The race to find alien life, intelligent life, or any life whatsoever, outside of the Earth has been the ambition of many an astrobiologist. In spite of the fact that no solid confirmation of extraterrestrial life has ever been confirmed, it appears that each new space mission launched has a mission to find life. So where are the most likely places to find extraterrestrial life outside of the Earth?
Over 22,000 meteorites have been documented on Earth and many of them have been found to contain organic compounds, some even containing what astrobiologists believe are fossil remains of bacteria in meteorites. If the fossilised remains of bacteria are confirmed then this gives excellent evidence to support the theory of panspermia, the idea that life came from outer space and planets exchanged life. Bacteria on Earth are known to survive in extreme environments, from sulphuric volcanoes to icy depths of the Arctic.
Mars has long been a target for extraterrestrial life hunters, but its arid and barren landscape has turned our attention away from finding little green Martian men to finding simpler life forms. One of the key ingredients to life is water, and with the recent discovery of water on Mars, the likelihood of finding life, albeit micro bacterial, has significantly increased. Because Mars's surface is pretty much uninhabitable with a thin atmosphere, cold temperatures, oxidising environment and harmful UV radiation, it's a pretty tough place to live. Below the surface, however, conditions may be more favourable and that is where we are potentially going to find life today. Perhaps in its past, there was life on the surface, but today it's going to be living below ground.
Europa has recently emerged as one of the top locations in the Solar System for potential conditions for supporting life. It is believed that an ocean could be hiding beneath Europa's icy surface, one that may contain high levels of hydrogen peroxide which can break down to form an oxygen-rich environment.
It is thought that bacteria could be living under the surface clustered around hydrothermal vents driven by geothermal energy, much like we see on Earth, or from gravitational forces caused by Jupiter's gravity.
There are numerous technical difficulties with sending a probe to Europa to analyse the liquid ocean, mostly concerning the depth of ice that it will encounter - up to 100km. NASA's Juno probe is due to go into orbit around Jupiter in 2016 will probably be tasked with helping to pin down potential landing sites for a Europa lander with its high-resolution cameras.
NASA scientists had declared Callisto a "dead and boring moon" until the discovery of a possible salty ocean beneath its surface.
NASA's Galileo spacecraft did a flyby of Jupiter's second largest moon in 1996 and 1997 and found that Callisto's magnetic field varied, indicating currents. In 2001, Galileo detected that an asteroid had struck the moon, forming the Valhalla impact basin. Usually, such an impact would cause intense shock waves to ripple through the planetary body, but Galileo couldn't find any evidence of this, leading scientists to theorise that a watery ocean could have softened the blow.
In keeping with the theme that water might equal life, astronomers believe that if such an ocean exists on Callisto, it's possible that complex life might also be in it.
Could this frigid moon provide a welcoming environment for life? Scientists are taking a closer look at this Saturn moon and finding more and more potential building blocks for very basic life there, despite Titan's surface temperature of -185Â° C.
NASA's Huygens probe detected what looked like liquid methane on the mini planet's surface in 2005. In May 2010, two teams of scientists announced that NASA's Cassini orbiter showed Titan is harbouring an unusual chemical combination consisting of hydrogen and acetylene.
When Cassini did a fly-by through one of Enceladus's geysers spewing ice and gas in 2009, the probe detected carbon, hydrogen, nitrogen, oxygen and even salt - all key elements for supporting living organisms. What's more, the temperature and density of the plumes could indicate a warmer, watery source beneath the surface.
Some estimates show that the Milky Way alone harbors around 400 billion stars and countless exoplanets, and that's just within our own galaxy. So there could potentially be billions of habitable cosmic bodies out there.
Kepler has discovered many, many, exoplanets, some of which are located within the habitable zone of their star, including KOI-1686.01 (Kepler Object of Interest) which is a similar size to Earth and it sits in roughly the same orbit from its parent star where liquid water can exist. Other potential exoplanets include Kepler-62e and Gliese 667 Cc.
The Orion Nebula is a stellar nursery in the Milky Way which has recently been investigated as a potential gold mine for finding life.
In May 2010, the European Space Agency's Herschel Space Observatory announced that the Orion Nebula showed signs of having life-enabling organic chemicals. Looking through the data collected by the telescope, astronomers were able to detect a pattern of spikes for various life-supporting molecules: water, carbon monoxide, formaldehyde, methanol, dimethyl ether, hydrogen cyanide, sulphur oxide and sulphur dioxide.
While life may not exist in the nebula itself, these organic compounds have to potential to seed nearby exoplanets with life giving compounds.
Life Around Dying Stars
In 2005, an international team of astronomers discovered that dying red giant stars could act like a defibrillator and bring icy planets back from the dead. This rebirth could also lead to new breeding grounds for life, albeit challenging.
Red Giants pose their own problems for life, including the possibility of tidal lock and severe solar radiation flares, but in terms of detectability, they show particular interest for planet hunters as a transiting terrestrial-class world in the habitable zone of a red dwarf is going to block a larger part of the star's light than a similarly sized world orbiting a larger star.
The vast Depths of Outer Space
The universe is an unimaginably vast space filled with planets, stars, nebulae, gas, dust, comets - and it's impossible for us to ever explore it all.
Are we even looking for the right "stuff"? Astrophysicist Stephen Hawking theorises that life could exist that is not carbon-based. If that's the case, is it possible that we have already found "life" and missed it as we were looking for "ourselves"?
The quest to find life beyond this planet continues. If aliens are found, let's hope they're friendly.