Scientists say that we should be on the lookout for purple planets, a possible sign of primordial extraterrestrial life. But what about the biosignatures of exoplanets that are dying? A recent study concluded that it is indeed possible — we just have to recognize the signs.
Astrobiologist Jack O'Malley-James at the University of St. Andrews in Fife, Scotland and his colleagues are concerned about what habitable planets might look like in their old age. Furthermore, his team also wants to know how long we can expect to detect life on such a planet over the course of its habitable lifetime.
To find the answers, O'Malley-James ran some computer models of various climates and biospheres of possible exoplanets, including those approximately 2.8 billion years older than Earth.
As their research notes, the diversity of life and population sizes would be significantly reduced during a planet's end-stage. The death of a planet's biosphere would commence with planets dying off, followed by rising temperatures and subsequent wearing away of silicon-loaded rocks known as silicates — an effect that would increase their absorption of carbon dioxide. The next phase would be the extinction of all animals, from large vertebrates right down to the small ones; invertebrates would be the last to go. Fascinatingly, a habitable planet will experience a conclusion similar to its beginning, one crawling in microscopic organisms.
Writing in Astrobiology Magazine, Charles Q. Choi explains how these changes might influence a planet's detectability:
The scientists calculated the extinction of higher plants would lower atmospheric oxygen and ozone levels to concentrations undetectable by astronomers by about 1.11 billion years from now. Still, this drop in oxygen could mean levels of the volatile compound isoprene could build up in the air, potentially serving as a biosignature until plants go extinct. Isoprene is a biological substance that normally has a very short lifetime in the atmosphere, since it quickly reacts with oxygen.
The death of plants and animals would also generate large amounts of decaying matter that would release compounds such as methanethiol into the atmosphere. This gas is only known to come from biological sources — although sunlight rapidly breaks this gas down, the resulting gas, ethane, could serve as a potential biosignature until all plants and animals go extinct.
Methane could also be a biomarker when all other biomarker gases become undetectable in a dying planet's atmosphere. In fact, far-future levels of methane in Earth's atmosphere could be 10 times higher than the present — methane-producing bacteria get more of the carbon dioxide they need as fuel because plants are no longer there to remove the carbon dioxide. Still, the researchers caution life is not the only source of methane — volcanoes and chemical reactions involving volcanic rocks can generate the gas as well.
Another consideration, of course, would be the influence of extraterrestrial intelligence on the biosphere, including elevated levels of carbon dioxide. But as O'Malley pointed out to Choi, "It's certainly possible that intelligent life could play a role in mitigating these changes to the far-future environment, perhaps by some form of geoengineering...or even moving the planet out to orbit in a cooler position. Predicting what that would do to a planet's biosignatures would be quite a challenge, but it may simply make the planet's biosphere appear younger than we would expect given the age of the planet."