A New Equation Estimates the Chances of Life Arising on a Given Planet
An astrobiologist and a chemist team up on a new equation to describe the conditions needed for life to arise on a planet.
A pair of researchers have come up with a new equation to help estimate one of the great unknowns in our Universe: the probability of life arising on a given planet.
They hope their formula could advise astronomers where to look (and what to look for) if they're seeking extraterrestrial life, and also guide chemists on Earth exploring the origins of life.
"In essence, you're viewing the planet like a really big test tube to make a life form," one of the researchers, chemist Lee Cronin from the University of Glasgow, told Motherboard. "By doing that, if we can work out what the likelihood is that there are planets out there with life on them and also work out what are the things to look for, we should be able to give the astronomers—the astrobiologists—basically a checklist of where to look."
The more chemical "building blocks" a planet has, the better
In a paper published in PNAS, Cronin and astrobiologist Caleb Scharf from Columbia Astrophysics Laboratory build on the well-known Drake equation, which was formulated by American astronomer Frank Drake in 1961 and which presents which factors you need to take into account to estimate the hypothetical number of communicative civilizations in our galaxy (these include things like the formation of stars suitable to develop life; fraction of planets with suitable environments; and fraction of civilizations that can develop technology sufficient to alert us to their presence).
In a phone call, Cronin told me that he and Scharf wanted to "ask a more basic question" in the context of upcoming planet-hunting telescopes such as the James Webb Space Telescope, which is planned to launch in October 2018.
That question: What's the frequency of "origin-of-life" events on a given planet?
By "origin-of-life event," also known as abiogenesis, the authors mean an event whereby the beginnings of something living originate from something inorganic—the domino that leads down the path to, say, cells and, if you're really lucky, complex organisms.
So what do you look for? On the left side of Scharf and Cronin's equation you have the probability of origin-of-life events; on the right side you start with the number of "building blocks"—essentially just different molecules—available on the planet. The researchers deliberately keep the definition of "building blocks" quite open; on Earth, life can be broken down to things like proteins and lipids, but Cronin noted that alien life could be based on different chemistries. "It could be on another planet you have polymers made out of silicon and sulfur and selenium and tellurium—weird things that we just don't see in our chemistry on Earth," he said.
The number of building blocks is divided by the amount required for each organism, and the equation also takes into account the fraction of building blocks available at any one time (as some might be, say, locked deep in the planet or tied up in minerals) as well as the probability of these building blocks assembling themselves to make an origin-of-life event.
You can't stick numbers into those values if you don't have that information about a planet, but that's not entirely the point. Like the Drake equation, a main idea behind the formula is to describe the parameters that could determine the probability of life arising.
In this case, one of the main takeaways is simple: the more chemical "building blocks" a planet has, the better.
"If you've got a planet with lots of interesting chemical building blocks, it's probably a safer bet to say, 'OK, there's probably more chance of life happening here than, say, on the Moon,'" summarised Cronin.
Given this, the researchers came to one conclusion that could be immediately helpful to alien-hunters. They suggest that the exchange of materials between planets—such as Earth and Mars—could greatly increase the number of chemical building blocks available and therefore up the odds of life occurring. So a pro-tip: look for planets that are in multiplanet systems.
But the equation also has implications closer to home and, indeed, in Cronin's own chemistry lab. For a while now, Cronin has been attempting to create some kind of life form from chemical building blocks.
He hopes this work will motivate chemists to look at the origin of life not just through a historical lens but to actively do experiments to see what it takes to cook up chemistry into something complex enough it could be a precursor to life.
"The problem with the origin of life area right now is there are lots of people doing history, but not many people actually actively trying to make new life forms, and I believe that should be the endeavour we're all working for, not the history lesson," he said.