Best possible world

OLIVER KING considers our prospects for discovering intelligent life on another planet. Could it be the big scientific discovery of your lifetime?

The relationship between scientific discoveries and our self-image as human being has long been fraught. Copernicus shifted the earth from the centre of the universe to the periphery. Darwin downgraded humans from the special creation of an omnipotent being to one of the crowd. Demonstrating that the laws of physics apply to objects on earth as much as they do to planets and stars, Newton levelled the playing field between humans and the rest of the universe. As I write, advances in human genetics continue to demystify the human condition, presenting the physical body as the sum total of its constituent parts. What connects these discoveries is the implication that human beings are not a unique part of the wider universe, as was once believed. In fact, the reverse is true. Constructed out of the same type of matter and subject to the same laws, human beings exist on the same terms as any other object in the universe.

Yet until now human beings have considered their intelligence to be unique amongst the objects with which they share the universe. It is commonly (though by no means universally) held that we face no competition on earth to our status as intelligent being. And as for the universe beyond our planet? Well, the closest we have come to facing serious competition to our monopoly on intelligence is on the silver screen. But it may not always be like this. The discovery of life, perhaps wholly unlike that known or imagined by human beings, would likely shake up humans' self-perception as much as – if not more than – the scientific developments of the past millennia. It is the possibility that such a discovery might be imminent that I will consider in this article.

There are two principal means of searching for life on other planets. The first and perhaps the most familiar approach involves sending probes to other planets in our solar system to detect life directly. The second and arguably the one that is most likely to succeed is the attempt to detect evidence of life outside our solar system in the electromagnetic spectrum.

The most likely candidate for hosting life in our solar system is thought to be one of Jupiter's moons, Europa. The presence of ice and probable sub-surface liquid water has made it a major target for probes. Time and energy has been devoted to developing technologies that would be capable of detecting alien life on Europa. Proposals have ranged from nuclear-powered pods which would melt their way through the surface ice (called Icepick, proposed by Larry Klaes from the Optical SETI project) to probes which would crash an impactor into the surface and then swoop through the debris plume (called the Europa Ice Clipper). However, such projects require expensive and complex probes and launch vehicles. Cassini-Huygens, the probe currently orbiting Saturn, is about the size of a bus and significantly more expensive. So far budgeting constraints at NASA have kept these probes in the realm of theory.

The most abundant element in the universe, hydrogen, emits very weak radio waves at 1.42 GHz. H.C. Van de Hulst first predicted the existence of this signal towards the end of the second world war as part of a project initiated by his supervisor, the Dutch father of radio astronomy, Jan Oort. It proved to be a boon to astronomers as its dust-penetrating properties allowed astronomers to study previously invisible parts of our Galaxy. Alien civilisations choosing to broadcast their existence to the rest of universe can do so at almost any frequency in the radio spectrum. However, if they hope to be detected by other civilisations they would want to broadcast at a frequency that astronomers would likely be studying. Hydrogen's special distinction as the simplest and most abundant element and its dust penetrating property make it the frequency of choice for studying the structure of our Galaxy, and hence marks 1.42 GHz as the position of choice on the radio dial for interstellar communication. The familiar and long-standing SETI project scans the sky looking for the signatures of intelligent life around this frequency for exactly this reason. Thus far, it has been unsuccessful.

So how else can we search for alien life? Life as we can conceive it needs planets. There was a time when the discovery of each new extrasolar planet would be reported with great fanfare in the media. The first planet to orbit a sun-like star was discovered as recently as 1995. The pace of planetary discovery has accelerated so dramatically that the average planet discovery today does not even warrant a press release. Astronomers discovered 31 new extrasolar planets in the first five months of 2007 alone. In an 18 year survey of over one thousand nearby sun-like stars, nearly 10% of these were discovered to have large, Jupiter-like planets orbiting them.

The most popular planet-hunting technique involves looking for a tell-tale sign of a planet's presence. Picture a hammer thrower. As the athlete spins, she wobbles from side to side: the larger the weight being tossed, the more pronounced the wobble. Planets have a similar affect on the stars they orbit, only over a longer time-scale. By observing stars over many years, planet-hunters hope to detect these minute shifts in a star's position. This technique is biased towards the detection of large planets because they exert more influence on the star they orbit. This explains the large detection rate of Jupiter-like planets. The Gaia satellite, due to launch in late 2011, will produce a flood of planet detections as it carries out this type of measurement on about one billion stars in our Galaxy.

Looking for unsteady stars is not the only technique for detecting extrasolar planets. One of the more imaginative techniques, born of methods used in the study of distant galaxies, treats the target star as a gravitational lens acting on light from a background star. Einstein's theory of General Relativity predicts that large bodies, like a star, bend passing light rays and can hence act as giant lenses in space. When light from a background star passes through the gravitational lens of a planet-less star it is spread into a characteristic pattern. If the foreground star has planets the shape of the gravitational lens differs, along with the pattern that passing light is spread into. By detecting the shape of this pattern astronomers can work out the number and distribution of planets around a star. This technique was suggested by Dr Bohdan Paczynski from Priceton University in 1991, and was first successfully used in 2004 to observe a planet orbiting a star 17,000 light years away by the MOA and OGLE experiments.

But how can we detect life on these extrasolar planets? Humans have only been capable of producing radio signals for a mere fraction of the time that life has existed on this planet. It has been suggested that we could look for the signs of life in the chemical balance of the planet's atmosphere. Living systems on earth have fundamentally altered the chemical equilibrium of our atmosphere to a form that cannot exist without them. Any planet which contains the same balance of key elements is hence almost certain to contain life. If we could work out all these chemical signatures of life, we would know what to look out for in the atmosphere of an extrasolar planet. To do this, we need to be able to distinguish the light emitted by the host star from the light reflected by the target planet. This is incredibly difficult. Most host stars will be one billions times brighter that their planets, and the two will appear to be as far apart as a person's eyes seen from a distance of 50 km.

Nonetheless, techniques capable of doing so exist in theory. Very soon, they will be implemented in practice. The European Space Agency's Darwin satellite and NASA's Terrestrial Planet Finder should be capable of detecting and analysing the atmospheres of extrasolar planets. They launch in 2015 and 2014, respectively.

All this assumes that life on other planets actually exists. An attempt to quantify our chances of detecting intelligent life was carried out by the astronomer Frank Drake in 1960. He devised an equation which estimates the number of extraterrestrial civilisations in our Galaxy that we might be able to contact. It contains seven parameters, starting with the rate of star formation in our Galaxy followed by factors which take into account the fraction of stars which have planets, the fraction of planets which develop life, the fraction which then develop intelligent life, etc. These fractions are usually highly speculative. Estimates of the number of contactable civilisations range from zero to many tens of thousands, depending on the person performing the calculation. A category of organisms, dominantly microbial, called extremophiles exist here on earth which can thrive in a stunning range of conditions previously considered barren, from environments with radiation levels 500 times higher than that sufficient to kill humans, to boiling water. The surprising resilience of some microbial life and the staggering number of stars and planets in our Galaxy prompt many to believe that while intelligent life may be rare, non-intelligent life could well be common.

So does this mean we are on the brink of discovering life on other planets? In only a decade from now astronomers will be directly observing planets around other stars, looking for the signs of life. No one can be certain of how many planets astronomers will have to search before life is discovered. And this is to assume that life is indeed “out there” in the first place, waiting to be discovered. But, should they succeed, be prepared. It will be the biggest scientific discovery of your lifetime.

Oliver King is a DPhil research student in astrophysics. He designs and builds instruments for radio telescopes, and uses them to study the radio emission from our galaxy and beyond.