If it is just us... seems like an awful waste of space - Carl Sagan

In May 1950, the great physicist Enrico Fermi and some colleagues were walking down for lunch from their lab at Los Alamos when they got to discussing flying saucers because there had been a rash of "sighting" reports. Then somebody mentioned a cartoon in The New Yorker which showed little alien beings carrying away trash cans - a comment on the recent trend of trashcan stealing in the city. The scientists were still shooting the breeze and settling down for lunch when suddenly Fermi asked "But, where is everybody?" Everybody burst out laughing, but they all knew what he was talking about - he was referring to extra-terrestrial life.

This is now known as Fermi's Paradox which goes like this: given that there are about 70 sextillion (seven followed by 22 zeroes) stars in the visible Universe, and each would have planets, and even if one in a million managed to evolve life in the 13. 7 billion years since the Universe was formed, then there should be at the very least a million civilisations in the Universe. So how come we have never seen any or heard even a whisper from any one of them?

Ever since they started gazing at the cosmos, people have wondered about life elsewhere. Science fiction imagination thrives on swamp monsters, flesh-eating plants, "little green men", and intelligent swarms of microbes. In the initial years of space exploration even scientists were convinced that within our solar system life might exist on Venus or Mars.

But since the 1980s, Mars and Venus have been ruled out. Venus is like a pressure cooker, with an average surface temperature of 460 degrees Celsius, surface atmospheric pressure 93 times that at sea level on Earth and the "air" comprising of over 95 per cent carbon dioxide. Something like what Earth would be if global warming continues indefinitely. Mars is the opposite. Its surface temperature ranges between -87 and +20 degrees C, its surface pressure is just two-thirds of Earth.

But meanwhile, spectacular advances in observation technology, powered by orbiting observatories like the Hubble and Chandra, led to the discovery of the most probable hosts for extra-terrestrial life - exoplanets, that is, planets attached to stars other than our Sun. Now there is a dedicated observatory in the sky - the Kepler mission - which has identified 2, 321 unconfirmed planetary candidates associated with 1, 790 stars in the first 16 months of its work. Of these, 777 have been confirmed as true exoplanets. So, the potential hosts are rising in numbers, just as theory predicted.

But all these exoplanets are too far out in deep space for any physical verification. One has to wait for a message or some sign of intelligent activity. Hence the search for life turned back to our own backyard - the solar system. Here is a look at the current top candidates for the search, and the reasons why...

MARS |
But wasn't Mars ruled out? Yes, but here's the thing: life could have existed in the past, and some remnants can be found, not on the surface but below the surface.

Mars is dry and has a very thin atmosphere, leaving its surface exposed to deadly solar radiation. But it also has huge icecaps on the two poles. And, there may be underground vents or reservoirs of water, that essential ingredient of life as we know it. In the past, the atmosphere may not have been so sparse and life might have existed, at least in microbial form. For that matter, it may exist underground in these aquifers. Curiosity is looking for just this kind of circumstantial evidence - minerals that may have dissolved in water once.

VENUS |
Once a favorite candidate, its insane atmosphere and temperature, its sulfuric acid clouds and angry volcanoes make it one of the most hellish environments around. So, why include Venus?
Recently, scientists have proposed that extreme microbes could exist in the dense clouds that swirl all over the planet. At 50 kilometers above the surface, pressure would drop to Earth-like levels and water vapor would condense into small acidic droplets. Acid-loving bacteria may possibly live a trapeze artist life there. There's no evidence yet, except for dark patches in these clouds, but worth investigating.

TITAN |
If Venus is a hot hell, Saturn's moon Titan is a frozen hell. It is the only moon in the solar system that has a nitrogen- and methanedominated atmosphere, analogous to Earth. But the mean surface temperature is a daunting -180 degrees C.

Until the Cassini spacecraft surveyed it in 2004, and the Huygens probe landed on its surface in 2005, Titan was a mystery because of the permanent orange smog that wreathes it. Huygens found a bizarre world of methane lakes and seas, floating on a surface made of water ice and rocks with some frozen ammonia thrown in.

Some scientists have suggested that the sun's weak light just might create acetylene in the atmosphere which can be used as food by microbes swimming in frigid methane lakes below. However, it is possible - say some scientists - that some time in the future when the Sun gets bigger and hotter, life might evolve on Titan's surface.

EUROPA |
Jupiter's moon Europa is an ice-ball with average surface temperatures at -170 degrees C. It is a rocky body covered entirely with ice, crisscrossed by lines thought to be cracks caused by volcanoes and quakes. There is a very thin atmosphere of oxygen.

Although legendary space probes like the Pioneer and Voyager pairs sent back fly-by pictures of Europa, it was only after the probe Galileo started its eight-year-long Jupiter mission in 1995 that the structure of Europa was revealed.

Scientists have evidence that the surface ice gives way to water further down, making a water/ice wrap 160-km thick. In the underground ocean, life might have evolved, probably near vents of hot gases from a heated core. Colonies of bacteria and tube worms have been found on the Earth near hydrothermal vents in deep ocean trenches like the Galapagos Rift in the Pacific or Lake Vostok in the Antarctic.

ENCELADUS |
This tiny moon of Saturn is full of intriguing possibilities because most of its surface is covered with water ice, cracked at many points giving the moon its characteristic 'Tiger Stripes'. It is geologically active, shooting huge geysers of water. The Cassini probe flying by in 2005 found traces of carbon, hydrogen, nitrogen and oxygen - the building blocks of life - in the flimsy atmosphere.

CERES |
The largest asteroid in the solar system, Ceres is going around the sun between Mars and Jupiter. It is just 4 per cent of the mass of our moon. It has a rocky core surrounded by a thick ice cover. Scientists think that life may have evolved on it in hydrothermal vents deep inside the liquid water ocean hidden by the ice crust.

IO, CALLISTO AND GANYMEDE
Of these three Jupiter moons, Io is one of the most extreme worlds with over 400 hyperactive volcanoes spewing sulfur fumes and lava on to a frozen sulfurous surface. There is virtually no atmosphere, allowing high radiation to hit the surface as measured by the space probe Galileo. Scientists believe that there might be underground sanctuaries with some leftover water from earlier times. Shielded from deadly radiation, microbes could possibly develop there.

Little is known about Callisto and Ganymede, except that both show slight variations in magnetic fields. This may mean that there are underground oceans, which are probable habitats for microbial life. Both are frozen bodies with trace atmospheres. Ganymede is the largest moon in the solar system, having twice the mass of our moon. Callisto is the most battered moon of all, entirely covered with craters formed due to unknown celestial bombardment 4 billion years ago. Both were studied by the Gallileo and Cassini space probes.

As can be seen from this, no evidence of life has been found anywhere yet. And potential candidates are mostly relying on the presence of hydrothermal vents in icy oceans for being considered. In fact, there are other candidates who have similar features but too little is known about them - like Triton, the Neptunian moon made of frozen nitrogen, water ice and dry ice, famous for its 'cantaloupe terrain'. We only know what Voyager 2 saw of this curious object as it flew by in 1989.

In the coming years, much more will become known as various space probes approach their destinations - Dawn will reach Ceres and New Horizons will reach Pluto in 2015, Cassini will closely observe Titan and Enceladus this year, Juno will arrive at Jupiter in 2016, while Stardust-NeXT and Rosetta will study passing comets.