The star 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 Dr Paul Rimmer, a postdoctoral researcher with a joint affiliation at Cambridge’s Cavendish Laboratory and the MRC LMB, and the paper’s first author. “It brings us just a little bit closer to addressing the question of whether we are alone in the universe.”
In the lab UV lamps were used to generate the precursors to lipids, amino acids and nucleotides. These are all essential components of living cells.
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 characterize many more planets that lie within the abiogenesis zone.
Using a known reliable pathway for photochemically building up the prebiotic inventory in large yields, we show that hotter stars serve as better engines for prebiotic chemistry. Investigating the race between light and dark bisulfite chemistry, we find, based on our requirement for >50%. yields, that, even for early Earth, the prebiotic inventory would need to be built up in places where the surface temperature is below ~ 20∘C.
Because of the efficiency of the bisulfite photochemistry, rocky planets within the liquid water habitable zones of K dwarfs can also lie within the abiogenesis zone, so long as the temperature is very close to 0∘C. We applied our results to a catalog of potentially rocky exoplanets within the liquid water habitable zones of their host stars. For gas giants within the liquid water habitable zone, there is a tantalizing possibility that some of their larger moons may be primed for life (20).
The abiogenesis zone we define need not overlap the liquid water habitable zone. The liquid water habitable zone identifies those planets that are a sufficient distance from their host star for liquid water to exist stably over a large fraction of their surfaces. In the scenario we consider, the building blocks of life could have been accumulated very rapidly compared to geological time scales, in a local transient environment, for which liquid water could be present outside the liquid water habitable zone. The local and transient occurrences of these building blocks would almost certainly be undetectable. The liquid water habitable zone helpfully identifies where life could be sufficiently abundant to be detectable.
For main sequence stars cooler than K5 dwarfs, the quiescent stellar flux is too low for the planets within their habitable zones to also lie within their abiogenesis zones. Planets within the habitable zones of quiet ultracool dwarfs may be able to house life, but life could not presently originate as a result of photochemistry on these worlds, although it possibly could have done in the past, if these stars emitted much more strongly in the UV before they entered into the main sequence or if they had been much more active in the past. Our results are only valid for the stars as they are now.
Given that the macromolecular building blocks of life were likely produced photochemically in the presence of ultraviolet (UV) light, we identify some general constraints on which stars produce sufficient UV for this photochemistry. We estimate how much light is needed for the UV photochemistry by experimentally measuring the rate constant for the UV chemistry (“light chemistry”, needed for prebiotic synthesis) versus the rate constants for the bimolecular reactions that happen in the absence of the UV light (“dark chemistry”). We make these measurements for representative photochemical reactions involving Embedded Image and HS−. By balancing the rates for the light and dark chemistry, we delineate the “abiogenesis zones” around stars of different stellar types based on whether their UV fluxes are sufficient for building up this macromolecular prebiotic inventory. We find that the Embedded Image light chemistry is rapid enough to build up the prebiotic inventory for stars hotter than K5 (4400 K). We show how the abiogenesis zone overlaps with the liquid water habitable zone. Stars cooler than K5 may also drive the formation of these building blocks if they are very active. The HS− light chemistry is too slow to work even for early Earth.
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