Is There Life In The Universe?
When we look up at the night sky, what are the chances that somebody else is looking back at us?
Modern humans evolved around 200,000 years ago and we have been looking up at the night sky ever since – it’s humanity’s oldest relationship with science. But there’s always been a fundamental question that we still have no answer to – is there life elsewhere in the Universe?
This question is deceivingly complicated. To begin with, we must ask: How unique is our planet? How likely is complex life? How unique is our Solar System?
Given that we live in a galaxy with about 200 billion stars, 40 billion of which have Earth-sized planets around them, it’s not a far-fetched thought that there may be somebody else out there.
But despite actively scanning the skies for signs of intelligence for more than thirty years, the silence continues and we remain alone.
On 5 September 1977, NASA launched Voyager 1 and Voyager 2 missions in a bid to study the gas giants in the outer Solar System.
When that main mission ended, the secondary mission gave the Voyager spacecrafts a whole new definition. Both spacecraft were equipped with two golden records containing greetings from Earth in 55 languages alongside pictures, sounds, and a galactic map giving directions to Earth. This was humanity’s postcard to any potential civilisations in the Galaxy.
In 2013, NASA confirmed that Voyager 1 had reached interstellar space making it the farthest spacecraft to date. In about 40,000 years, Voyager will pass by a star called Gliese 445. This entire act of sending messages on a space probe demonstrates hope that one day we may realise that we aren’t alone.
Since 1984, a project called SETI (Search for Extra-Terrestrial Intelligence) has been scanning the sky for radio signals.
Of all the types of light, radio waves travel the furthest without scattering or absorption, so they are the best way to communicate across the stars. Any signal coming from within a thousand light years could be detected by one of the 42 radio telescopes that make up the Allen Array.
But to date, no repeatable or verified signal from extra-terrestrial beings has ever been found.
The Drake Equation
In 1961, astronomer Frank Drake dared to turn questions into numbers and invented one of astronomy’s most widely known equations – the Drake equation.
This equation is an attempt to estimate the number of extra-terrestrial civilisations in the Milky Way, N. There are seven factors in total that multiply together to give you N.
Drake arrived at 10,000 as his number for N, but the question remains, if there are 10,000 civilisations out there, why don’t we hear from them?
The answer to this potentially lies in something called the Fermi Paradox. Back in the 1950s, physicist Enrico Fermi noted that if there are intelligent civilisations out there, then inevitably they should travel and spread across the stars, given enough time. Why don’t we see this? One interesting possibility is that intelligent civilisations may never live long enough to contact us.
Exoplanets and signatures for life
To date, Kepler has found over 2,300 confirmed exoplanets and has over 4,000 candidates to verify. Kepler particularly focused on finding planets orbiting their host star in a region known as the habitable zone – which means that the planet is the right temperature to have liquid water on its surface. Currently we have only one example of life and that’s on Earth. Every life-form on the Earth requires liquid water to survive so it makes sense to search for water elsewhere if we want to find alien life.
The Kepler space telescope also studies the atmospheres of these planets – looking for any organic molecules or water signatures.
With the upcoming launch of the massive James Webb Space Telescope, we’ll be able to study these alien atmospheres with even greater precision.
Multi-cellular life on Earth
Simple life on Earth is believed to have begun in hydrothermal vents at the bottom of primordial oceans over 3.5 billion years ago.
The most commonly accepted theory for the formation of multi-cellular life is called the fateful encounter hypothesis. This states that one day millions of years ago, two simple cells merged together. The host cell attacked the invading cell but somehow it survived. Over time, the cell reproduced and started working collectively. These multi-cellular organisms are called eukaryotes. These organisms survived meteorite impacts, rising oxygen levels, and global ice ages.
But surviving both the merging process and mass extinction events is incredibly difficult and rare. The probability of this successful merger is so small that life may have arisen purely by accident.
Does this make us unique?
But at the rate that we’re exploring other planets with rovers and spacecraft, and discovering Earth-like worlds around other suns, we’re closer than ever to the possibility.
About the author: Kimran Dhaliwal is a physics student at the University of Leicester and works as a Science Interpreter at the National Space Centre.