A study proposes that stars which give off sufficient UV light could kick-start life
Scientists have identified a group of planets outside Earth's solar system, where the same chemical conditions that may have led to life on Earth exist.
The researchers, from the University of Cambridge and the Medical Research Council Laboratory of Molecular Biology (MRC LMB) in the UK, found that the chances for life to develop on the surface of a rocky planet like Earth are connected to the type and strength of light given off by its star.
Their study, published in the journal Science Advances, proposes that stars which give off sufficient ultraviolet (UV) light could kick-start life on their orbiting planets in the same way it likely developed on Earth.
This is where the UV light powers a series of chemical reactions that produce the building blocks of life.
The researchers identified a range of planets where the UV light from their star is "sufficient to allow these chemical reactions to take place", and that lie within the habitable range where liquid water can exist on the surface.
Dr Paul Rimmer, a postdoctoral researcher and the paper's first author, said: "This work allows us to narrow down the best places to search for life.
"It brings us just a little bit closer to addressing the question of whether we are alone in the universe."
The new paper is the result of a collaboration between the Cavendish Laboratory and the MRC LMB, bringing together organic chemistry and exoplanet research.
'Exoplanet' is the term given to planets that orbit around stars other than the Sun.
It builds on the work of Professor John Sutherland, a co-author on the current paper, who studies the chemical origin of life on Earth.
In a paper published in 2015, Professor Sutherland's group proposed that cyanide, although a deadly poison, was in fact a key ingredient from which all life on Earth originated.
In this hypothesis, carbon from meteorites that slammed into the young Earth interacted with nitrogen in the atmosphere to form hydrogen cyanide.
This hydrogen cyanide rained to the surface, where it interacted with other elements in various ways, powered by the UV light from the sun.
The chemicals produced from these interactions generated the building blocks of RNA, the close relative of DNA, which most biologists believe was the first molecule of life to carry information.
In the lab, Prof Sutherland's group recreated these chemical reactions under UV lamps.
Dr Rimmer said: "I came across these earlier experiments, and as an astronomer, my first question is always what kind of light are you using, which as chemists they hadn't really thought about.
"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 two groups performed a series of laboratory experiments to measure how quickly the building blocks of life can be formed from hydrogen cyanide and hydrogen sulphite ions in water when exposed to UV light.
They then performed the same experiment in the absence of light.
"There is chemistry that happens in the dark: it's slower than the chemistry that happens in the light, but it's there," says senior author Professor Didier Queloz, also from the Cavendish Laboratory.
"We wanted to see how much light it would take for the light chemistry to win out over the dark chemistry."
The researchers then compared the light chemistry to the dark chemistry against the UV light of different stars.
They found that stars around the same temperature as our Sun emitted enough light for the building blocks of life to have formed on the surfaces of their planets.
Cool stars, on the other hand, do not produce enough light for these building blocks - except if they have powerful solar flares.
Planets that receive enough light to activate the chemistry and could have liquid water on their surfaces are in what the researchers have called the 'abiogenesis zone'.
Among the known exoplanets in the abiogenesis zone are several detected by the Kepler telescope, including Kepler 452b, a planet that has been nicknamed Earth's 'cousin'.
However, it is too far away to probe with current technology.
According to recent estimates, there are as many as 700 million trillion terrestrial planets in the observable universe.
"Of course, being primed for life is not everything and we still don't know how likely the origin of life is, even given favourable circumstances - if it’s really unlikely then we might be alone, but if not, we may have company," Prof Sutherland adds.