How should humans go about searching for alien life? After all, astronomers have already identified thousands of candidate planets – but in new research, scientists have come up with a way to refine what we're actually looking for.
Researchers say it could be the amount of ultraviolet (UV) light coming from a host star that triggers the development of life, and, on this basis, have identified a group of planets that may have developed life in a similar manner to how it occurred on Earth.
"Life as we know it requires a variety of molecular structures that perform various functions within the cell," explains astrophysicist Paul Rimmer of the University of Cambridge at the Conversation.
"These include DNA, RNA, proteins, and cell membranes, which are made up of relatively simple building blocks (lipids, nucleotides, and amino acids). For a long time it was a mystery where those building blocks came from, but recently there have been major breakthroughs in determining how they arose on the surface of the early Earth."
"For example," Rimmer says, "shining ultraviolet light on hydrogen cyanide (a chemical compound that exists in nature) in water, along with an negatively charged ion (an atom that has lost electrons) such as bisulfite, leads to simple sugars."
In the right conditions, hydrogen cyanide - which is abundant in protoplanetary discs - and a negatively charged ion can create large concentrations of many of the building blocks of life. But they need enough UV light to be able to do so, or they wind up as an inert compound.
A research team demonstrated this experimentally in 2015 by shining UV light on hydrogen cyanide to produce lipids, amino acids, and nucleotides, all of which are components of living cells - but when they didn't use enough light, the reaction didn't occur.
Rimmer and his team used this as a basis for their research. They compared the amount of UV light used in the 2015 experiment to the light emitted by stars orbited by Kepler candidates - exoplanets that may be able to host life.
The research team plotted the amount of UV light available to the planets' orbits to calculate where these chemical reactions could be activated - what they term the abiogenesis zone.
The criteria for a Kepler candidate are that the planet has to be rocky, which essentially means below a certain size limit; and that it is orbiting in what is termed the "habitable zone".
This means that it is not so close to the star that any liquid water would have evaporated, nor so far that any liquid water would have completely frozen. Obviously there would need to be more conditions for habitability, but those are the ones that astronomers can look for without being able to see the planet directly.
What the researchers found is that the habitable zone and the abiogenesis zone don't always overlap. Here on Earth, we're in the perfect spot in orbit around just the right kind of star - and indeed it's stars that are the same temperature as our Sun that produce the right amount of UV light, the team found.
So this might rule out life on the heretofore promising Ross 128 b, but it's not all bad news. After all, an exoplanet NASA called "Earth's cousin" in 2015 - Kepler 452b - falls both within the habitable zone and the abiogenesis zone.
It is possible that extraterrestrial life may be totally different elsewhere, but since we don't know what other forms it might take, the best course of action is to look for what we know.
"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," said 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."
The research has been published in the journal Science Advances.