We have discovered 3663 (and counting) planets around distant stars. About 50 are in the 'Goldilocks zone' which supports liquid water. A new study has whittled the number believed habitable down to just three.
And Earth is one of them.
Until now, the chief measure of a planet's potential habitability has been whether it is the right temperature for liquid water to exist.
But life — as we know it — needs more than just muddy puddles.
It needs a spice rack of vital ingredients.
And behind them all is the baking power of UV light.
A study published in Science Advances says ultraviolet rays power the basic chemical reactions needed to produce the building blocks of life. And, once built, recent discoveries on comets hint that these have a tendency to assemble themselves in ever more advanced organic structures.
So they set about creating a new set of criteria to define the most promising life-promoting planets. After filtering out those stars which didn't produce the right 'flavour' of UV, only a handful of contenders remained.
"This work allows us to narrow down the best places to search for life," lead author of the study Dr Paul Rimmer says. "It brings us just a little bit closer to addressing the question of whether we are alone in the universe."
The cookbook for primordial soup hasn't been written. But scientists have figured out a fair bit of the recipe behind it all.
It contains some surprises.
For example, one of its key ingredients is cyanide …
Researchers believe that carbon in meteorites slamming into a planet's early atmosphere can react with nitrogen to form hydrogen cyanide.
This then rains down on the surface, collecting in pools of water.
Hydrogen cyanide, when mixed with the likes of hydrogen sulfite, can be baked by UV light into sugars, polymers and phosphates. These are the building blocks of RNA. RNA, like DNA, can carry evolutionary information. DNA just carries twice as much.
Co-author of this paper Professor John Sutherland — who specialises in the chemical origins of life on Earth — has been seeking to understand what initiates these chemical processes.
Working with astronomer Dr Rimmer, however, produced a fresh perspective.
"My first question (was) what kind of light are you using, which as chemists they hadn't really thought about," Dr Rimmer says. "I started out measuring the number of photons emitted by their lamps, and then realised that comparing this light to the light of different stars was a straightforward next step."
The astronomers and biochemists conducted a series of tests to see what the most efficient UV 'temperature' was to cook up a primeval soup.
"There is chemistry that happens in the dark: it's slower than the chemistry that happens in the light, but it's there," senior author Professor Didier Queloz says. "We wanted to see how much light it would take for the light chemistry to win out over the dark chemistry."
With no light, the process produced inert substances.
Those with UV light created the right stuff — but at different rates according to brightness.
This productivity curve was then applied to the light of different stars known to be hosting planets in their 'Goldilocks Zone'.
Cool stars — such as red dwarfs — don't produce enough light to activate the chemical reactions, the study found. But those with a similar temperature to our own were ideal.
This produces the 'abiogenesis zone' — Goldilocks planets where there was enough UV light to activate organic chemistry.
Put simply, it's where the chances are best you might find primordial soup.
"Of course, being primed for life is not everything, and we still don't know how likely the origin of life is," Professor Sutherland says. "Even given favourable circumstances, if it's really unlikely then we might be alone. But if not, we may have company."
When plotted on a chart, two places come up trumps.
Earth.
Kepler 62e is right on the edge.
And then there's Kepler 452b.
When discovered in 2015, it was dubbed Earth's 'cousin'.
This latest discovery could elevate it to 'sibling' status.
It's 1400 light years away, though. So it's beyond the ability of even our best telescopes to have a clear look at.
We know it is about 1.5 times as big as Earth. Like Earth, it sits in the 'Goldilocks zone' of its star.
And, like Earth, it fits neatly near the centre of the new 'abiogenesis zone'.
"I'm not sure how contingent life is, but given that we only have one example so far, it makes sense to look for places that are most like us," Dr Rimmer says. "There's an important distinction between what is necessary and what is sufficient. The building blocks are necessary, but they may not be sufficient: it's possible you could mix them for billions of years, and nothing happens. But you want to at least look at the places where the necessary things exist."
Cooler red dwarf stars — the most common type in the known universe — are not without hope. The researchers say regular flares have the potential to kickstart the chemistry. And young red dwarfs are known to be restless. But they're also long-lived, entering sedentary adulthood that could allow molecular life to flourish.
But that's far harder to calculate.
And, of course, life out there may be nothing like that down here.
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