How can we locate other life in the universe?

Written by TomBurnett88
The question that has intrigued humanity since we first looked up at the stars is perhaps also fundamental in our understanding of where we as a species stand.

The question that has intrigued humanity since we first looked up at the stars is perhaps also fundamental in our understanding of where we as a species stand.  Is there anybody else out there?
In my opinion, based on the vast expanse of the observable Universe, the answer is almost certainly yes.  If would be a terrifyingly depressing thought if this tiny blue dot in an ocean of darkness was the only bastion of life in the Universe.  
But what exactly are we looking for?  What are the criteria for a planet to be able to host life as we know it?  And how can we hope to find that life?
A solid rocky planet
Most of the planets outside our system that we have detected so far are gas giants. For life as we know it to form we would need to find planets with a rocky, solid surface, such as the four terrestrial planets in our own system (Mercury, Venus, Earth and Mars).
The fact that most of the planets we’ve detected are gas giants is not necessarily a hindrance to life in the universe, it is simply due to limitations in our technology that confirmed gas giants are the majority. Since such planets tend to be much bigger, they are easier for us to detect than smaller Earth-like ones. New telescopes are coming online all the time and this is a technical obstacle that we are slowly overcoming.
Water (or another liquid)
Water is a pre-requisite for life as we know it.  We drink it to sustain ourselves and our bodies are made up largely of it. Life on Earth first formed in our world’s vast bodies of water, and it seems logical to assume that this would also be the case elsewhere.
It is this theory that leads to us establishing whether planets are in the goldilocks zone around their star. This is the distance from a star where a planet can host liquid water on its surface. Too close to the star and it merely evaporates, too far away and it freezes. Earth inhabits this zone around the Sun. The exact distances vary between types of star.
Of course these criteria are not mutually exclusive. Some planets and moons further out may have water beneath their surfaces, and just because a rocky planet is within the goldilocks zone does not mean that it hosts bodies of water. But they are as a good place as any to start our search.
An atmosphere
Life as we know it needs to breathe, the chemicals it breathes would be part of the atmosphere.  Not having an atmosphere creates a sterile, dead, world, thoroughly hostile to life – such as the moon. We know of many planets with an atmosphere, but again having one does not guarantee life. Venus has an atmosphere and it is hard to envisage a more horrific planet as far as human survival is concerned.  
Most large animals on Earth produce methane to such a level that it is detectable in the planet’s atmosphere.  If we could detect large levels of such chemicals in an exoplanets atmosphere it could be an indication that there is life, at quite an evolved level, on its surface.
Being able to measure the composition of atmospheres on planets billions of miles away sounds like a tall order, and it is. Science however is catching up. The ATLAST space telescope, set to be deployed a million miles away from Earth by 2030, will greatly assist in assessing the atmospheres of distant worlds.
Within two decades we may find out that a planet ten light-years away is playing host to a herd of alien-buffalo. Unlikely maybe, but far from impossible. These are all ways that we could assess whether a planet is likely to host life, among others such as Tectonic plate movement and the presence of a large moon. What level that life is likely to be on such worlds is anyone’s guess.  
But what can we do to increase our chances of finding intelligent life?
SETI, the search for extraterrestrial intelligence, monitors the skies constantly, crossing a wide variety of radio wave lengths. Their hope is that they will detect a far-flung signal from some corner of the stars. So far, despite a few promising signals that were later disproved, they have found nothing.  
We’ve sent signals intermittently ourselves, as well as unintentionally beaming countless hours of television and radio out across the galaxy. For all we know a civilisation 70 light years away has just now started searching for signals themselves and the first thing they’ve heard of Earth is the ranting of Adolf Hitler (not a great tourist draw). 
Again, a lack of signals does not mean intelligent life isn’t out there and chattering away. There are dozens of reasons we may not be picking them up – lack of frequencies, they may not use radio, they may communicate telepathically, we just don’t know.
We don’t know much about how abundant life elsewhere in the Universe is, but I’m sure it is out there.
What do you think? Have your say in the comments section below.
 Sweetie187/ Flickr