'Biosignature' study could help astronomers detect alien life on other planets
Researchers from the University of Washington believe the evidence of oxygen – a biosignature – is key in the search for life outside our solar system.
But just because a planet has oxygen does not mean life is sustainable or present there. For example, Venus contains both oxygen and carbon dioxide, but its atmosphere is too hot and acidic to sustain life – so the oxygen is a 'false positive.'
Ruling out such false positives is essential for scientists searching for life, and the researchers claim they have a found a way to do so easily. That process hinges on the process of spectroscopy, the study of spectral features of light visible through a planet’s atmosphere when it transits or passes in front of its host star.
“We wanted to determine if there was something we could observe that gave away these ‘false positive’ cases among exoplanets,” lead author Edward Schwieterman, a doctoral student in astronomy, said in a statement. “We call them ‘biosignature impostors’ in the paper.
“The potential discovery of life beyond our solar system is of such a huge magnitude and consequence, we really need to be sure we’ve got it right – that when we interpret the light from these exoplanets we know exactly what we’re looking for, and what could fool us,” Schwieterman continued.
Previous research has found that some worlds can create oxygen 'abiotically,' or by non-living means. This is most likely in the case of planets orbiting low-mass stars, which are smaller and dimmer than our sun and the most common in the universe.
Building on that research, the study's authors were able to identify an abiotic method which occurred when a star's ultraviolet light splits apart from carbon dioxide molecules, freeing some of the oxygen atoms to form into O2, the type of oxygen present in Earth’s atmosphere.
But the key indicator that such oxygen may not indicate life was found when the researchers used computer modeling to determine that the process produces not only oxygen, but also significant and potentially detectable levels of carbon monoxide.
“So if we saw carbon dioxide and carbon monoxide together in the atmosphere of a rocky planet, we would know to be very suspicious that future oxygen detections would mean life,” Schwieterman said.
The researchers also found an indicator for abiotic oxygen resulting from starlight breaking down atmospheric water, allowing hydrogen to escape and leaving vast quantities of oxygen – more than Earth has ever had in its atmosphere.
Schwieterman noted that in such cases, oxygen molecules collide with each other frequently, producing short-lived pairs of oxygen molecules that become O4 molecules, with their own unique signature.
“Certain O4 features are potentially detectable in transit spectroscopy, and many more could be seen in reflected light,” Schwieterman said. “Seeing a large O4 signature could tip you off that this atmosphere has far too much oxygen to be biologically produced.”
“With these strategies in hand, we can more quickly move on to more promising targets that may have true oxygen biosignatures,” he continued.
Schwieterman and his colleagues believe the research will help scientists using the James Webb Telescope – set for launch in 2018 – study the possibility of life on exoplanets.
The research, funded by the NASA Astrobiology Institute, was published in Astrophysical Journal Letters.