It’s one of the first big questions someone learning about space, especially a young scientist, will ask: Why haven’t aliens visited? This article contains a lot of speculation based on the ideas of leading scientists. At any time, we could discover a new scientific fact or idea that will completely change the whole way we think about the cosmos. However, this is a summary of the most popular ideas so far.

Why do we think aliens would exist?

This is a good starting question. The reason many scientists think aliens – that is to say, life like the animals on Earth, but on other planets – must exist is because of probability. Even if they’re extremely rare, the universe is so big, and there are so many planets like the Earth, that it seems reasonable to assume that extra-terrestrial (alien) life will have evolved somewhere. In fact, if the universe is infinite, then they must have! Aliens may have evolved like animals – alive and complex, but not advanced enough to try contacting our planet. When we talk about aliens that we think are like us, we describe them as “intelligent life”. We don’t think aliens exist in our solar system – we’ve explored our planets quite closely, and haven’t found a shred of evidence. While it’s possible we may discover something unexpected, most scientists work off of the assumption that we need to look elsewhere for our neighbours.

How would intelligent life contact us?

There are a number of ways that aliens might get in touch with the human race. They could visit Earth in spaceships, much like in movies. Or, if they’re anything like us, they might send space probes, just as we have sent rovers to Mars or satellites to Jupiter and Saturn. There are a couple of types they might use. A “Von Neumann” probe uses resources it finds to replicate itself – in a few years you might have a whole flock of them reproducing across a solar system. A “Bracewell” probe, meanwhile, is sent out into space and designed to talk to aliens on behalf of whoever designed it.

Alternatively, they may try to get in touch by sending radio signals. The important thing to remember is that space is big; even if aliens orbited the closest star to us, and travelled at the fastest speed allowed by physics, it would still take them four years to reach us. For that reason, they may try to send us a radio signal instead. An alien civilization wouldn’t really be able to gain anything from sending us a radio signal. It’s just a way to say “hello!” to us from a huge distance. They may try to describe themselves, or share some of their knowledge or history, depending how complex the signal is.

The organisation SETI (Search for Extra-Terrestrial Intelligence) exists for this reason. They listen out for signals from space using huge satellite dishes. Occasionally, they send out signals from Earth too – us saying “hello!” to anyone who might be listening. We haven’t heard anything yet, but who knows what the future may bring?

The Fermi Paradox, and its solutions:

Italian-American physicist Enrico Fermi pointed this out in the 1950s. There are half a trillion stars in our galaxy alone, and billions of galaxies with similar numbers of stars throughout the universe. Many of these stars have planets that would be suitable for aliens to live on; water, a good temperature, a similar size, and so on. So why haven’t they got in touch? There are a number of possible solutions that have been put forward over the years:

They might not be clever enough

It may be the case that even though alien animals evolve in many places, none become intelligent enough to build things like radio telescopes; or even if they have, they simply haven’t thought of the idea yet.

Alternatively, we might not be clever enough. After all, we’re assuming that aliens will try to contact us using radio signals. However, they might have discovered a new way to communicate across the universe – and assume we’re using that too.

They might not be loud enough

The reason we think we should be able to hear alien civilizations is because we use radio signals, which leak out into space. We think they probably use the same. However, as a species, we have been moving towards other, “less leaky” ways of talking to each other, such as fibre optic cables. Perhaps aliens have been doing the same, so very little noise is coming from them; imagine trying to work out what song someone was listening to if they were using headphones across the street.

They might not have enough resources

There are many times when we have discovered a new barrier to exploration of space – for instance, the speed of light. It may be that we have simply underestimated the amount of material it takes to effectively get in touch with another world, a bit like not having enough credit on your phone to call your parents.

They might have been destroyed by natural disaster

The human race is bombarded by floods, earthquakes, tornadoes and hurricanes every day. It’s possible that aliens have been wiped out by natural disaster – or even destroyed by a meteor, like the dinosaurs.

They might have been destroyed by themselves

Some thinkers have proposed that aliens might be doomed to destroy themselves when they reach a certain level of advancement; and with climate change, nuclear weapons, and discrimination on our own planet, you can see why. Don’t panic, though – it’s equally possible that that isn’t true, and that alien species live for an extremely long time!

They might live underwater

Despite so much of our planet being water, our planet’s 32% landmass could theoretically be unusually large. Perhaps on other planets, it’s usual for more or less the entire surface to be covered in water, and aliens haven’t developed features that suit space travel, such as hands and large brains. Alternatively, perhaps alien intelligent mostly occupies subsurface oceans, such as those theorised to exist under the icy surface of Europa. Radio signals would struggle to escape these surfaces, and civilizations would effectively be invisible. They would, however, be largely protected from impacts and stellar radiation.

They might live on too big a planet

It’s already difficult to accumulate enough power to get off of Earth. If aliens live on a bigger planet, they might not be able to escape its larger gravity. That wouldn’t prevent them from sending radio signals, but it would mean they couldn’t travel in a spaceship or send a space probe.

They might have deliberately left us alone

The “zoo hypothesis” is the idea that aliens have left us alone on purpose. It’s like how when people explore other countries, they are asked to leave the animals alone, and not interfere in their ecosystems. However, it would only take one species of many to change their mind for this to break down, so this seems unlikely.

They might just not want to meet us

We assume that because we want to contact aliens, they’ll want to contact us. But perhaps they don’t like other planets as part of their culture. Perhaps they don’t want to colonize or explore elsewhere in the way that we do. Perhaps they don’t even have scientific curiosity.

We might be too young

We’ve only been broadcasting radio signals since the very end of the 1800s. We might not have made enough of an impact on the universe to be detected yet; we’ve done the cosmic equivalent of clearing out throats before the speech begins.

We might be extremely rare

This is Professor Brian Cox’s theory, and one I find convincing. The process of evolution is extremely complex, and to some extent relies on chance. For that reason, intelligent life developing in the way we did may be extremely unlikely; perhaps as little as one-per-galaxy. While there may be the slim chance of picking up on communication from them, we’d never be able to visit due to the literally astronomical distances involved – it would take many millions of years at the very least.

So which is the answer?

All of the ideas here necessarily make a huge number of assumptions about the way an aliens species would communicate, operate, and evolve. With no evidence available to tell us about aliens or their absence, we cannot be sure why we haven’t met aliens yet. If they don’t exist, that’s a reminder how special life on Earth is. If they do, it would be extremely exciting to communicate with them and perhaps learn more about the cosmos. Either way, the very idea of aliens means that we should always try to keep learning, whether to protect our own species or discover new ones.