What’s the meaning of life? How do we search for alien life? And why haven’t we found it yet? Exploring the possibilities of biology among the stars.
What does life look like?
It’s a question that has fascinated people for centuries: inspiring dreams, nightmares, books, and films. Are we alone in the universe?
To look for life in space, we first need to define life. This is surprisingly difficult. We know life when we see it, but it’s hard to pin down. We can list things that life does: reproduce, consume energy, and grow, for example. But a fire does all of these things – does that mean a fire is alive?
All life on Earth is composed of cells that carry DNA. Viruses complicate things – carrying DNA or RNA, they can only survive and replicate with a host. The life of viruses is a matter of debate.
NASA uses a working definition to search for life in the universe: “Life is a self-sustaining chemical system capable of Darwinian evolution”. The geneticist Trifonov came up with a simpler definition in 2011: life is “self‐reproduction with variations”. It’s not perfect, but it helps us know what to look for.
Extremophiles and the origin of life on Earth
Even on Earth, life thrives in environments too extreme for human survival. These environments might have extremes of temperature, salinity, radiation, or pressure. Organisms living in such challenging conditions are known as extremophiles.
Hydrothermal vents on the ocean floor spew scalding hot water, superheated in the Earth’s crust to up to 752°F (400°C). Microbial organisms use the chemicals from this water to produce energy, forming communities of life. The Pompeii worms that cling to their side are the most temperature-resistant multicellular organisms and can withstand temperatures of up to 176°F (80°C).
We believe that life on Earth could have started around these hydrothermal vents. Some of the earliest evidence of life on Earth, around 4 billion years old, comes from rocks that formed around such vents. Studying the origin of life on Earth can inform how we think about life in space.
Tardigrades are interesting extremophiles in the search for life in space. These microscopic water creatures have been sent into space, where they survived exposure to intense radiation, and the vacuum of space. We may be able to learn more about surviving in space from these tiny water bears.
Civilizations
Finding life elsewhere in the universe would be exciting. But what would be even more dazzling would be finding complex, multicellular life in space. Intelligent life can think, understand, and learn, but such intelligence might look different to our own.
Consider the octopus, for example: it has intelligence but also a totally different nervous system from vertebrates. Its distributed nervous system means its arms can make decisions without involving the brain.
If advanced technological civilizations are out there, we could theoretically measure their advancement on ‘the Kardashev scale’. This scale classifies civilizations based on the amount of energy they can use. Type I civilizations can use the energy that reaches their planet from the sun. Type II civilizations can use all the energy radiated by their sun. And type III civilizations can control their entire galaxy and its energy. From our point of view as a type I civilization, a type III civilization would probably look almost like gods.
Not as we know it
It is hard to overcome our biases about what life looks like. Life on Earth traces back to a single origin, so we don’t know whether it has developed according to universal laws or by accident and chance. Extra-terrestrial life doesn’t need to follow our expectations.
All life on Earth depends on water as a solvent. But other liquids exist in the universe, which could possibly act as solvents for life. A strong candidate is ammonia – it’s abundant and supports a range of chemical reactions. Different solvents would provide different limitations and probably change how life looked. If life evolved in a sea of non-transparent liquid, for example, it might not evolve eyes as no light could reach them.
Organisms could use different wavelengths of light for photosynthesis, to better suit the light from their star. Plants on Earth appear green because they absorb red and blue light to use for photosynthesis. Perhaps photosynthesis using different wavelengths could result in purple plants?
There are many different speculations on what life in the universe could look like. When searching for this life, we need to work hard to keep an open mind and overcome our Earth-based biases.
The Drake equation
The Drake equation uses algebra to estimate the number of alien societies in the Milky Way galaxy. It was designed to start a debate, rather than provide a definite answer. It was created by Frank Drake in 1961 and discussed at the first meeting on the search for extraterrestrial intelligence (SETI).
The equation puts a number (N) on the number of alien civilizations in the galaxy broadcasting in the electromagnetic spectrum. To get to N, it feeds in a number of variables: including the rate of star formation, the number of habitable planets, the fraction of planets that develop intelligent life, and how long an alien society might survive. Estimates for the Drake equation range from N=1 to N=10,000. If N=1, we are currently the only society broadcasting into the universe. N=10,000 is Drake’s current preferred estimate.
With what we currently know, the Drake equation can’t be solved – there are too many unknown variables. But, as they found at the meeting in 1961, it makes for interesting discussions.
The Fermi paradox
Suppose for a moment that the probability of intelligent life in the universe is high. Where is everyone else? This is the heart of the Fermi paradox, a puzzle named by astronomer Enrico Fermi and a conversation he apparently had one lunchtime.
Perhaps complex life is rarer than we imagine. After all, life on Earth has existed for at least 4 billion years, but for most of that time, it was basically just algae.
Maybe intelligent societies are spread so far apart that they will never be able to communicate.
Perhaps civilizations reach a certain level of technological advancement and destroy themselves. Humanity did this very nearly using nuclear technology in the Cold War.
Perhaps other civilizations are hiding – could there be some unknown threat in the universe that we should be hiding from too?
Or perhaps there’s no paradox at all. Perhaps we really are alone in the universe.
Biomarkers – the signatures of life
Advanced civilizations may be able to give us a wave to let us know they’re out there. But it’s harder to search for microbial life, which probably hasn’t developed space communications.
To look for extraterrestrial microbial life, we can look for biomarkers: molecules that can be traced back to a biological origin. For example, microbes could be broadcasting signals by changing the gases in their planet’s atmosphere.
One of these signatures of life could be the abundance of oxygen in a planet’s atmosphere. Oxygen molecules are very reactive, and these O2 molecules won’t last long in an atmosphere without being replenished. One of the best ways we know to replenish O2 molecules is through biology. So the presence of these molecules could be an indicator of life. Unfortunately, these molecules are very difficult to detect in exoplanet atmospheres. However, a new technique has been developed to detect colliding O2 molecules, and this, combined with advanced space telescopes, might make them easier to find going forward.
Our attempts to communicate
SETI, the Search for Extraterrestrial Intelligence, has been in progress since the late 1950s. We have scanned the sky, looking for signals that could come from extraterrestrial societies – chiefly radio waves. We have heard nothing: The ‘Great Silence’ as it is known.
In addition to listening, we have also been attempting to speak. Our first attempt to do so was the Arecibo Message. A radio signal beamed to a globular cluster of stars around 25,000 light years away. It’s densely packed with information: including a stick figure of a man, a diagram of our solar system, and a description of DNA, among other things. Of course, we don’t know if this message would reach a civilization able to decode it – and how they might react if it does. An updated message has also been developed for broadcast in the future.
The Voyager 1 and 2 spacecraft both carried golden records loaded with information in case an interested alien found them. These golden discs contained sounds and images to showcase the richness of life on Earth.
What does a habitable planet look like?
When we search for planets that could sustain life, we tend to focus on planets that are a bit like Earth. That means planets in the habitable zone of their star – orbiting at a distance that would allow liquid water on the surface, given the right pressure conditions. We’ve also concentrated on trying to find planets around sun-like stars – emitting a broad spectrum of light, reasonably stable and long-lived.
Earth-size planets are difficult to detect in the habitable zone of sun-like stars, thanks to their relative sizes. But we’ve found planets that might harbor life around other stars – the red dwarf Trappist 1, for example, has an astonishing 7 planets in its habitable zone. Only 40 light years away, we’ve studied the Trappist system intensely.
Could moons support life? The Jupiter Icy Moons Explorer (JUICE) mission will launch in 2023 to make detailed observations of some of Jupiter’s moons and assess their suitability for life as we know it. If life is substantially different from that on Earth, it becomes harder to say what kind of place might support it.
Could there be life on Venus?
It has surface temperatures of hundreds of degrees and an atmosphere of corrosive sulfuric acid. Pressure on the ground is equivalent to the pressure you’d feel 0.5 miles (900 meters) undersea on the Earth. From a human perspective, Venus is almost ridiculously inhospitable.
And yet, in 2020, a team of researchers found traces of phosphine in Venus’s atmosphere. This is considered a biomarker, very hard to make with chemical processes but known to be produced by microbial life on Earth. Could there be life on Venus? Scientists speculated about microbial life drifting in the thick, Venusian atmosphere, gaining energy from sulfur and entering dormancy to protect it from the worst Venus had to offer.
Some research has supported the presence of phosphine in Venus’s atmosphere, and some have disputed it. On balance, Venus is unlikely to harbor microbial life. But it’s not impossible – and it reminds us that an open mind is an essential piece of equipment in the search for life in the universe.
