In this possibly infinite universe, could Earth truly be the only inhabited planet? Are we really that special, or is the universe actually teeming with life? Could there be advanced civilisations out there trying to make contact right now? In July 2015, Russian entrepreneur Yuri Milner and renowned physicist Stephen Hawking announced an ambitious new initiative to search for communications from advanced alien worlds.
Breakthrough Listen is described by the National Radio Astronomy Observatory as “the most powerful, comprehensive, and intensive scientific search ever for signs of intelligent life in the universe”. The initiative has set aside $100 million (£66 million) over ten years to listen for signals from the nearest million stars in the Milky Way, and from the nearest hundred galaxies around us. Led by a team of internationally renowned experts that includes Astronomer Royal, Lord Martin Rees, the project will use some of the world’s largest and most powerful telescopes. The search is based on the idea that among the hundreds of billions of stars in our close galactic neighbourhood, there are thousands of planets similar to our own. With the right environment and optimal chemistry, many scientists believe that life could evolve on some of these distant Earths.
If life exists on other planets, so too might intelligent life, who like us, could be interested in exploring the universe around them, and in making contact. This is not the first time that Search for Extraterrestrial Intelligence (SETI) experiments have been attempted. Dr Frank Drake, author of the Drake Equation and one of the scientific leads on the Breakthrough Listen project, was among the first to start scanning for extraterrestrial life back in 1960. The Breakthrough Initiative builds upon more than 50 years of experience, allowing the team to look further and wider than ever before.
So far, we have no proof that life has ever existed on any planet other than Earth, but if we can find just one example elsewhere, it will completely change the way that we view the universe. As Frank Drake said at the Breakthrough launch, “Right now there could be messages from the stars flying right through the room, through us all. That still sends a shiver down my spine. The search for intelligent life is a great adventure. And Breakthrough Listen is giving it a huge lift.”
“What is the likelihood that only one ordinary star, the Sun, is accompanied by an inhabited planet? … To me, it seems far more likely that the universe is brimming over with life.”
– Carl Sagan, Cosmos
The search for intelligent life
We have begun searching for signs of life in our own Solar System, but the search for intelligent life is different. We can reach our neighbouring planets and moons with probes and rovers, allowing us to sample the atmosphere and the soil directly to find even the tiniest traces of biological materials. But to find out whether there is life beyond the reaches of our spacecraft, scientists must take a different approach. We cannot yet tell whether primitive life exists on distant planets, but if advanced, intelligent civilisations have developed the technology to send messages out into space, we might be able to detect their signals.
Signs of life
What do we actually look for when searching for aliens?
The search for intelligent life focuses less on what aliens might be made of, and more on how they might communicate. Distant planets in other star systems are too far away to see clearly, but we can pick up signals released into space. But how do we know what to listen for? We live in the same universe, so we share the same fundamental physics and chemistry. Communications have to reach over vast distances, travelling through the dust and gas of the universe without being lost or degraded, and scientists think that it is most likely that they would be sent using radio waves or powerful optical lasers.
Listening out for every single signal across the entire electromagnetic spectrum would be impossible, so to detect these communications, we need to try to think like aliens. This was first attempted in 1959 by two scientists from Cornell University; Giuseppe Cocconi and Philip Morrison suggested focusing in on a specific frequency, the 1,420 MHz ‘hydrogen line’. Hydrogen is the smallest and most abundant element in the universe, and when its energy state changes it creates a characteristic spectral line, which is always at a frequency of 1,420 MHz. This falls into the microwave radio region of the electromagnetic spectrum, and is able to travel through dust and gas that block the path of visible light. Looking at the universe in this frequency allows us to see through dark clouds that normally block our view.
Cocconi and Morrison reasoned that civilisations more advanced than our own would also have used hydrogen line emissions to map the universe around them. If intelligent life forms also realise that other civilisations might be tuning in to this special frequency, they might use it to try and send a message. Frequencies either side of the hydrogen line are also monitored, in case alien life forms choose to reserve 1,420 MHz for scientific use, and some SETI experiments, including Breakthrough Listen, also monitor for pulses of laser light in case they are used instead of radio.
As the SETI Institute points out, “optical SETI requires that any extraterrestrial civilisation be deliberately signalling in the direction of our Solar System.” This could happen by chance, but if aliens are signalling right at us, they might already know we are here.
“To my mathematical brain, the numbers alone make thinking about aliens perfectly rational” – Stephen Hawking
We’re over here!
The Arecibo Message was a coded image sent out, in 1974, in the direction of 300,000 stars in the nearby M13 star cluster, over 40 years ago. It was constructed by shifting the frequency of the broadcast to spell out binary 0s and 1s. In less than three minutes, the message attempted to paint a picture of life on Earth for any intelligent life that might be watching.
How to hunt for aliens
The first step in the search for life is to define what life actually is. This is still a topic of debate, but it is generally agreed that living things are complex and organised. They use resources from their environment to generate energy, and build molecules for replication and growth. They react to their surroundings, adapt and reproduce, all of which requires complex chemistry.
The most abundant elements in the universe are hydrogen and helium, but helium does not form molecules with other elements, and hydrogen can’t make complex molecules on its own. Oxygen and carbon are the next most plentiful, and together with hydrogen are the most abundant elements in Earth’s organisms.
It might seem a bit egocentric to assume that life elsewhere in the universe will be based on the same components as life on Earth, but a closer look at the chemistry reveals why scientists are so focused on finding carbon and water. Carbon can make four bonds to other elements, providing the scaffold that allows complex molecules to be made. This property can be matched by silicon, but the chemistry is not quite the same. While we exhale carbon dioxide, a silicon-based equivalent might exhale sand.
Water provides a solvent in which these large, complex molecules can dissolve, enabling them to interact. Water is also good at maintaining stable temperatures, and the fact that ice floats means that lakes don’t freeze solid. These properties are hard to match, although ammonia and hydrogen fluoride come close.
Given what we know about the chemistry and composition of the universe, scientists are searching for planets and moons in the so-called ‘Goldilocks zone’ or ‘habitable zone’, where liquid water might exist. If these conditions can support life on Earth, why not elsewhere?
“I think life is common in the universe. We may be the only civilisation in the Milky Way. There will be other civilisations in the universe” – Brian Cox
Are we alone in the universe?
There are billions of stars in the universe, and some astronomers think it’s likely that each one in the Milky Way galaxy has at least one planet. The director of the Space Telescope Institute in Baltimore, Matt Mountain, told NASA: “What we didn’t know five years ago is that perhaps ten to 20 per cent of stars around us have Earth-size planets in the habitable zone.” Being in the right zone is one thing, but being home to life is another. And being home to intelligent life with the technology to send signals out into space is something quite different again.
On Earth, moving from single-celled organisms like bacteria, to complex, multicellular organisms, like worms, fish, and humans took around 2.5 billion years, and it only happened once. As Professor Stephen Hawking pointed out in a lecture entitled Life in the Universe, “This is a good fraction of the total time available, before the Sun blows up.” Assuming that life can get past this bottleneck, at least one species then needs to become intelligent enough to want to communicate with the universe. If this is possible, where is everybody? This question, known as the Fermi Paradox, was asked by Enrico Fermi in 1950. He argued that technologically advanced civilisations could colonise entire galaxies in just ten million years, fractions of the age of the Milky Way, so we really should have seen evidence of them by now.
It could be that there really are no other intelligent life forms in the galaxy, but there are dozens of other explanations. One of the most widely discussed is the idea that intelligent life might not survive long enough to make contact; it could be that asteroid impacts, supernova blasts, natural disasters and warfare wipe intelligent life forms out before they have a chance to explore. Ultimately, the lifespan of a civilisation is limited by the life of its parent star, unless of course, the life forms find a way to leave.
“Right now there are maybe only 10,000 civilisations we can detect in the galaxy. That’s one in ten million stars. We have to look at ten million stars before we have a good chance of succeeding.” – Frank Drake
Are we being buzzed?
Fast radio bursts (FRBs) are brief, high-energy pulses of electromagnetic waves that have been appearing in scientific data gathered at the Parkes Telescope since the early 2000s. The bursts contain high and low frequency wavelengths, which travel at different speeds through space, and the delay between the arrival of the highest and lowest frequency waves can be used to calculate the distance to the source. Strangely, the ten FRBs all had delay times nearly divisible by 187.5. There is no natural object known to be able to do this, leading scientists to speculate about a possible alien source. However, other signals, called perytons, have since been found to have much more local origins – scientists discovered that they could produce the same interference patterns by opening the door of the microwave oven.