Scientists have realized that spotting signs of life in an alien planet's atmosphere is quite tough. A new study suggests that identifying the prominent "biosignature" target, ozone, is hard to detect from afar since it might get trapped near the equators of Proxima b, TRAPPIST-1d and other potentially habitable worlds that orbit close to their host stars.
"Absence of traces of ozone in future observations does not have to mean there is no oxygen at all," study lead author Ludmila Carone, of the Max Planck Institute for Astronomy in Heidelberg, Germany, said in a statement. "It might be found in different places than on Earth, or it might be very well hidden."
The ozone is an unstable molecule that consists of three oxygen atoms. Speaking from the earth’s perspective, ozone is generally produced in the atmosphere after ultraviolet (UV) radiation from the sun splits "normal" diatomic oxygen (O2).
Most of the Earth's O2 is generated by living organisms that include plants and photosynthetic microbes, it, therefore, serves as a sort of secondary biomarker, at least for Earth-like life.
The atmospheric flows on Earth distribute most ozone relatively evenly into our planet's famous ozone layer, which helps shield life from harmful UV radiation. In a hypothetical case, aliens studying Earth from afar with powerful telescopes would have a good chance of detecting the gas.
However, the situation is different on Proxima b, TRAPPIST-1d and other tidally locked worlds — those that always show the same face to their parent stars, and therefore have a "dayside" and a "nightside" — according to Carone and her colleagues.
According to modeling work performed by the researchers, it turns out that planets with orbital periods of 25 Earth days or less have airflows that tend to concentrate ozone in an equatorial band.
"We all knew from the beginning that the hunt for alien life will be a challenge," Carone said. "As it turns out, we are only just scratching the surface of how difficult it really will be."
The study also suggests that worlds like Proxima b don't have a global ozone layer. That may or may not have a significant negative effect on their habitability, Carone said.
"Proxima b and TRAPPIST-1d orbit red dwarfs, reddish stars that emit very little harmful UV light, to begin with," she said in the statement. (Tidally locked planets pretty much have to orbit dim dwarf stars to be habitable; worlds that orbit so close to sunlike stars are far too hot to host life as we know it.)
"On the other hand, these stars can be very temperamental, and prone to violent outbursts of harmful radiation, including UV," she added. "There is still a lot that we don't know about these red dwarf stars. But I'm confident we will know much more in five years."
In a period of five years, astronomers will have a lot more data derived from telescopes such as NASA's $8.8 billion James Webb Space Telescope, which is scheduled to launch in early 2019. This will inform their inferences about the habitability of red-dwarf planets. It will also help them understand the advances in modeling techniques over this time span.