Identified: Exoplanets where life could develop like Earth

Scientists have identified a group of planets outside our solar system where the same chemical conditions that may have led to life on Earth exist. The researchers, from the University of Cambridge 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 host star. The 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, where the UV light powers a series of chemical reactions that produce the building blocks of life.

The researchers have identified a range of planets where the UV light from their host 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 planet's surface. "This work allows us to narrow down the best places to search for life," said Paul Rimmer, a postdoctoral researcher with a joint affiliation at Cambridge and the Medical Research Council Laboratory of Molecular Biology in the UK.

"It brings us just a little bit closer to addressing the question of whether we are alone in the universe," Rimmer said. In a paper published in 2015, Sutherland's group at the MRC LMB proposed that cyanide, although a deadly poison, was in fact a key ingredient in the primordial soup 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.

The 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. The researchers 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 to be formed, except if they have frequent powerful solar flares to jolt the chemistry forward step by step. Planets that both receive enough light to activate the chemistry and could have liquid water on their surfaces reside in what the researchers have called the abiogenesis zone. 

Among the known exoplanets which reside in the abiogenesis zone are several planets detected by the Kepler telescope, including Kepler 452b, a planet that has been nicknamed Earth's 'cousin', although it is too far away to probe with current technology. Next-generation telescopes, such as NASA's TESS and James Webb Telescopes, will hopefully be able to identify and potentially characterise many more planets that lie within the abiogenesis zone, researchers said.