Alien life could be found within the next few decades, according to NASA scientists leading the exhaustive search.

In recent years, capabilities have snowballed; the discovery of Pluto in 1930 was once thought a once-in-a-lifetime feat, yet not even 100 years later, over 3,500 exoplanets from thousands of star systems have since been located.

Experts say the search is heavily guided by characteristics of our own planet, helping scientists to weed out inhospitable worlds from those that may have promise for life, such as the icy moons Enceladus and Europa.

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The detection of water completely changed scientists' opinions on the two icy moons, explains research scientist Morgan Cable. 'We thought Enceladus was just boring and cold until the Cassini mission discovered a liquid water subsurface ocean,' said Cable.

GOLDILOCKS ZONE In astronomy and astrobiology, the habitable zone is the range of orbits around a star in which a planet can support liquid water. This habitable zone is also known as the ‘Goldilocks’ zone, taken from the children’s fairy tale. The temperature from the star needs to be 'just right' so that liquid water can exist on the surface. The boundaries of the habitable zone are critical. If a planet is too close to its star, it will experience a runaway greenhouse gas effect, like Venus. But if it's too far, any water will freeze, as is seen on Mars. Since the concept was first presented in 1953, many stars have been shown to have a Goldilocks area, and some of them have one or several planets in this zone, like 'Kepler-186f', discovered in 2014. Advertisement

'Before we go looking for life, we're trying to figure out what kinds of planets could have a climate that's conducive to life,' said Tony del Genio of NASA's Goddard Institute for Space Studies.

'We're using the same climate models that we use to project 21st century climate change on Earth to do simulations of specific exoplanets that have been discovered, and hypothetical ones.'

There are many factors that contribute to a planet's potential habitability, including proximity to its star.

This dictates whether the planet has the right conditions to sustain liquid water; if it's too close, or too far, the surface could be dry and barren, or completely frozen.

And, from what's known about life on Earth, water is key.

'Everywhere we look, whether it's a desert or Antarctica or the deepest parts of the ocean or the deepest parts of Earth's crust that we've explored, as long as there's a tiny speck of liquid water, there's life,' NASA explains in a new video, How to Find a Living Planet.

'And because of that, it's been central to NASA's search for habitable environments elsewhere.

'It's why scientists get excited about water spewing up from the icy moons of Europa and Enceladus in our outer solar system.

'Not only could they have water, they could have global oceans like the ones we have here on Earth.'

The detection of water completely changed scientists' opinions on the two icy moons, explains research scientist Morgan Cable of NASA's Jet Propulsion Laboratory in Pasadena, California.

'We thought Enceladus was just boring and cold until the Cassini mission discovered a liquid water subsurface ocean,' said Cable.

Characteristics of our own planet help scientists weed out inhospitable worlds from those that may have promise for life, such as the icy moons Enceladus and Europa (shown)

The researchers are also looking for certain atmospheric gases, NASA explains.

In particular, they're looking for environments that contain both oxygen and methane.

When these two gases are found in the same atmosphere, 'you've got something special,' the space agency says.

'There are ways to build up oxygen and methane in a planetary atmosphere, but the only way you could have both in the same atmosphere at the same time is if you produce them both super rapidly,' said Shawn Domagal-Goldman, of NASA's Goddard Space Flight Center.

NEWLY FOUND PLANET JUST 11-LIGHT YEARS AWAY COULD BE THE ‘CLOSEST KNOWN HOME FOR LIFE’ Astronomers working with the European Southern Observatory's High Accuracy Radial velocity Planet Searcher (HARPS) at the La Silla Observatory in Chile found that the red dwarf star Ross 128 is orbited by a low-mass 'exoplanet' every 9.9 days. The star was named after the Californian astronomer Frank Elmore Ross who discovered it. The Earth-sized world is expected to be temperate, with a surface temperature that may also be close to that of the Earth. Ross 128 is the 'quietest' nearby star to host such a temperate exoplanet. A planet the same size of Earth and with a similar surface temperature may be 'the closest known comfortable abode for possible life'. The newly discovered world, named Ross 128b, was found orbiting a red dwarf star 11 light-years away from Earth (artist's impression pictured) With the data from HARPS, the team found that Ross 128b orbits 20 times closer than the Earth orbits the sun. But, despite the proximity, Ross 128b receives only 1.38 times more irradiation than the Earth. As a result, Ross 128 b's equilibrium temperature is estimated to lie between -60 and 20°C, thanks to the cool and faint nature of its small red dwarf host star, which has just over half the surface temperature of the sun. While the scientists involved in the discovery consider Ross 128b to be a temperate planet, uncertainty remains as to whether the planet lies inside, outside, or on the cusp of the habitable zone, where liquid water may exist on a planet's surface. Advertisement

'And the only way we know how to do that is through life.'

In the search for alien life, it's also important to identify false possible, the experts explain.

When studying a potential habitable planet, 'life has to be the hypothesis of last resort,' Cable said.

'You must eliminate all other explanations.'

It's hoped that efforts with advanced instruments will help to uncover more details about exoplanets that appear just a group of pixels in the images.

By collapsing images from NASA's Earth Polychromatic Imaging camera on the National Oceanic and Atmospheric Administration's Deep Space Climate Observatory (DSCOVR), for example, one researcher is looking to simulate interference that might be seen in an exoplanet mission.

By collapsing images from NASA's Earth Polychromatic Imaging camera on the DSCOVR craft, for example, one researcher is looking to simulate interference that might be seen in an exoplanet mission

'I think that in 20 years we will have found one candidate that might be it,' says del Genio.

In a nod to his assumptions as a young scientist that planetary discoveries would be rare, the researcher also added, 'But my track record for predicting the future is not so good.'

Many have their sights on Enceladus and Europa.

'It's been 20 years away for the last 50 years. I do think it's on the scale of decades,' said research scientist Andrew Rushby, of NASA Ames Research Center.

'If I were a betting man, which I'm not, I'd go for Europa or Enceladus.'