Do Aliens Exist?

Asking scientists if they believe we're being visited by aliens in little flying saucers is likely to get you a few funny looks. Yet there are a number of scientists interested in the question "Do aliens exist?". The key point here, is that the two questions are completely different.

Many scientists believe there is life elsewhere in the universe. Some even believe it's likely to exist on other places within our solar system, such as on Jupiter's moon Europa. That's not to say they believe in UFO's - this life could be as simple as bacteria.

The Arecibo Observatory is a radio telescope near Arecibo, Puerto Rico.
Image credit: NAIC 

Scientists have been searching of evidence of intelligent life on other worlds for more than a century - a search known as SETI. The most famous aspect of this search is probably SETI@home, which allows people to download an application onto their home computers, to analyse data from the Arecibo radio telescope (left). The program has been running for more than a decade (since 1999) and has attracted more than 5 million participants. They are searching for a radio signal that might be coming from an intelligent species, elsewhere in our galaxy - the Milky Way.

But is there likely to be another civilisation out there for us to detect?


The answer to this question is summed up nicely by the famous Drake Equation. Don't worry, I'm not about to launch into complex mathematics. This equation describes the number of civilisations in a galaxy which are able to communicate. It has seven terms, which are multiplied together to give us the answer. So all we have to do, is work out:

1. the average rate at which new stars are being created within the galaxy
2. the percentage of those stars that have planets 
3. the average number of planets capable of supporting life around each star that has planets
4. the percentage of planets capable of supporting life, on which life develops
5. the chance that life developing on a planet leads to intelligent life
6. the percentage of civilisations that transmit radio waves into space
7. the number of years civilisations capable of transmitting continue to do so

You've probably guessed, some of these terms are extremely difficult to work out. So... how close are we to the answer? Not very. We have estimates for them (you can find details of estimates for each term on Wikipedia, if you're interested), but only have "good" accuracy on the first one, and are working hard on the next two. The last four are essentially just reasoned guesses. So depending on how generous or mean you want to be with the values, we could be in a galaxy literally teeming with millions of intelligent communicating civilisations - or all alone in the Universe.

Artist's impression of the ESA JUICE mission
Image credit: ESA/AOES

The search for life on Mars has been going on for quite some time, but has leaned more towards the search for evidence of past life rather than existing life, as we learned more about the conditions on the planet. Robotic missions such as the NASA Curiosity Rover, which we had a wonderful guest post about back in August, are busy exploring the surface of Mars right now, and one of the things they're tasked with is proving whether or not Mars was ever capable of supporting life.

In ten years time, Europe will launch the Jupiter Icy moons Explorer – JUICE (right), as part of ESA’s Cosmic Vision 2015-2025 program. After an 8-year journey to the Jupiter system, it will study the moons of this giant planet. Three of those moons; Europa, Ganymede and Callisto, are suspected of having giant water oceans - not on the surface, but deep down inside. And where there's liquid water, scientists will be looking for signs of life. But first we have to find out whether or not these underground oceans exist at all.

JUICE will also seek to help answer the questions "what are the conditions for planet formation?", and "how does the Solar System work?" - two of the biggest questions in planetary science.

Even other stars are being looked at eagerly, as we discover more and more "exoplanets" in what is known as the "Goldilocks zone". Planets which are too close to their parent star are likely to be too hot to sustain life, while planets which are too far away are probably too cold. But just in the middle is a zone which is "just right", and astrobiologists are looking keenly for planets which fall into this category. But life is a complex thing, and we've found life in all sorts of strange and unexpected places right here on Earth - often places we believed life was impossible. So how large this habitable zone actually is, also remains an open question :)

EDIT (July 2014):
This video is now also available in video form, thanks to our partnership with Sheffield Hallam University.