Researchers have detected a bunch of planets outside our solar system where the same chemical conditions that may have opened the way for life on Earth exist.
The team, from the University of Cambridge and the Medical Research Council Laboratory of Molecular Biology (MRC LMB), discovered that the chances for life to originate on the surface of a rocky planet similar to Earth are related to the nature and strength of light emitted by its host star.
Their research, reported in the Science Advances journal, suggests that stars which emit adequate ultraviolet (UV) light could stimulate the origin of life on their orbiting planets in a way similar to the probable development of life on Earth, where the UV light powers a sequence of chemical reactions that synthesize the building blocks of life.
The team has detected a range of planets where the UV light from their host star is adequate to enable these chemical reactions to occur, and that is located within the habitable range in which liquid water can exist on the surface of the planet.
“This work allows us to narrow down the best places to search for life,” stated Dr Paul Rimmer, a postdoctoral researcher with a joint affiliation at Cambridge’s Cavendish Laboratory and the MRC LMB, and the first author of the paper. “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 outcome of a progressive collaboration between the Cavendish Laboratory and the MRC LMB, uniting organic chemistry and exoplanet studies. It builds on the study of Professor John Sutherland, a co-author on the current paper, who conducts research on the chemical origin of life on Earth.
In 2015, Professor Sutherland’s team at the MRC LMB published a paper proposing that cyanide, known to be a lethal poison, was actually an important constituent in the primordial soup from which all life on Earth developed.
The hypothesis states that carbon from meteorites that hit the young Earth reacted with nitrogen in the atmosphere to form hydrogen cyanide. The hydrogen cyanide reached the surface of the Earth through rains, where it reacted with other elements in different ways, powered by Sun’s UV light. The chemicals created as a result of these reactions produced the building blocks of RNA, the close kin of DNA which is considered by most biologists to be the first molecule of life to carry information.
In the lab, Sutherland’s team recreated the chemical reactions under UV lamps, and produced the precursors to amino acids, lipids, and nucleotides, all of which are fundamental constituents of living cells.
“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,” stated Rimmer. “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 teams carried out a sequence of laboratory experiments to measure how rapidly was it possible to produce the building blocks of life from hydrogen cyanide and hydrogen sulfite ions in water upon exposure to UV light. Then, they conducted 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,” stated 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 same experiment conducted under dark conditions with the hydrogen cyanide and the hydrogen sulfite led to an inert compound which could not be used to form the building blocks of life, whereas the same experiment performed under the lights led to the formation of the required building blocks.
Then, the scientists compared the light chemistry to the dark chemistry with respect to the UV light from different stars. The amount of UV light available to planets in orbit around these stars was plotted to ascertain where it is possible to activate the chemistry.
They discovered that stars with a temperature same as that of our sun emitted sufficient light for enabling the formation of the building blocks of life on the surfaces of their planets. In contrast, cool stars do not produce sufficient light for the formation of these building blocks, except when they have frequent powerful solar flares to advance the chemistry forward gradually. Planets receiving sufficient light for the activation of the chemistry and possibly having liquid water on their surfaces are located in the abiogenesis zone, as termed by the researchers.
Of the familiar exoplanets located in the abiogenesis zone are various planets identified by the Kepler telescope, including Kepler 452b, a planet that has been labeled Earth’s “cousin,” even though it is highly distant to be detected using existing technology. Futuristic telescopes, such as NASA’s TESS and James Webb Telescopes, will probably be able to detect and prospectively characterize many more planets located within the abiogenesis zone.
Most certainly, probabilities are also that if life exists on other planets, it has developed or will develop in a way completely different from life on Earth.
“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,” stated Rimmer. “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.”
Recent predictions indicate that there are nearly 700 million trillion terrestrial planets in the observable universe. “Getting some idea of what fraction have been, or might be, primed for life fascinates me,” stated Sutherland. “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.”
The Kavli Foundation and the Simons Foundation funded the study.