we ourselves might be descended from alien life. The controversial panspermia hypothesis suggests that life is carried across the universe on rocks or comets, and even as free-floating organisms. Within our solar system, material can certainly be transported between planets and moons when an asteroid or comet impact blasts chunks of rock or dust into space. If there have been microbial hitch-hikers tough enough to withstand the extreme conditions, maybe life on Earth has been influenced by life on other worlds. Conversely, life on Earth could have influenced life elsewhere.
“It is even conceivable that we ourselves might be descended from alien life”
Seeing stars
Astrobiologists can call on a variety of techniques to search for exoplanets (see diagram). Mostly these sense the presence of a planet only indirectly.
One of these methods tracks a star’s motion to see if it wobbles under the influence of the gravitational pull of an orbiting planet. This radial velocity is done by monitoring slight shifts in the wavelengths of light in a star’s spectrum. Stellar wobbles typically add up to a change in speed of less than a few metres per second, and measurements at thousands of different wavelengths may be needed to spot this. It is thanks to this technique that the first detection of an exoplanet was achieved in 1995, and it has since detected more than 400 new worlds.
A second technique will only find planets whose orbits happen to pass directly in front of their parent star, as viewed from Earth. When the planet itself passes in front of its parent, or “transits”, it briefly blocks a tiny fraction of starlight. Measuring the changes in starlight during this transit reveals the planet’s diameter. Using this powerful method, NASA’s orbiting Kepler telescope has found more than 1000 likely exoplanets in one small patch of our galaxy. Sometimes we can even detect light from a planet as it passes behind a star.
Another technique exploits gravitational microlensing – a consequence of the fact that large masses distort space-time. If light from a distant star passes close to another stellar system on its way to Earth, the distortion to space-time caused by the intervening star can act like a lens, dramatically magnifying the light. An exoplanet in a suitable orbit can show up thanks to the changes it creates in the magnified light.
Finally, it is also possible to image exoplanets directly. This has been compared to spotting a firefly next to a searchlight, yet advances in telescopes and image processing have allowed astronomers to do just that. So far, the only exoplanets seen this way are Jupiter-sized or bigger, and often much hotter. Eventually, we should be able to see smaller, cooler planets too.
Read next article: “Astrobiology: Hot topics in the search for ET“
