Topic of the Moment – the search for life

As IOP’s London and South East Branch hosts a talk on astrobiology, we look at the search for life beyond the Earth.

Credit: Grand prismatic spring by Jim Peaco, National Park Service
The bright colours are caused by bacteria living around the edge of the spring

Life is abundant on the Earth: the planet harbours another 10–14m species besides our own – and that’s still only about 1% of all those that ever existed but are now extinct.

Living organisms are found in just about every imaginable part of this world, from geothermal hot springs to buried under Antarctic ice. It’s produced some particularly hardy critters: the Tardigrade, a type of micro-animal, can survive the vacuum of space, and one bacterium, Deinococcus radiodurans, can easily withstand more than 1000 times the dose of radiation that would kill a human. But Earth is also the only place where life is known to exist – at least for now.

Astrobiology harnesses all the natural sciences and engineering to try to find out if life elsewhere – whether it originated in one place and spread throughout the universe, or arose multiple times independently.

This doesn’t necessarily mean little green men, the search for which is another matter entirely. Even a microbe would be major discovery – if we can find them.

It’s assumed that life depends on certain factors or conditions. One is the presence of carbon – all forms of life we know of on the Earth based on carbon, and it easily bonds to atoms of other elements to form complex molecules. The assumption is referred to by its critics as ‘carbon chauvinism’, but even the next most promising element, silicon, is unlikely to be versatile enough to support the processes necessary to life.

Another assumed necessity is water, as a solvent to facilitate chemical reactions. It was also once thought that life would ultimately be dependent on access to sunlight – plants use the sun’s energy to convert carbon dioxide and water into sugar via photosynthesis, and are eaten by animals, which are eaten by other animals. But in 1977 the crew of a US Navy research submersible discovered sea creatures living near undersea volcanic vents; the base of their food chain was not plants, but a type of bacteria who eked out an existence by oxidising chemicals such as hydrogen sulphide leaking up from the Earth’s interior.

Voyages to other worlds to seek out new life and new civilisations are still quite unfeasible, so Earth-bound astrobiologists study some of the harshest locations on the planet to learn more about the type of environment in which life can survive and thrive.

But there are, at least, unmanned probes capable of seeking out space-plankton. Within our solar system there are several places on which the search for life is focused. The most well studied thus far is our near neighbour, Mars. It probably once had an ocean that has since evaporated into space, and liquid water may still transiently exist on the surface. Successive space probes have looked for signs of life, or evidence that it once existed, but turned up no convincing evidence. However the best locations to study may be underground, which has not yet been explored.

It’s also going to have to be below the surface on the other candidates within our solar system – some of the moons of Jupiter and Saturn.  Ganymede and Europa – respectively the largest and smallest of Jupiter’s Galilean moons – are thought to have underground oceans, and the latter also has plumes of water at its poles. Saturn’s moon Enceladus has a liquid ocean under its frozen exterior and is geologically active – so probably has thermal vents like those on Earth around which ‘extremophile’ life sometimes clusters. The European Space Agency and NASA are both planning, or weighing up the possibility of, missions to these worlds, such as the Europa Clipper.

The search isn’t just limited to our own solar system, however – we can also look for signs of life on planets orbiting other stars. Again, searches are biased towards conditions similar to our own, focusing on Sun-like stars – bigger ones are too short-lived and smaller ones just aren’t hot enough. Life on the surface of an exoplanet will leave tell-tale chemical signatures in its atmosphere, which, as the resolution of our telescopes improves, should become detectable. We can already spot organic material – the building blocks of life – in deep space, so it seems inevitable that if life is out there it will be detected sooner rather than later. One chief scientist at NASA expects success within a decade.