Are we alone in this universe, or is it teeming with life?
We've been looking for extra-terrestrial life for years, but so far none has been found. A lot of attention has been focused on looking for life on Mars - first with telescopes, and then using more sophisticated techniques. In 1975 the remotely-controlled Viking probe was sent to Mars to search for life. The successful landing the following year was an amazing achievement, but despite hopes, no life was found.
Mars was barren of life … or was it?
A lot of excitement was generated in the 1990s when a team of scientists from the Natural History Museum and the Open University found that a meteorite that had fallen to Earth from Mars (called ALH 84001) contained a high concentration of organic compounds. They couldn't tell whether these compounds had formed on Mars or on Earth.
In 1996, a team from NASA examined the meteorite and discovered structures that resembled fossilised microbes. It was a discovery that caught the imagination of the whole world.
Could this be evidence of life on Mars?
Unfortunately, the huge excitement was crushed by further study, which revealed that the structures could easily have been formed by processes on Earth. It seemed that they probably hadn't come from space after all.
Despite this, the search for signs of life on Mars continues. There's evidence of water on Mars, past and present, and scientists believe that life could still exist... below the planet's surface. Scientists are also looking for life in other parts of the solar system…and beyond!
One current theory of how life emerged on Earth is that it evolved around hydrothermal vents deep in the oceans. Some researchers now think that Jupiter's moon Europa might also have hydrothermal vents at the bottom of its oceans, and if so, life might be found there.
NASA is planning to send a remote-controlled probe to Europa which will land on the surface ice, drill through to the oceans, dive to the bottom of the sea and look for signs of life!
Although we all think of aliens as big, multicellular creatures like ourselves, it's probable that if we do find extra-terrestrial life, it will be a lot simpler. We're far more likely to find a tiny, slimy microbial blob - if we're lucky!
It's also possible that life might exist in a form that we simply do not - or cannot - recognise.
Despite this, scientists are still looking for life of an altogether more intelligent nature. The Search for Extra Terrestrial Intelligence - commonly known as SETI - is a world-wide collaboration in which radio-telescopes are used to scan the skies for signals from aliens that must not only be biologically complex, but technologically advanced.
It's highly unlikely that we'll ever detect such a signal from another, alien civilisation. But so what? It's a relatively cheap process to look for signals, and the impact of finding one would be momentous! So scientists are having a go. We can all get involved in looking for signals by joining Team SETI and downloading the SETI screensaver.
Some of the experts in this field
Barrie Jones is head of astronomy in the Planetary Sciences Institute at the Open University. He has a wide range of interests within planetary sciences, including the atmospheric phenomena associated with solar eclipses, and the orbits of planetary and smaller bodies.
He's also interested in the detection of extraterrestrial life (of course!) and in aspects of the design of space telescopes for imaging exoplanets.
He's a keen proponent of SETI, the Search for Extra Terrestrial Intelligence.
Monica Grady is an expert on meteorites and is curator of meteorites at the Natural History Museum. She was one of the scientists involved in the study of the Martian meteorite (ALH 84001), which in the 1990s was thought to contain evidence of life.
Monica is also a member of the UK Exobiology Network, a collaboration between individuals from different scientific disciplines who are interested in the search for extra-terrestrial life.
Julian Hiscox is an RNA virologist at the School of Animal and Microbial Sciences at the University of Reading. That means he's an expert in the genetic material (RNA) of viruses, including how it replicates.
This is relevant because he's interested in looking at the factors involved in the earliest stages of the evolution of life - what is the smallest, simplest possible unit that replicates and can be described as life? If we can understand these processes on Earth, we might be able to apply them to other parts of the universe.
Julian is a member of the UK Exobiology Network, and is particularly interested in the search for life on Mars, and on Jupiter's moon, Europa.
Bob Lambourne is an astronomer in the department of Physics and Astronomy at the Open University. He's an expert in astronomy, particle physics, relativity and cosmology.
He's also a keen sci-fi enthusiast. His mixture of expertise enables him to draw parallels and comparisons between the science fact and the science fiction involved in the search for extra-terrestrial life.
Life On Other Worlds and How To Find It
Rare Earth: Why Complex Life Is Uncommon In The Universe'
Peter Douglas Ward, Donald Brownlee
Jean Heidmann, Storm Dunlop
Cambridge University Press
'SETI Pioneers: Scientists Talk About Their Search For Extra-terrestrial Intelligence
David W. Swift
University of Arizona Press