If we are going to look for life, we need to be able to define what it is that distinguishes living from non-living. Unfortunately for us, life defies simple definition.

There is no neat sentence that sums up what life is, no mathematical formula, no straightforward schematic. Instead we have resorted to describing life, with lists of characteristics that living things have. These familiar characteristics can be found in any biology text, and include cellular organisation, ability for growth and reproduction, heredity, metabolism, movement, and response to stimuli.

While all living organisms on Earth exhibit these characteristics, vexingly, so do some non-living entities. Fire can be said to metabolise, that is convert energy from one form to another, but fire is not alive. Crystals can reproduce, but they are not alive. Viruses are seemingly living when they take over the machinery of a host cell, but by themselves are not alive.

Although there are difficulties with the way we answer this most fundamental of questions, without some idea of what constitutes life, we will find it very difficult to go and look for it. So, while the clumsy definitions that we currently employ have a range of limitations, we do not have a great deal of choice but to use them in our search to see if we are the only creatures in the universe that exhibit this peculiar set of characteristics.

In the search for life beyond Earth, it's also important that we have some understanding of how and where life on Earth originated. As we can be 100% certain that life has emerged once in the universe, discovering the origins of life on this planet has the potential to tell us a great deal about the occurrence of life on others.

There are a number of theories on how life began on Earth. It may have cooked up in a primordial soup of increasingly complex compounds on the Earth's surface 3.5 billion years ago. Alternatively, it could have originated many miles underground in the exceedingly hot and chemically volatile regions of the Earth's still forming crust. It may have even arrived from space, riding in on one of the vast number of meteorites that impacted the surface of the newly formed earth. We can not be sure.

The latter theory, widely known as panspermia, has for the most part been widely disregarded. Recently, in light of findings such as the discovery of amino acids in the Murchison meteorite, and evidence of microfossils in a meteorite of Martian origin, the theory has undergone a resurgence of popularity.

The primordial soup theory, while still popular, is losing some support in favour of the idea that life may have evolved deep in the Earth's crust.

Evolution of life on the surface of the relatively young Earth would have had a lot of obstacles to overcome, not least of which was frequent bombardment by meteorites and radiation. Although the subterranean environment would have provided shelter from bombardment, and allowed early life a reasonably uninterrupted chance to establish, the extreme conditions present there were thought to be too harsh for life to exist. Now, with the relatively recent discovery of a totally new order of life, known as Archaea, this belief is being reviewed.

Archaean microbes live in environments of extreme temperature, pressure, salinity and pH. Broadly termed extremophiles, the different groups have been given equally inventive names to describe their particular habitat. Thermophiles live in temperatures of 50-80°C, while hyperthermophiles have been found in the temperature range 80-115°C. On the other end of the scale are the psychrophiles, which live at temperatures of around -2°C. Halophiles live in very saline environments. Barophiles live in high pressure environments (up to 110 Mpa). Acidophiles live in conditions where pH ranges from 0.7-4, while alkalophiles can be found in pH ranges of 8-12.5.

The interest in these organisms, apart from the very novelty of their existence, is that the inhospitable conditions in which they thrive may be similar to what Earth was like 1 billion years or so after its formation. The discovery of extremeophiles lends a great deal of support to the theory that life may have emerged on Earth in the high pressure, high temperature, chemically volatile depths of the planet, and only emerged once things had settled down on the surface.

If this is the case, and life could have emerged in such unfriendly conditions on Earth, why couldn't the same be said for other planets that until now were thought not to be suitable for life?

It was recently estimated that there are 70 thousand million million million observable stars in the universe, not to mention those that are beyond our detection. Given this, it is my personal belief is that we are not alone in the universe. There's no real science behind this belief, but to me the size and numbers involved seem to indicate that there is more than a fair chance that there is life, intelligent or otherwise, somewhere out there. Otherwise, it would be an incredible waste of space.

There are, of course, many people who are more scientific in their approach to determining the existence of life beyond earth than I am. One such person is Frank Drake. Currently Chairman of the Board of the SETI Institute, in 1961 he developed the now famous Drake equation, which for the first time attempted to quantify the probability of detecting life (in this case, intelligent life) beyond Earth.

The Drake equation basically states that the number of civilizations we could detect will depend on the rate at which stars like our sun form, then the fraction of these stars that form planets, then the number of these planets that are hospitable to life, then the number of these planets where life actually emerges, then the number of these planets were life evolves to develop intelligence, then the fraction of these planets where interstellar communication evolves and, finally, the time that communication is carried on for before these intelligent civilizations die out or stop trying. More succinctly, the equation looks like:

Not only does the Drake equation convert the question of the existence of extraterrestrial neighbours from one of metaphysics to hard science, but it gives those looking for life beyond Earth a place to start.

It's accepted that life on Earth is highly unlikely to be representative of all life in the universe, but we have to start somewhere.

The most basic requirement of life on Earth is the presence of liquid water. Water is important to life because, in liquid form, it is an excellent medium for carrying chemical and biological compounds. It is also stable as a liquid over a wide temperature range, a temperature range that (conveniently) accommodates a wide range of biological processes. In identifying places where life may exist, astrobiologists are looking for signs of water, particularly in liquid form.

Astrobiologists are also looking for the right cosmic chemistry in their search for life. The presence of organic (carbon) compounds, while not conclusive, could be suggestive of life. Atmospheric concentrations of certain substances could also be indicative of living organisms. Oxygen and methane, for example, are both found in our atmosphere, but are both highly reactive molecules. Their individual presence suggests that molecules are being constantly produced to replenish the numbers in the atmosphere, and the source of this replenishment could be life.

Given that life did emerge and evolve on Earth, it seems a logical step to look for Earth-like planets as potential hosts for extraterrestrial life. These planets would be of a similar age and size to Earth, and orbit a similar distance from sun-like stars - far enough away from the star that any water present doesn't evaporate, but close enough that it doesn't freeze.

If there are highly evolved life forms out there we may even intercept signals from them. This search is the whole premise of the SETI program - the Search for Extraterrestrial Intelligence. Rather than looking for chemical and biological artefacts, SETI scientists are aiming to make contact with ETI through radio astronomy.

Of course, finding all of these things does not mean that we should not expect to find life forms (particularly evolved or higher life forms) that are in any way similar to life as we know it. The Earth's biota is the result of a set of unique conditions shaping the products of the natural life giving processes - the laws of chance dictate that finding a planet whose population has survived five great extinction events, not to mention geological, meteorological , physical, chemical and biological conditions that ensued as a result of each other, is exceedingly slim, and even if we did, the probability of life beyond Earth following exactly the same evolutionary pathway is too remote to contemplate.

Although it may seem an odd place to look for our extraterrestrial neighbours, there are a vast number of astrobiological projects taking place here on Earth. Apart from being easier to access and a whole lot cheaper to study than sites in deep space, the terrestrial laboratory that is our planet provides an array of fascinating opportunities for astrobiologists. Extremophile studies may help to unlock the origin of life on Earth, and so offer insights into life beyond it. Animal communication studies utilising information theory, which allows the complexity of a given signal to be measured, will hopefully allow us to identify the long awaited signal from space once it comes from random noise.

Other studies that are being undertaken involve examining materials from space that we find here on Earth. Over 22,000 meteorites have been discovered on Earth, including 28 of Martian origin. As mentioned earlier, studies of these meteorites have broadened our ideas about the beginnings of life, and about its distribution in the solar system and beyond.

These lines of enquiry are but a few of the many being examined on Earth in the search for life beyond it. NASA's astrobiology site gives details of many more.

Mars has always been a favourite source of speculation when it comes to extraterrestrial life. Its proximity means that it is also a target for scientific expeditions. Since 1960 there have been 34 missions to Mars.

Of the successful ones (16 have failed), four have involved landing spacecraft on the surface of Mars. In 1971 the first Martian landing was accomplished by the Soviet Mars 3 mission. Although only broadcasting information for 20 seconds, landing a craft on another planet was a huge success. NASA followed with the successful deployment of two orbiter-lander pairs in 1976 - Viking 1 and Viking 2. The landers conducted experiments looking for signs of life, but found no conclusive proof at their landing sites. Most recently, the Carl Sagan Memorial Station lander and Sojourner rover of NASA's 1997 Pathfinder mission collected information suggesting that Mars was at one time warm and wet - conditions suitable for life.

Mars is again the destination du jour with three separate craft winging their way to the red planet. The European Space Agency (ESA) launched its Mars Express mission in June 2003, with the primary objective being the search for subsurface water. The Mars Express spacecraft is carrying the Beagle 2 lander which will perform exobiological and geochemical research after it lands on the Martian surface in December 2003. NASA's Mars Exploration Rover program is also looking for signs of water, and has two separate rovers on their way to Mars. Spirit, launched in June 2003, and Opportunity, launched in July 2003 are set to arrive at their destination in January 2004.

In addition to the missions landing on the surface of the red planet, there have been a number of orbiting spacecraft sent to try and unlock some of its mystery. At present the Japanese spacecraft Nozomi is on its way there. Although plagued with problems since its launch in 1998, it is hoped that Nozomi will make it to Mars where it will study the upper Martian atmosphere. A summary of all missions to Mars, past and present, is on the NASA website.

Europa is one of the four large "Galilean satellites" orbiting Jupiter. Although it is the smallest of these satellites, Europa is still the sixth largest satellite in the solar system, only slightly smaller than our own moon. Europa has a relatively smooth, icy surface under which there is good evidence for the presence of liquid or semi-liquid "oceans". As liquid water is one of the key signs of potential life beyond Earth, Europa has caused a great deal of excitement in astrobiological circles.

Pioneer 10 and 11, and Voyager spacecraft have flown by Jupiter, but Galileo has given us the most information about Europa. Galileo was launched in October 1989, and after arriving at Jupiter in July 1995, made 11 orbits of Jupiter and its moons over the two year period of its prime mission. In addition, a probe was sent plummeting through the Jovian atmosphere early in the mission, where it recorded 58 minutes of data before being destroyed by the harsh conditions it encountered. In 1997 after the prime mission was completed Galileo completed an additional 14 orbits, eight of which were around Europa.

Titan is Saturn's largest moon, and it is believed that the atmospheric composition (nitrogen, methane, ammonia and argon) and surface conditions might be similar to those that we would have found on Earth when life was first emerging.

Pioneer 11 made the first direct observations of Saturn in 1979, with the two Voyager spacecraft following in 1980-81. These spacecraft took photographs of Titan (although the hazy atmosphere of the moon obscured the surface) and obtained atmospheric pressure and composition readings.

The latest mission to head to Saturn is Cassini-Huygens, an international collaboration between NASA and the ESA. Scheduled to reach Saturn in the second half of 2004, the craft consists of the Cassini orbiter (NASA), and the Huygens probe (ESA). On arrival, the Huygens probe will be deployed to the surface of Titan, where it will relay information about what it finds to the Cassini orbiter. This part of the mission is expected to last for four hours. The Cassini orbiter will continue to orbit Saturn and its moons for another four years.

Although more than 100 planets have been found orbiting stars outside of our solar system, they have all been more "Jupiter-like" than "Earth-like". At present, we do not have sensitive enough equipment to detect the presence of relatively tiny planets like Earth. A number of missions are being planned in an attempt to overcome these limitations such as NASA's Terrestrial Planet Finder which it is hoped will be implemented in 2006, and the ESA's Darwin mission, to be launched in 2014.

Perhaps the most well known search for life beyond earth is the Search for Extraterrestrial Intelligence. Projects under the SETI banner are not just looking life beyond earth, but highly evolved, intelligent life.

The search is based on the premise that the intelligent civilizations will be either deliberately or inadvertently transmitting signals that we will be able to detect on earth. The largest program being undertaken at present is Project Phoenix. Starting in 1995 at the Parkes Radio Telescope in Australia, the program is now based at the world's largest single-dish radio telescope at Arecibo in Puerto Rico. It involves the systematic scrutiny of space in the vicinity of sun-like stars. To date approximately half of the target stars have been investigated with no success. However, there are still an awful lot of stars to go...

The search for life beyond earth is potentially one of the most exciting, illuminating and confronting pieces of science ever to be undertaken. Its success will change the face of science and life as we know it forever. The journey through space and time that this success could take us on has profound implications, but none more so I suspect, than the realisation that at the end of the day, there's no place like home. Maybe then we'll give our own planet the care and attention it deserves.

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