Will we know life when we see it?

A group of leading researchers in astronomy, biology and geology gathered under NASA's Nexus for Exoplanet System Science, or NExSS, to take stock of our knowledge in the search for life on distant planets and lay the foundation for advancing the related sciences. .

"We are moving from life theory elsewhere in our galaxy to a robust science that will ultimately give us the answer we seek to this profound question: Are we alone?" said Martin Still, an exoplanet scientist at NASA headquarters in Washington.

In a set of five articles published last week in the scientific journal astrobiologyNExSS scientists have compiled an inventory of the most promising signs of life, called biosignatures. The paper authors include four scientists from NASA's Jet Propulsion Laboratory in Pasadena, California. They considered how to interpret the presence of biosignatures, should we detect them on distant worlds. A major concern is to ensure that science is strong enough to distinguish a living world from a sterile planet posing as one.

The evaluation is presented as a new generation of space and ground telescopes are being developed. NASA's James Webb Space Telescope will characterize the atmospheres of some of the first small rocky planets. Other observatories are planned, such as the giant Magellan telescope and the Extremely Large telescope, both in Chile, to carry sophisticated instruments capable of detecting early biosignatures on distant worlds.

Through their work with NExSS, scientists are looking to identify the tools needed to detect the potential life of future NASA flagship missions. Detection of the atmospheric signatures of some potentially habitable planets may possibly come before 2030, although determining whether the planets are truly habitable or have life will require further study.

Since we will not be able to visit distant planets and collect samples anytime soon, the light that a telescope will observe will be all we have in search of life outside of our solar system. Telescopes can examine the light reflected on a distant world to show us the types of gases in the atmosphere and their "seasonal" variations, as well as colors like green that could indicate life.

These types of biosignatures can all be seen on our fertile Earth from space, but the new worlds we examine will differ considerably. For example, many of the promising planets we have found are around colder stars, which emit light in the infrared spectrum, unlike the strong visible light emissions of our sun.

"What does a living planet look like?" said Mary Parenteau, an astrobiologist and microbiologist at NASA's Ames Research Center in Silicon Valley and a co-author. "We need to be open to the possibility that life can arise in many contexts in a galaxy with so many different worlds – perhaps with a life of purple color instead of the familiar life forms dominated by the green on Earth, by example, considering a wide range of biosignatures. "

Scientists claim that oxygen – the gas produced by photosynthetic organisms on Earth – remains the most promising biosignature of life elsewhere, but it is not infallible. The abiotic processes on a planet could also generate oxygen. Conversely, a planet devoid of detectable levels of oxygen could still be alive – which was exactly the case of the Earth before the global accumulation of oxygen in the world. atmosphere.

"On primitive Earth, we could not see oxygen, despite an abundant life," said Victoria Meadows, astronomer at the University of Washington in Seattle and senior author of one of the articles. . "Oxygen teaches us that seeing or not seeing a single biosignature is insufficient evidence for or against life – the general context is important."

Rather than measuring a single feature, NExSS scientists say we should look at a series of traits. A planet must be able to support life through its features and those of its parent star.

NExSS scientists will create a framework that will quantify the probability that a planet will have a life, based on all available evidence. With the observation of many planets, scientists can begin to classify more broadly the "living worlds" that show common features of life, compared to "non-living worlds".

"We will not have" yes "or" no "to find life elsewhere," said Shawn Domagal-Goldman, an astrobiologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and co-author. "What we will have is a high level of trust that a planet appears alive for reasons that can only be explained by the presence of life."

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