Exoplanet habitability connected to host star’s light

Ability to produce life’s building blocks depends on level of ultraviolet light received from the host star.

The chances of life developing on rocky, Earth-like exoplanets may be determined by both the type and intensity of the light emitted by the planets’ host stars, according to a new study published in the journal Science Advances.

A group of scientists at Cambridge University and at the Medical Research Council Laboratory of Molecular Biology (MRC LMB) identified a list of rocky exoplanets whose stars emit sufficient ultraviolet light for the chemical reactions that produce the building blocks of life to occur.

On Earth, ultraviolet light from the Sun started this series of chemical reactions.

All the exoplanets the researchers identified are located in their stars’ habitable regions, with temperatures that allow liquid water to exist on their surfaces.

“This work allows us to narrow down the best places to search for life,” said Paul Rimmer of both Cambridge University’s Cavendish Laboratory and MRC LMB. “It brings us just a little bit closer to addressing the question of whether we are alone in the universe.”

The latest study is part of an ongoing collaboration between the two above institutions combining exoplanet research with organic chemistry. It builds on a 2015 paper, also published in Science Advances, in which scientists theorized that meteorites containing carbon impacted early Earth, releasing that carbon, which then interacted with atmospheric nitrogen to produce hydrogen cyanide.

Although hydrogen cyanide itself is toxic to life, its interaction with various elements on Earth’s surface via power from ultraviolet sunlight generated the building blocks of RNA, theorized to be the first molecule of life to carry information. RNA is closely related to DNA, the self-replicating material that carries genetic information in practically all organisms.

In a series of laboratory experiments, the researchers exposed water containing hydrogen cyanide and hydrogen sulphite ions to ultraviolet light and to no light to find out how quickly the building blocks of life would form.

“There is a chemistry that happens in the dark: it’s slower than the chemistry that happens in the light, but it’s there. We wanted to see how much light it would take for the light chemistry to win out over the dark chemistry,” explained Didier Queloz, also of Cavendish Laboratory.

Under darkness, the hydrogen cyanide and hydrogen sulphite produced an inert compound that could not form the building blocks of life. However, under ultraviolet light, they did produce these building blocks.

The scientists then compared the light chemistry used in the experiment to the ultraviolet light of various stars as well as the amount of light available to those stars’ planets and found that stars with roughly the same temperature as our Sun emitted sufficient light to form life’s building blocks.

Planets with temperatures that allow liquid water on their surfaces that also receive the appropriate amount of ultraviolet light to start this chemical process were designated as being in the abiogenesis zone.


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