The hoary academic admonition that absence of evidence is not evidence of absence has received fresh application in the field of astrobiology.
Modelling of atmospheric turbulence on Proxima-b – a planet orbiting red dwarf star Proxima Centauri and thought to be a good candidate for hosting life – shows that tell-tale chemical signatures may be hidden by gases cycling between the poles and the equator.
In a paper published in the journal Monthly Notices of the Royal Astronomical Society, researchers led by Ludmila Carone of the Max Planck Institute for Astronomy in Germany suggest that unusual air flows could be trapping ozone – considered a possible indicator of oxygen-producing life – at the planet’s equator, hiding it from Earth-bound observers.
Carone and her colleagues draw the same conclusion for another hot favourite in the astrobiological life stakes, one of the planets orbiting the red dwarf star TRAPPIST-1.
The modelling is based on the short duration of the orbits of the target planets around their respective host stars. All of them make the journey in under 25 Earth-equivalent days. As a result, the planets are “tidally locked”, with one side permanently facing the star, while the other faces away.
The temperature gradients thus engendered would have a pronounced effect on any ozone that may be released into the atmosphere. Major air flows in the stratosphere are likely to proceed from the poles to the equator, sequestering the ozone in a way that makes it undetectable to present technology.
“Absence of traces of ozone in future observations does not have to mean there is no oxygen at all,” says Carone.
“It might be found in different places than on Earth, or it might be very well hidden. We all knew from the beginning that the hunt for alien life will be a challenge. As it turns out, we are only just scratching the surface of how difficult it really will be.”
The team also found that the models had implications for possible future human missions to such planets. Ozone in the Earth’s atmosphere plays a critical role in reducing the amount of ultraviolet light from the sun reaching the surface.
Even assuming the presence of ozone-enabling life, Proxima-b and the TRAPPIST-1 planets do not have such a protective layer, although the air flow may result in a narrow band of it girdling the equator.
The absence of a blanket ozone layer, however, says Carone, does not necessarily mean conditions incompatible with human survival.
“Proxima-b and TRAPPIST-1d orbit red dwarfs, reddish stars that emit very little harmful UV light to begin with,” she explains. “On the other hand, these stars can be very temperamental, and prone to violent outbursts of harmful radiation including UV.”