This story was first published on 20 October 2017 and is part of our Cosmos Classic series.
What would aliens look like? It’s a more serious question than it sounds at first. Would we even be able to recognise extraterrestrial life if we found it? As we’ll see, not all experts agree.
1: Brian Cox: purple slime
Cox suggests that life in the universe is likely abundant. Intelligent life, however, is another matter.
In his 2015 book Human Universe (written with Andrew Cohen), Cox writes there is a sense of “chemical inevitability” to life in the universe. But he adds that complex life on Earth only arose after the emergence of eukaryotes – cells containing organelles.
Research suggests eukaryotes developed as a result of one primitive cell – called a prokaryote, like a bacterium – absorbing another, two billion years ago. (Mitochondria and chloroplasts are descendants of independent prokaryotes that entered symbiotic relationships with larger cells.)
The advent of eukaryotes on Earth was a vanishingly unlikely development. Cox calls it an “evolutionary bottleneck” and thinks it so unusual that it might have happened only once in the universe.
“One can easily imagine that the 20 billion Earth-like worlds in the Milky Way could all be covered in prokaryotic slime,” he writes. “A living galaxy, yes, but a galaxy filled with intelligence? … I’m not so sure.”
2: Chandra Wickramasinghe: bacteria
Together with the late Fred Hoyle, mathematician, astronomer and astrobiologist Wickramasinghe developed the astrobiological theory known as panspermia, which that holds life on Earth arrived as microbes travelling through space on meteors and interstellar dust.
He suggests that microbial life may have landed during the Hadean period, four billion years ago, when lots of smallish meteorites smacked into the planet.
In a paper published in August 2017, he suggests ET might also have been deposited on the moon. Recent evidence indicating the presence of water, he says, “reopens the possibility that microbial life might exist close to the lunar surface”.
3: Paul Davies: indescribable
In a 2016 paper, The “Hard Problem” of Life, written with Sara Imari Walker, astrobiologist Paul Davies begins with a well-known problem. Given that all life on Earth arose from a single common ancestor, we have no way of knowing what aspects of it are law-like – found in all life, across the universe – and which are specific only to our own biosphere.
It follows that life elsewhere need not involve Earth-like biology or chemistry. Indeed, write Davies and Walker, it is possible that it “will not ultimately be reducible to known physical principles”.
The pair suggested that the true essential for life is information – which somehow “calls the shots”.
Those things that we assume to be fundamental – replication and metabolism – might be features only of “Earth-like” biologies. There is a risk that should we ever encounter ET, we might not recognise it because we lack “a general-purpose set of criteria for identifying it”.
4: Chris Hadfield: oddly familiar
“It’s so wildly different to the life we’re used to,” he says. “There was such wild experimentation through the four billion years of life on Earth.”
We might find life on Mars, or Enceladus, or Europa, he notes, and it might look like an organism that died out during a mass extinction down here.
“But I might have that completely wrong,” he adds. “Some of the strange examples they came up with on Star Trek might be a better representation.”
5: Jason Wright: highly skilled builders and engineers
Wright, an astrophysicist at the Pennsylvania State University in the US, suggests we need to be looking for the things ET builds.
He suggests focusing the search for intelligent ET on exoplanets. Rather than look for traces of biology, though, we should look for technology.
We should search for giant machines, which are likely to be “be detectable by their waste heat in the mid-infrared”.
Such structures, he says, could include enormous energy-collectors (known as Dyson spheres), satellites, and defensive shields.
Looking for them, he notes, need not be expensive. The search can “piggyback on work likely to happen in the future, anyway, as natural anomalies are discovered in the course of exoplanetary science”.
6: Neil deGrasse Tyson: not huge, and hopefully paying attention
In his book Death By Black Hole (2005), Tyson says microbial life is possibly humming away throughout the universe.
Intelligent life, on the other hand, is likely to be very scarce. However, the Copernican Principle – the key idea that we and our planet are not special – implies it must be around somewhere.
Tyson says ET must be limited to a maximum size. It could not be as big as the solar system – 10 light-years across. Even assuming nerve impulses at the speed of light, “if it wanted to scratch its head, then this simple act would take 10 hours to accomplish”.
He hopes that some intelligent ETs are sending out messages into the cosmos, rather than, like us, scanning in the hope of hearing an alien transmission. If not, he writes, “everybody would be listening, nobody would be receiving, and we would collectively conclude that there is no other intelligent life in the universe.”