If life does exist anywhere else in the universe, it may only be fleeting. Now scientists are researching how signs of life might look on dying planets.
Astronomers have discovered hundreds of distant alien planets in the past two decades. Future missions could detect potential signs of life called biosignatures on those worlds, such as oxygen or methane in their atmospheres.
Astrobiologist Jack O'Malley-James at the University of St. Andrews in Fife, Scotland and his colleagues noted that biosignatures of life on Earth have not remained the same over time, but have altered considerably over its history. This led the researchers to speculate about how Earth and other planets might look in the future.
"Astrobiology as a field seems to put a lot more focus on the origins of life and how to find life beyond Earth, but less emphasis is put on the end of life, which is what got me interested in finding out more about how biospheres on other planets might meet their ends, and by extension, how long we could expect to detect life on a habitable planet over the course of its habitable lifetime," said O'Malley-James, the lead author of the study.
The scientists were testing a computer model of the climates and biospheres — the overall life — of possible exoplanets.
"That was when the idea came about to run this model forward in time to see when all water and all life would disappear from the planet," O'Malley-James said.
The Sun is a middle-aged star, currently about 4.6 billion years old. In the later stages of its evolution, about 2 billion to 3 billion years from now, the Sun will grow much hotter, leading to much higher surface temperatures on the future Earth and thus far harsher environments for any last life to grow and survive on the planet.
The research team modeled the biosignature gases Earth's biosphere would generate up to 2.8 billion years from the present.
Swansong Biospheres II: The final signs of life on terrestrial planets near the end of their habitable lifetimes
Authors:
O'Malley-James et al
Abstract:
The biosignatures of life on Earth do not remain static, but change considerably over the planet's habitable lifetime. Earth's future biosphere, much like that of the early Earth, will consist of predominantly unicellular microorganisms due to the increased hostility of environmental conditions caused by the Sun as it enters the late stage of its main sequence evolution. Building on previous work, the productivity of the biosphere is evaluated during different stages of biosphere decline between 1 Gyr and 2.8 Gyr from present. A simple atmosphere-biosphere interaction model is used to estimate the atmospheric biomarker gas abundances at each stage and to assess the likelihood of remotely detecting the presence of life in low-productivity, microbial biospheres, putting an upper limit on the lifetime of Earth's remotely detectable biosignatures. Other potential biosignatures such as leaf reflectance and cloud cover are discussed.
pop sci write up.
In 2 billion years’ time, life on Earth will be confined to pockets of liquid water deep underground, according to PhD astrobiologist Jack O’Malley James of the University of St Andrews. The new research also suggests that though the hardiest forms of life may have a foothold on similar worlds in orbit around other stars, evidence for it may be very subtle. O’ Malley- James presented the findings at the National Astronomy Meeting in St Andrews, Scotland.
All species have finite lifetimes, with each eventually facing an event that leads to its extinction. This can be sudden and catastrophic, like the giant impact that wiped out the dinosaurs, or a slow and gradual process. Ultimately, a combination of slow and rapid environmental changes will result in the extinction of all species on Earth, with the last inhabitants disappearing within 2.8 billion years from now.
The main driver for these changes will be the Sun. As it ages over the next few billion years, the Sun will remain stable but become steadily more luminous, increasing the intensity of its heat felt on Earth and warming the planet to such an extent that the oceans evaporate. In his new work, O’Malley James has created a computer model to simulate these extremely long-range temperature forecasts and has used the results to predict the timeline of future extinctions.
Within the next billion years, increased evaporation rates and chemical reactions with rainwater will draw more and more carbon dioxide from the Earth’s atmosphere. The falling levels of CO2 will lead to the disappearance of plants and animals and our home planet will become a world of microbes. At the same time the Earth will be depleted of oxygen and will be drying out as the rising temperatures lead to the evaporation of the oceans. A billion years after that the oceans will have gone completely.
"The far-future Earth will be very hostile to life by this point", said O'Malley-James. "All living things require liquid water, so any remaining life will be restricted to pockets of liquid water, perhaps at cooler, higher altitudes or in caves or underground". This life will need to cope with many extremes like high temperatures and intense ultraviolet radiation and only a few microbial species known on Earth today could cope with this.
The new model not only tells us a lot about our own planet's future, but it can also help us to recognise other inhabited planets that may be approaching the end of their habitable lifetimes.
