The hunt for alien life has just got a lot bigger. Green Bank, the world's largest radio telescope, listens for alien radio signals from advanced civilisations in deep space. So far, it seems that if there are aliens in the Milky Way, they're staying quiet.

But now the Breakthrough ListenLeading astrophysicists, including Stephen Hawking, set up this project in 2015 to dedicate thousands of hours of telescope time to search for extraterrestrial life. The main telescopes are Green Bank in the US and Parkes Observatory in Australia. project to find intelligent life has widened its search of our galactic neighbourhood: from 1,400 to 280,000 stars. This 200 fold increase is made possible by the groundbreaking work of the Gaia spacecraft to create a 3D map of the Milky Way.

Of course, scientists do not expect all aliens to be sending radio signals. Humans only began to do this in the last century, and astrobiologistsThe search for extraterrestrial life is often narrowed down to so-called Goldilocks zones, planets that are not too hot and not too cold. believe most alien life will not be intelligent. They also say we need to be looking a lot closer to home - in our own solar system and maybe even on Earth itself.

The Nasa spacecraft Perseverance is on its way to Mars, with similar missions planned for Jupiter's icy moon of EuropaMany astrobiologists consider Jupiter's smallest moon to be the most likely place to find extraterrestrial life. An enormous ocean may lie beneath its water-ice crust, with underwater volcanoes creating thermal vents. and Saturn's largest moon, Titan. We may soon have a better idea of whether we are alone in the solar system.

But when these probes reach their destination, will they know what to look for? "You can't hunt for something if you have no idea what it is," says Nasa astrobiologist Lynn Rothschild. Unfortunately, with over 100 competing theories, scientists cannot agree on a definition of life.

Life on Earth is carbon based and has adapted to a watery environment. However, Nasa defines life as "a self-sustaining chemical system capable of Darwinian evolutionThe English naturalist first called it "descent with modification". Spelling it out in detail: the process by which organisms change over time as a result of changes in heritable physical or behavioural traits.". This broader definition could include life composed of siliconScientists have managed to create carbon-silicon proteins in the laboratory, but complex silicon aliens would have very unusual biochemistry. Due to how silicon reacts with oxygen, one theory suggests silicon-based life forms would breathe out quartz rock., one of the most abundant elements in the universe.

If silicon-life is possible, astrobiologists speculate that we may discover aliens here on Earth, hiding in the Earth's crust in a shadow biosphereThe biosphere is the entire system of life on Earth. Scientists have speculated that there may be another biosphere on Earth with a different biochemistry, one that has so far gone unnoticed by biologists. of living rock.

Life may also have evolved to exist without water. The planetary scientist Stuart Bartlett suggests that the oily lakes of methane and ethane on Titan could be home to organisms that "drink petrol".

But Bartlett goes even further, saying we need to completely rethink our definition of life. Together with astrobiologist Michael Wong, he has devised a new concept: lyfe. This is a process that draws energy from its environment, grows exponentially, regulates itself and has the ability to learn.

This expanded definition allows scientists to think radically about what life might look like. Terrestrial organisms draw their energy from the sun, but what if extraterrestrials exist that are powered by kinetic or magnetic energy?

The idea of individual aliens may also be too limiting and geocentricThe popular image of "little green men" is a good example of this. Our ideas about what alien life looks like are based on our experience and expectations of life on Earth.. Bartlett argues lyfe is a process not a being and we should be prepared to think of whole planets as living systems, an idea first developed by James LovelockThe scientist who proposed the Gaia Hypothesis, the idea that the Earth should be viewed as one complex self-regulating system..

But Lynn Rothschild wonders "whether life is just what we define it to be". Whatever weird and wonderful phenomena we discover in deep space, or under our feet, we will continue to debate whether each is alive or not.

Some say no, aliens are not all around us. Scientists have been listening for signs of intelligent life for half a century without any success. They are now moving the goalposts and changing the definition of life. But this makes the search for aliens meaningless because then they could be whatever we want them to be - even silicon microbes hiding on Earth.

Others say yes, in science you must expect the unexpected. The word "alien" literally means a stranger or something unfamiliar. We should not assume that aliens will look like us. We have evolved over billions of years to adapt to living on Earth. Life on planets with different atmospheres and chemistry will take completely different forms, which are probably beyond our imagination.