WHY WE WILL
1. Science has shown the "building blocks" for life - water, organic chemicals - are widespread in the universe.
Planetary bodies in our solar system such as Mars, our moon and several Jovian moons have been shown to support significant amounts of water. Asteroids have been shown to contain complex organic molecules that could form the building blocks for life, and these fall as a cosmic "rain" on planetary surfaces.
2. Nasa's Kepler space telescope has identified thousands of Earth-like planets orbiting distant stars, and some estimates now claim that 10 to 20 per cent of stars may support Earth-like planets.
Couple this with the knowledge that billions of stars out there potentially support planets and the odds start to look good.
Nasa has an on-going commitment to planet hunting with the new James Webb telescope and so the pace of discovery for new planets is certain to increase.
3. Microscopic bacteria and other microbes on Earth have been shown to thrive under extreme conditions such as in boiling hot pools and in Antarctica's icy deserts.
These are good analogues for the hostile surface of other planets.
Microbes have even been shown recently to survive space travel on the outside of the International Space Station.
4. Nasa and the European Space Agency have advanced plans to send landers to Mars within the next few years with the direct aim of searching for traces of life. Although likely a "dead" planet now, Mars may once have supported primitive microbial life in its extreme habitats.
Nasa's chief scientist Ellen Storfan has made a bold prediction that they will deliver "definitive" proof of alien life by 2030.
5. Our Earth will be uninhabitable within the next few hundred million years and so we need a new home. As an optimist I have to believe we will find one and that likely means encountering other life forms.
In a nice interaction between science and faith the Pope has stated he would be happy to baptise an alien - should we meet them - and this hints at the need to approach this issue from multiple perspectives, including scientific, religious, philosophical and societal.
WHY WE WON'T
1. Some argue the series of extremely fortuitous events that led to life on Earth is unlikely elsewhere.
Life on Earth required the right ingredients, such as water and organic chemicals, likely from asteroid impact; stability over time, as we have been free of catastrophic asteroid impact since the lunar cataclysm and a radiation shield, as radiation is fatal to all life and so the Earth's magnetic field and atmosphere were vital to evolution of life.
Further, we've had plate tectonics to recycle carbon and other elements and the evolution of oxygenic photosynthesis to create conditions for complex aerobic life to evolve.
2. Scientists can not agree how life itself originated, or what alien life might be like, so we are not quite sure what signs of primitive life to look for .
Although it's widely agreed bacteria were the first life on Earth, scientists cannot yet agree how the first cells evolved, and the exact role of DNA, RNA and proteins in this evolution.
There have also been a few instances of "crying wolf", such as the Clinton announcement of microbial fossils in a Martian meteorite on Earth that is now widely discredited.
3. Enrico Fermi reasoned that intelligent aliens do not exist because we have not heard from them (Fermi's Paradox).
Basically this theory suggests that in our galaxy if one civilisation developed interstellar travel capability it would only take a few generations for the whole galaxy to be contacted, yet this has not happened.
4. Humans are not making much effort to make themselves known -- Seti (The search for extraterrestrial intelligence) has looked at only a fraction of our local cosmic real estate, and space probes are only just leaving our solar system.
The budget for this type of science is tiny. Kepler can scan thousands of stars at a time but this is still a minute part of the known universe.
Even Nasa's budget for tracking potentially catastrophic asteroid impact to Earth, via the Nasa Near Earth Object Programme, allows survey of only a tiny fraction of asteroids that pass through our solar system.
5. Humanity may not last long enough to meet aliens.
Although Earth will in theory be habitable for hundreds of millions of years more, natural disasters, pollution, climate change and conflict may make it uninhabitable sooner so humans may not survive.