A NASA scientist leading the search for life on other planets claims a major breakthrough could be made within the next 20 years.
The rate at which potentially habitable planets are being identified is soaring, helping scientists to narrow down their search for extraterrestrial life.
When the first exoplanet was discovered in 1992, many regarded it as a once-in-a-lifetime event, but 25 years later, over 3,500 exoplanets from thousands of star systems have been located.
Several of these are orbiting within their star's "habitable zone" - where surface temperatures range from nought to 100 degrees centigrade, allowing liquid water to pool on the surface.
The presence of liquid water massively increases the chance that life as we know it exists on that planet.
According to Tony del Genio, co-lead of NASA's NExSS project at the Goddard Institute for Space Studies in New York, the discovery of alien life may only be a couple of decades away.
"I think that in 20 years we will have found one candidate that might be it," he said.
Del Genio recognises that alien life may well exist in forms and places so bizarre that it might be substantially different from Earth.
However, he believes that, in this early phase of the search, "We have to go with the kind of life we know".
Every cell we know of - even bacteria around deep-sea vents that exist without sunlight - requires water, so scientists are focusing their attention on worlds that have the potential to support liquid water.
These include Jupiter's moon Europa, which has a liquid water ocean under an icy crust, similar to the oceans under the Earth's Arctic and Antarctic ice fields.
"When we visit Europa, we want to go to very young places, where material from that ocean is being expressed on the surface," said Morgan Cable, research scientist at NASA's Jet Propulsion Laboratory.
"Anywhere like that, the chances of finding evidence of life goes up - if they're there."
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But water isn't the only thing scientists have to go on. They're also looking for chemical signs of life, such as two or more molecules in an atmosphere that shouldn't be there at the same time.
They are also using using space telescopes to take high-resolution photos of potentially habitable planets, to get a better idea of their composition.
While del Genio is optimistic that NASA is on the cusp of an alien discovery, other scientists are not convinced.
"It's been 20 years away for the last 50 years," said research scientist Andrew Rushby, of NASA's Ames Research Center.
However, he admitted: "I do think it's on the scale of decades," adding: "If I were a betting man, which I'm not, I'd go for Europa or Enceladus."