Certainly, most of our depictions of aliens seem to assume that extraterrestrial life will be very similar to us:

• It'll be physical - made of matter
• It'll walk on legs and pick things up with hands (or tentacles, or suckers)
• It'll talk using some kind of mouth and see using eyes
• It'll take on energy by eating food
• It'll need to breathe an atmosphere in order to live
• It'll be social and have a developed society - they’ll be coordinated and work together

These assumptions might be true, but they also might be completely false.

The words ‘alien’ and ‘extraterrestrial’ simply means ‘of or from outside the Earth or its atmosphere’ (terra is a Latin word which means ‘Earth/land’ and the word 'extra' in this case means ‘outside or beyond’). So alien life doesn’t just mean the non-Earth equivalent of humans - it could be any kind of living thing. It might be another kind of being, or it could be teeny tiny microbes, or invisible particles of something we’ve never even thought of, or giant elephant-marshmallows.

One thing we do know is that the way a life-form looks and how it works are driven by its environment. So imagine a planet like Jupiter - one that’s enormous, has extreme temperatures (about -145 °C in its clouds and around 24,000 °C in its core!) and is mostly made of gas.

This image of Jupiter was taken in April 2014 by the Hubble telescope. Image credit: NASA, ESA, and A. Simon (Goddard Space Flight Center).

Could a life-form like a human evolve on a planet like this? It’s pretty unlikely. There wouldn’t be any need for legs (there's nothing to stand on), or hands (there's nothing to pick up), or the ability to eat solid food (because the only solid items are you and your fellow life forms). On this planet, it'd be easier to imagine a life form that floats in the atmosphere. One that ‘feeds’ on radiation or raw hydrocarbons. One that communicates (if it needs to communicate) by touch, or by leaving chemical traces of itself in the clouds, or by emitting radiation pulses. All in all - something very different to us.

We don't even have to imagine far-off planets to think about different types of life form. We assume that humans are a good solution to the problem of designing a life form that fits our earthly environment - but what about some of the other life-forms on our planet? Some of them are very different to us but all of them are equally good at being inhabitants of the Earth.

Imagine this - there’s an organism that can:

• Live for thousands of years
• ‘Eat’ by directly absorbing radiation
• Remain alive though most of it is dead, inert matter
• Survive huge wounds and tolerate having large pieces of itself cut off
• Live outdoors at all times and survive in the harshest climates

What would you think? Is it an alien? Is it an exotic life form from another planet? Or is it just an ordinary tree?

If one planet, and one evolutionary chain, can produce life forms as physically different as humans and trees, we have to think deeply about how different alien life could be. Trees and humans have a surprising amount in common - our chemistry is based on complex carbon molecules, our bodies are made of cells, those cells need both water and sugar in order to stay alive, etc. So imagine how different an alien could be if it shares none of these things with us.

Another thing to consider is that alien life might be so alien we don't even realise it’s alive...

"Finally, they came to the granite. "This is alive," Leavitt said. "It is living, breathing, walking, and talking. Only we cannot see it, because it is happening too slowly. Rock has a lifespan of three billion years. We have a lifespan of sixty or seventy years. We cannot see what is happening to this rock for the same reason that we cannot make out the tune on a record being played at the rate of one revolution every century. And the rock, for its part, is not even aware of our existence because we are alive for only a brief instant of its lifespan. To it, we are like flashes in the dark." (The Andromeda Strain, Michael Crichton)

In sum, trying to predict what aliens might look like is difficult when we don’t know what their environment will be like. They might have three legs or green skin and they might even have feathers.

However, the latest research suggests that we can use Darwin’s theory of natural selection to predict how aliens evolve, and it’s very similar to how we evolved. Scientists think that we evolved from simpler organisms during a sequence of events known as major transitions. These major transitions are bought about when extreme conditions allow a group of separate organisms to evolve into a single complex organism. Like us, the theory predicts that these complex organisms will have separate organs, each with a specific function, which are supported by mechanisms such as a nervous system and a blood supply. So even if they don’t look like us on the outside, they’re probably a lot like us on the inside.

What do you think?

Have you ever wondered how many packets of jelly would it take to fill an Olympic swimming pool?

How about how many piano tuners there are in Chicago? How many pizzas will be ordered in the UK this year? If it’s worth turning off the tap while you brush your teeth? And how many planets with intelligent alien life there are in our Galaxy? 

These problems and ones like them are called Fermi Problems.  They are named after Enrico Fermi, an Italian physicist who was known for his ability to make quick, accurate guesses as to the answer of complex calculations with little or no prior information.

His trick was to break the question down into several components, roughly estimate each of those components and assume that the over-estimate of some aspects would be cancelled out by under-estimating others.  Let’s look at this method in relation to the jelly-filled swimming pool problem.

Firstly we need to break this problem down into several components... namely, what is the size of an Olympic swimming pool and how much jelly can one packet make? You might be tempted to google for this information, but Fermi could solve this question without looking anything up. He approximated numbers to the nearest power of 10, to guess how big or small or common something was. Under his logic, reasonable estimation could be that I am 200 cm tall; the distance between my left and right fingertips is equal to my height; my heart beats 100 times a minute, and I have 100,000 hairs on my head.

Applying this in the context of jelly – I know one packet makes enough jelly to feed my family dessert, so I know there is roughly 1/2 litre of jelly and that is 500cm^3 in volume. What about an Olympic pool? Well, I’d expect that to be deep enough to cover my head (200 cm), wide enough to fit 10 lanes in, and for each lane to be my handspan wide (2000cm = 20m) and long enough for a good race (50m = 5000cm).  Let’s put this all together:

Number of packets needed to fill pool = Volume of pool / Volume of packet
                                                            = 5000 cm x 200 cm x 2000cm / 500cm^3
                                                            = 4 million packets

So how close are we? Well, a real Olympic swimming pool holds 2,500,000L of water and Harley’s jelly makes up 570ml of water, giving us 4.38 million packets needed. So we were pretty close!

We can also use this method to address other problems – for example here is a video on how many piano tuners there are in Chicago:

https://www.youtube.com/watch?v=0YzvupOX8Is

How does this link back to aliens?

In 1959, Frank Drake was working on systematic searches for signals from alien life and he wanted to make an estimate of how many detectable alien civilisations there should be in our Galaxy. He did this by approaching it as a Fermi problem – breaking the problem down into smaller components and roughly estimating those components.  

He defined N to be the number we want to find, the number of alien civilisations in our Galaxy capable of communicating with us.  Before we can have a civilisation, we need a planet capable of supporting life – so let F be the rate of formation of stars in our Galaxy, P be the fraction of those stars that have planets and E be the fraction of planets that can support life. Then F x P x E is the rate that planets that can sustain life form. But not all planets capable of forming life actually develop intelligent life, so we add two more factors: L the fraction of planets capable of forming life that actually produce life and I the fraction of the planets with life that goes on to form intelligent life.  So now we have F x P x E x L x I. Does all intelligent life start sending signals out into the Universe? Possibly not, so we add two more components: C the fraction of intelligent alien civilisations that release detectable signals and, crucially as species don’t live forever, T the length of time they send these signals out. Putting this all together we get the Drake Equation:

                N = F x P x E x L x I x C x T

Like with the jelly and piano tuner problems, all that is left is to estimate each of these values and plug them into the equation. However, these values are much harder to estimate – scientists who look into deep space have a good idea of how quickly stars form, and how many planets there might be, but we don’t have any examples other than Earth for considering the likelihood of intelligent life. 

When Drake tried his first estimates in the equation, he predicted our Galaxy would have between 1,000 and 100,000,000 civilisations – so where are they?

But 'Unidentified Flying Objects' are just that - unidentified. We’ve no real way of knowing what they are. Military planes, asteroids, space debris, a satellite flung out of orbit, a strange bit of weather, a trick of the light, video special effects… or life from another planet?

So here are some UFO sightings that have actually been caught on camera - take a look and make up your own mind…

1. London, UK https://www.youtube.com/watch?v=QDIF-ZwJbF0 Strange lights appeared over BBC’s Radio 1 building in London in June 2011.

2. France https://www.youtube.com/watch?v=fe-mvQshCRw A strange black object was spotted in the sky somewhere in the south of France in April 2007.

3. The US https://www.youtube.com/watch?v=_6Uibqf0vtA A tourist on a trip to Yosemite Park in California (quite near to Area 51, just over the border in Nevada) saw strange lights in the sky in 2002.

The story is never straightforward - there are so many possible explanations for what gets caught on camera. But are any of them actual alien sightings? Well, that's for you to decide.

But not every environment is able to produce life, as it takes quite a precise combination of astrophysical and geological conditions. And the probability of getting that combination of conditions is very, very small. Now we know that Earth has them - which is why we’re here, hurrah - but most scientists are doubtful that they exist anywhere else. This makes it pretty hard for there to be life anywhere else. 

This idea, called the Rare Earth Hypothesis, is a favourite of those arguing that aliens don’t exist. They’re saying that the odds suggest that Earth - and the life it supports - is one of a kind.

So let’s investigate the hypothesis a little - just what are the conditions of our Earth that have made it possible for us to exist?

How to grow a human

First things first: location, location, location. We needed to be in a Galaxy with the right types and quantities of the elements it takes to build a planet - iron, magnesium, carbon, oxygen, potassium, etc. We also had to find a spot clear of black holes, supernovae and X-ray/Gamma radiation spewing neutron stars (phew) as that stuff would have destroyed us before we even got started.

Plus we had to be positioned perfectly in amongst all the other planets so that we’d be close enough to get cover from any incoming asteroids but far enough away that their planetary masses don’t interfere with our gravitational field and orbit (so we can keep rotating around the sun.)

Speaking of the sun, that’s another key thing - light. We needed to get ourselves a star just the right size and just the right distance away that it could provide intensity of light without ionising the Earth’s atmosphere with radiation (which would slowly but surely kill us off). We definitely managed that perfectly - if the sun were even a speck closer we’d probably burn alive and a speck further away would see us freeze. This brilliance of distance is called The Goldilocks Principle because (sort of like the blonde girl’s porridge) it’s not too far or too near - it’s just right. And it’s stayed just right for the whole of the Earth’s lifetime - between 5-10 billion years.  

Size is also an issue. Of the eight planets in our solar system, Earth is the only one that’s the right size and structure for nurturing life. It’s able to keep a stable atmosphere on and around the planet and keep the right balance of different temperatures through the crust and core. This, in turn, greases the wheels of the tectonic plates in the upper layers of the Earth, keeping them moving. And although that might not seem like a big deal, scientists believe that the large-scale motion of those plates is essential for recycling carbon matter - making our air breathable and keeping us alive.

So are we a freak of nature?

Well there’s no denying that it’s pretty lucky we’re here - our planet definitely had to beat a lot of odds to produce us. But surely we’re proof of the fact that although our planet’s conditions are unusual and unlikely, they’re not impossible?

Professor Stephen Hawking (University of Cambridge) reminds us that probability only describes what’s likely to happen, not what will happen. And he thinks that if the Universe can form one planetary system like this, there’s no reason to think that there aren’t others. 

In fact, just a few years ago the discovery of Kepler-186f was announced, a planet 560 light-years away that’s the exact same size as Earth and orbiting a star (like our sun) at just the right distance. Ongoing research has shown that the number of potentially habitable planets is now 15.

Now that may be the first place we’ve found that’s even vaguely similar to our planet -  but the edge of our Universe does stretch over 46 billion light-years away. And there could even be other universes for all we know! So isn’t it actually quite hard for us to really say what’s rare and what’s common?

Professor Stephen Hawking reminds us that probability only describes what’s likely to happen, not what will happen. And he thinks that if the Universe can form one planetary system like this, there’s no reason to think that there aren’t others. 

Plus, even if we accepted that Earth was one of a kind, does that necessarily reduce the chances of aliens existing? It would mean that there wouldn’t be any other planets capable of producing human life - but other planets (or rocks, or stars) might be able to produce a different kind of life. And that’s the thing that makes it alien - it’s different.

So, the conditions of our Earth are pretty unusual - but are they rare, or not? And are they the only conditions that could produce life - or could aliens come into existence in any number of different environments? Just a few things to ponder...