Working out the odds - a mathematical approach
How likely is it that the universe is teeming with beings? The universe is massive, in the observable universe - that's the part we can see, the part close enough to us for its light to reach us - there are at least 200 billion galaxies, each galaxy has about one billion stars, and most stars have at least one planet, but often more.
The existence of planets around other stars was only a theory until 1992 when the first one was discovered. Now thanks to Earth based and space observatories like Kepler and TESS there are 4,000 confirmed exoplanets and thousands more possible candidate worlds.
On the face of it then you’d think there must be lots of life, but where is it? Why have we not seen or heard them? In 1950 physicist Enrico Fermi asked this very question.
Enter astrophysicist Frank Drake and the Search for Extra-Terrestrial Intelligence (SETI). In 1961 Drake proposed a formula to work out the chances of life in the Universe. It looks scary but is very simple:
N = R*fpneflfifcL
The formula looks at how many stars are formed in a galaxy (R), how many have planets (fp), how many have a suitable environment (ne) the number that might see life develop (fp), how many of those worlds see intelligence (fi), how many create detectable technology (fc) and finally how long they last (L).
The problem is that any figure you put in the right-hand side of the equation is a ‘best guess’ as there is a lot of uncertainty about each aspect. Results using this formula vary from 1 to 1,000,000 intelligent lifeforms in a galaxy! So there might just be us or there may be lots more intelligent life in the Milky Way, this is not really satisfactory and doesn’t tell us anything useful.
Detecting ‘little green men’
How else can we look for life? One of the parts of the Drake Equation looked at how many aliens create detectable technology. So how would we detect that and what might we be able to detect? Perhaps the easiest thing to search for are TV or radio transmissions.
Astronomers use radio telescopes to listen for these transmissions, these are large dishes that can hear across a wide range of frequencies. The first radio telescope was built in 1932 by Karl Jansky and consisted of antennae that look a bit like TV aerials. The first dish radio telescope was built by an amateur radio operator called Grote Reber and lived in his back garden!
Telling the difference between artificial and natural transmissions can be tricky, even today. The first example of this came in 1967 when Cambridge University postgraduate student Jocelyn Bell noticed ‘a bit of scruff’ in her radio data. This signal was very unusual, it was thought to be too regular and fast to be natural. Initially nicknamed ‘Little Green Men 1’ it was soon found to be a natural signal after the discovery of a similar source in another part of the sky. Instead of aliens Bell had discovered the first Pulsar – a rapidly rotating neutron star.
An intriguing and still unresolved example is the ‘Wow signal’ of August 1977. Astronomers at Big Ear Radio Observatory in the USA detected a 72 second burst, the signal increased in intensity and then decreased all within that period, something they thought an artificial signal might do. The signal was also much stronger than anything else recorded.
What was the Wow signal? No-one knows. It has never been detected again despite many attempts, its nature and origin remain a mystery.
If we can’t detect alien transmissions then maybe an advanced species could detect us instead? Marconi transmitted the first radio signal across the Atlantic in 1901 and the first radio station began transmitting from Detroit in 1920. There are 24 exoplanets around 100 light years away whose inhabitants could now be picking up signals form that first radio station. If they are there then they should be able to hear us and we should be able to hear their broadcasts. However the radio telescopes used by SETI have heard nothing so far.
Is there something in the air?
Is there any other way of looking for alien life? Luckily there is, by checking the atmospheres of exoplanets we may be able to detect signs of life and even any industries!
When an exoplanet moves in front of a star, some of the star’s light has to travel through the planet’s atmosphere. As it does the light is changed by molecules in the atmosphere, those changes can tell us what is in the air. Astronomers split the light into a spectrum, a rainbow of colours and bands, that show its composition. Comparing the light from the star with light from the star that passes through a planet's atmosphere will show differences, those differences tell us about the atmosphere on the planet and what it contains.
Astronomers look for methane, carbon dioxide and oxygen, all of which can be products of life. The most interesting is oxygen. On Earth the vast majority of our oxygen is produced by microbes in the oceans, without them there would be very little in our air and we would not be here. If oxygen is found in an exoplanet’s atmosphere it means there is some form of life: it may not be advanced life but it will be life.
Let’s say we detect a planet with lots of oxygen, how can we find out if it has advanced life? We will need to look for ‘technosignatures’ - the products of industrial processes – the smog of pollution. The best telescope to do that will be the recently launched James Webb Telescope. It is the largest telescope ever launched, with a mirror 6.5 metres in diameter, much bigger than the 2.4 metre mirror on the Hubble Space Telescope. It will view objects in infra-red instead of the visible light we and Hubble use. The beauty of infra-red is that when you split it into its spectrum it reveals more information than spectra from visible light, giving a lot more detail.
Is there alien life in our solar system?
How about life closer to home? People have imagined life on Venus (some thought it would be full of dinosaurs) and there are lots of stories about Martians but no evidence that they exist so far. Probes and rovers like Spirit, Perseverance and Opportunity have explored the surface and dug into the soil of Mars and have found nothing. There is water frozen into the soil and at the ice caps but no life, not even a microbe.
In the outer solar system, the best places to look are some of the moons of Jupiter and Saturn. Jupiter’s moon Europa has the smoothest surface of any object in the solar system. Astronomers believe there may be a massive salty ocean under the icy crust maybe as deep as 100 kilometres. The Hubble Space telescope has seen evidence of plumes of water vapour escaping into space. How can there be liquid water so far away? It’s thought that the moon is being ‘massaged’ by Jupiter’s gravity pulling its surface back and forth, generating heat.
Saturn’s moon Enceladus is the largest moon in the solar system. Like Europa, its surface is made from ice, and geysers of water vapour erupt into space. The Cassini spacecraft found evidence of an ocean 10 kilometres deep. For there to be liquid that far away from the sun means there must be a source of heat, deep down at the core.
On Earth creatures live in the depths of the ocean away from sunlight using heat coming from hydro-thermal vents, so perhaps something similar is happening at Enceladus and Europa. Missions are being planned to investigate both Europa (Europa Clipper, due to launch in 2024, and Lander) and Enceladus (Orbilander mission) to look for any signs of life.
Another of Saturn’s moons, Titan, is the only moon with a thick atmosphere. The Huygens probe landed on its surface revealing an atmosphere made from nitrogen and lakes of liquid methane. It may be too cold for life now, but as the Sun begins to die it will expand and warm the satellite, maybe for just long enough to let life begin.
So is there life out there?
Some people have speculated that if there are any aliens perhaps they don’t think we are advanced or sensible enough to contact and that’s why we haven’t seen any. It is a fanciful but unlikely idea.
Will we ever find any signs of life, intelligent or otherwise? At the moment it doesn’t seem likely but we will keep looking.
For now we live on the only planet in all the vastness of space with any life at all. Let’s hope if we do encounter any intelligent life from other planets it's a friendlier species than the Daleks!
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