The Rolling Stones’ Mick Jaggar crooned “Time Is On My Side” in the 1964 classic rock hit of the same title. Sadly, that’s not the case for habitable planets orbiting sun-like stars according to a recent computer simulation by astrobiologist Jack O’Malley-James of the University of St Andrews in the United Kingdom.
“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,” O’Malley-James predicts.
He says that we’ve got about 2 billion years left before the oceans will have evaporated leaving behind a desiccated sand dune landscape as alien-looking as that of Mars. The last vestiges of life on Earth will have retreated to the few scattered reservoirs of water left on our planet.
As is well-known from stellar evolution theory the sun will remain stable over the next few billion years but become steadily brighter as fusion reactions in the core change.
His modeling shows that within the next billion years, increased evaporation rates and future chemical reactions with rainwater will draw more and more carbon dioxide from the Earth’s atmosphere. The falling levels of carbon dioxide will lead to the extinction of plants and animals and Earth 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.
“The far-future Earth will be very hostile to life by this point.” O’Malley-James says. “All living things require liquid water, so any remaining life will be restricted to pockets of liquid water, perhaps at cooler, higher altitudes (as with the lakes on Titan, pictured right) or in caves or underground. This life will need to cope with many extremes like high temperatures and intense ultraviolet radiation.”
This gloomy forecast is sobering because there has been a lot of news about finding Earth-sized planets in the habitable zones around other stars. But what’s mostly overlooked is the temporal dimension. How old are the planets? What is their stage of evolution?
Though the sun burns as a main sequence star for 10 billion years, the window of opportunity for advanced life on Earth is about 25 percent of the sun’s lifetime, according to this latest model.
Those exoplanets where conditions have deteriorated to where life has moved underground (such as is likely the case with Mars) have feeble or no chemical biotracers to study from light-years away. “Dying Earths will have a nitrogen and carbon-dioxide atmosphere with methane being the only sign of active life,” O’Malley-James predicts.
Recent estimates for the number of Earth-like planets in the galaxy range from 17 to 100 billion. Let’s be especially conservative and say 10 billion are Earth clones. Most of these will orbit red dwarf stars that are far more long-lived that our sun. This leaves us with 1 billion Earths orbiting solar-type stars. But roughly 250 million of these are at a stage right now where they can support complex life according to O’Malley-James’ model.
Still, these are not bad odds for finding someone else out there in the galaxy.
I would further argue that alien civilizations orbiting a sunlike star are more likely to pursue interstellar colonization because of the comparatively short lifespan of their home star. And extraterrestrials living in binary systems (like Alpha Centauri) would further be motivated to pursue space-faring because they want to explore inhabitable planets orbiting the companion star.
Alternatively, aliens may chose to slowly move their home world into a wider orbit that backs off from a heating-up star. This requires no more science that a straightforward application of Newtonian mechanics. Newly forming stars are dynamically shuffling planets around all the time.
Therefore, the future is not so dour if we plan on either moving Earth to a cooler place in space, or leaving our home world behind to colonize another star.
Image credits: iStockPhoto, NASA