The short answer is "Nobody knows" - but it's interesting to try to make some guesses based on what we do know!
Scientists who have tried to make good guesses use an equation called 'The Drake Equation' which was named after Doctor Frank Drake who first tried to work this out about forty years ago. The idea is that if you know how likely it was for life to form on Earth - and if you know how many planets there are that are more or less like the Earth - then you can make a good estimate of how much life there is out there.
We can try to do that ourselves.
How many stars are there in the Universe?Edit
When you look up at the sky on a really dark night - far from the lights of cities and roads, you can see about five thousand stars and four or five planets. But nearly all of the stars and planets out there are far, far away and much too dim to see.
We know that there are eight planets orbiting the Sun and that our sun is a lot like the other stars out there in our galaxy. Our galaxy (which we call "The Milky Way" because it looks like a splash of milk across the night sky) has about three hundred billion stars in it. But the Milky Way is a rather small galaxy and there are many, many other galaxies.
With our very best telescopes we can see about a hundred billion galaxies. With a hundred billion galaxies - each one with hundreds or even thousands of billions of stars - astronomers calculate that there are about fifty thousand, billion, billion stars in the part of the universe that we can see! That's a big number. Written out in numbers it's 50,000,000,000,000,000,000,000 stars!
Do all of those stars have planets?Edit
Well, it's rather hard to tell for sure - but astronomers have looked carefully at some of the stars in our neighborhood and whilst planets that are orbiting other stars are too small to see - even with our very best telescopes, we can sometimes see a slight wobble in the light coming from a star that is caused by the gravity of one or more planets that are orbiting it.
Knowing that, we have found that many nearby stars have at least a few planets that are big enough to wobble the lights from the star enough so we know they are there. There are probably other planets too small to produce a big enough wobble for us to measure - so we don't know exactly how many planets to expect. But it seems that there are probably more planets than stars in the universe, so there could easily be 50,000,000,000,000,000,000,000 planets for life to try to get started on. That's a lot of places and we haven't counted moons, asteroids, comets and dust clouds!
How often does life actually get started on one of these planets?Edit
This something we can really only guess at. We know that life formed once on one planet because we are that life and we live on that planet - but how do we know whether the Earth is just an amazingly special place or whether there are Earth-like planets orbiting most of the stars in the sky? Unfortunately, we don't know.
- Many planets must be too hot - like Venus and Mercury in our own solar system. Life can't survive there because the chemicals that life needs don't work properly when things are that hot.
- Other planets might be too cold - so there would not be enough sunlight for plants and animals to use as energy. The outer planets of our Solar system - Pluto and Neptune - must be far too cold for life to exist there.
- It's possible that planets might not have the right mixture of life-giving chemicals. There is no life on our Moon because it has no atmosphere - in daytime it's too hot and at night too cold.
- Other planets may form and then be destroyed before life has had time to form - and on others, life might have formed a long time ago and then died out for some reason,
But there are plenty of planets to choose from. Next time you are on a beach or at a playground with a sand lot, take a handful of sand. There are an awful lot of tiny sand grains in a fistful of sand - but there are more stars and planets than there are grains of sand on all the beaches of all the world! Surely they can't all be too hot, too cold, too dry or too poisonous?
But we humans haven't yet looked closely enough in enough places where life might be to really know for sure what life needs. We've found bacteria living in boiling hot water near underwater volcanoes. There are tiny worms who live in the ice near to the South Pole. Bacteria have been found living inside rocks in the deepest mine shafts. There is hardly anywhere on earth where live can't survive somehow. Astronauts from Apollo 12 who visited the moon in 1969 visited the Surveyor III lander that had been sitting on the moon for more than two years. They found that bacteria that had hitched a ride on Surveyor when it was launched and were still alive after two years of living on the moon!
It seems then that life can survive and even prosper in some fairly hostile places - this shows that a planet may not have to be a comfortable place for humans in order to develop life. However, there doesn't seem to be any native life on the Moon beyond what humans have put there by accident and our robotic missions haven't found life on Mars or Venus or any of the comets we've looked at closely.
With at fifty thousand billion billion planets in the universe, even if each planet only had a one in a trillion chance of having life, there could still be billions of planets teeming with life out there in the universe. But if the chances are that bad then we might be alone in our galaxy with the nearest other creatures living hundreds of millions of light years away. Even if the odds for life are only a million to one for each planet, we humans and our robots might have to travel for thousands of years and search millions of planets and moons before we'd find them.
So how many aliens are there?Edit
That is where the drake equation comes into play:
R* × fp × ne × fl × fi × fc × L = The number of alien civilisations that we can talk to in our galaxy
This equation might look complicated, but it's actually quite simple when you understand it.
R* is how many stars appear a year. So, for example, if there was 10 stars being made every year, then you could scrible out R* and put at '10' there instead.
fp is the chances that a star has planets. If, on average, there was a 1 in 2 chance that each new star had a planet, then the number that replaced fp would be 0.5 (1÷2).
ne is the number of planets in each solar system that can have life. In our solar system, ne would be 1, because we only have one planet that we know has life on it, Earth.
fl is the chances that these planets have life. If all planets that could have life have life, then 1 would go in the place of fl
fi is the chances that the life on these planets is clever. A 1 in a hundred chance of life would give us 0.01, 1÷100.
fc is the chance that they will be able to talk to us. This number might be 1 (100%), because if they are clever they will eventually find out how to talk to other planets.
Finally, L is how long each alien civilisation will live (or want to talk to us), in years. Though nobody knows the real value of this, Frank Drake guessed 10,000 years.
Now, we can multiply them all together. 10×0.5×1×1×0.01×1×10000 = 500 alien civilisations in our galaxy. The values here are just guesses, though, and other guesses have ranged from only one civilisation in our galaxy to thousands.
Little Green AliensEdit
If so many planets in the universe does this mean that we should expect little green aliens to be popping out of their flying saucers soon? Well, perhaps not. For beings living in another galaxy outside of the Milky Way, if they noticed that we are here and set out to visit us - traveling at the speed of light, they still wouldn't get here for another 300 million years! They couldn't have set out millions of years ago to visit us because humans haven't been on around for nearly that long.
If we're going to be visited by aliens, they'd have to live on a nearby star and know how to fly very fast to get here. That means that if we are to meet alien cultures they must come from within the very few stars that are within a reasonable travel time. Life would have to be very common indeed for there to be intelligent beings close enough to meet us face-to-face.
SETI - The Search for Extra-Terrestrial IntelligenceEdit
One group of scientists are searching for other intelligent life in our galaxy by listening to the radio. With large radio telescopes they sweep the stars looking for radio signals with interesting patterns in them that would not come from natural radio sources. So far, they haven't found anything - but that may mean that they have to keep on looking - or it might be that the aliens are not transmitting anything in our direction with enough power for SETI's sensitive radio telescopes to pick up.
Panspermia - Life from Space?Edit
One other possibility is that life didn't just appear on Earth at all. If life were really common throughout the universe, it's possible that life arrived on Earth after it developed somewhere else. We know that meteorites found on Earth have come here from the Moon and from Mars - and probably from other planets too. We also know that debris in the form of little bits of rock and ice rain down on the Earth from space all the time. If life existed in that debris when the Earth was young, there is a good chance that we Earthlings originally came from some other planet entirely. It could be that all life on Earth originated on Mars and we are the alien invaders!
So what IS the answer?Edit
We still don't know for sure that there is life anywhere beyond the earth, and even if life were common in the universe, we don't know whether we'd ever be able to find it if there was.
The search for life beyond Earth is a very new thing. Humans have only been searching for about 40 years - and we've only been looking in the most likely places for 10 years. If we humans are going to find signs of life outside the Earth, it will probably happen in your lifetime. You may one day be amongst the first humans to encounter alien life or to hear a message from another world through SETI!