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Study: Earth to be livable 2-3 billion years more, but not for all

Sept. 17, 2013
Courtesy of the University of East Anglia
and World Science staff


Earth should re­main hab­it­a­ble for about two or three bil­lion years long­er—though glob­al warm­ing is help­ing to as­sure that only mi­crobes will be around by then, sci­en­tists are re­port­ing.

Fur­ther­more, that time span, though enor­mous, is much shorter than the Sun’s ex­pected ac­tive life­time. That’s be­cause a grad­u­al ex­pan­sion of the Sun, long pre­dat­ing its fi­nal de­mise, will kill eve­ry­thing off. Only a move fur­ther from the Sun could avert this.

“If we ev­er needed to move to an­oth­er plan­et, Mars is probably our best bet. It’s very close and will re­main in the [Sun’s] ‘hab­it­a­ble zone’ un­til the end of the Sun’s life­time—six bil­lion years from now,” said An­drew Rushby, an as­tro­bi­ol­o­gist at the Uni­vers­ity of East An­glia in the U.K. A hab­it­a­ble zone is the dis­tance from the Sun or oth­er star at which a plan­et’s sur­face tem­per­a­tures are suit­a­ble for liq­uid wa­ter.

A study by Rushby and col­leagues, pub­lished Sept. 18 in the jour­nal As­tro­bi­ol­o­gy, es­ti­mates Earth’s hab­it­a­ble life­time based on our dis­tance from the Sun and the tem­per­a­ture range al­low­ing for liq­uid wa­ter. The re­search­ers looked to the stars for in­spira­t­ion: us­ing re­cently found plan­ets out­side our so­lar sys­tem, or exoplan­ets, as ex­am­ples, they in­ves­t­i­gated the plan­ets’ po­ten­tial to host life.

“We used stel­lar ev­o­lu­tion mod­els to es­ti­mate the end of a plan­et’s hab­it­a­ble life­time by de­ter­min­ing when it will no long­er be in the hab­it­a­ble zone,” Rushby said. “We es­ti­mate that Earth will cease to be hab­it­a­ble some­where be­tween 1.75 and 3.25 bil­lion years from now. Af­ter this point, Earth will be in the ‘hot zone’ of the sun, with tem­per­a­tures so high that the seas would evap­o­rate. We would see a catas­trophic and ter­mi­nal ex­tinc­tion event for all life.”

Com­plex life forms will be more vul­ner­a­ble and die off soon­er than our fast-reproducing, adapt­a­ble and in­credibly nu­mer­ous mi­cro­bi­al sis­ters and broth­ers, he added. Our ear­li­er de­mise “is be­ing ac­cel­er­ated by an­thro­pogenic [human-caused] cli­mate change,” he ex­plained. “Hu­mans would be in trou­ble with even a small in­crease in tem­per­a­ture, and near the end only mi­crobes in niche en­vi­ron­ments would be able to en­dure the heat.

“Look­ing back a si­m­i­lar amount of time, we know that there was cel­lu­lar life on earth. We had in­sects 400 mil­lion years ago, di­no­saurs 300 mil­lion years ago and flow­er­ing plants 130 mil­lion years ago. An­a­tom­ic­ally mod­ern hu­mans have only been around for the last 200,000 years—so you can see it takes a really long time for in­tel­li­gent life to de­vel­op.

“The amount of hab­it­a­ble time on a plan­et is very im­por­tant be­cause it tells us about the po­ten­tial for the ev­o­lu­tion of com­plex life—which is likely to re­quire a long­er pe­ri­od of hab­it­a­ble con­di­tions,” he went on. “Look­ing at hab­it­abil­ity met­rics is use­ful be­cause it al­lows us to in­ves­t­i­gate the po­ten­tial for oth­er plan­ets to host life, and un­der­stand the stage that life may be at else­where in the gal­axy.

“Of course, much of ev­o­lu­tion is down to luck, so this is­n’t con­crete, but we know that com­plex, in­tel­li­gent spe­cies like hu­mans could not emerge af­ter only a few mil­lion years be­cause it took us 75 per­cent of the en­tire hab­it­a­ble life­time of this plan­et to evolve. We think it will probably be a si­m­i­lar sto­ry else­where.”

As­tro­no­mers have iden­ti­fied al­most 1,000 plan­ets out­side our so­lar sys­tem. Rushby and col­leagues looked at some of these as ex­am­ples, and stud­ied the evolv­ing na­ture of plan­etary hab­it­abil­ity over as­tronomical and ge­o­log­i­cal time. “In­ter­est­ingly, not many oth­er pre­dic­tions based on the hab­it­a­ble zone alone were availa­ble, which is why we de­cid­ed to work on a meth­od for this. Oth­er sci­en­tists have used com­plex mod­els to make es­ti­mates for the Earth alone, but these are not suit­a­ble for ap­ply­ing to oth­er plan­ets,” he said.

“We com­pared Earth to eight plan­ets which are cur­rently in their hab­it­a­ble phase, in­clud­ing Mars. We found that plan­ets or­bit­ing smaller mass stars tend to have long­er hab­it­a­ble zone life­times.

“One of the plan­ets that we ap­plied our mod­el to is Kep­ler 22b, which has a hab­it­a­ble life­time of 4.3 to 6.1 bil­lion years. Even more sur­pris­ing is Gliese 581d, which has a mas­sive hab­it­a­ble life­time of be­tween 42.4 to 54.7 bil­lion years. This plan­et may be warm and pleas­ant for 10 times the en­tire time that our so­lar sys­tem has ex­isted!

“To date, no true Earth an­a­logue plan­et has been de­tected. But it is pos­si­ble that there will be a hab­it­a­ble, Earth-like plan­et with­in 10 light-years, which is very close in as­tronomical terms. Howev­er reach­ing it would take hun­dreds of thou­sands of years with our cur­rent tech­nol­o­gy.” A light-year is the dis­tance light trav­els in a year.


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Earth should remain habitable for about two or three billion years longer—though global warming is helping to assure that only microbes will be around by then, scientists are reporting. Furthermore, that time span, though enormous, is much shorter than the Sun’s expected active lifetime. That’s because a gradual expansion of the Sun, long predating its final demise, will kill everything off. Only a move further from the Sun could avert this. “If we ever needed to move to another planet, Mars is probably our best bet. It’s very close and will remain in the [Sun’s] ‘habitable zone’ until the end of the Sun’s lifetime—six billion years from now,” said Andrew Rushby, an astrobiologist at the University of East Anglia in the U.K. A habitable zone is the distance from the Sun or other star at which a planet’s surface temperatures are suitable for liquid water. A study by Rushby and colleagues, published Sept. 18 in the journal Astrobiology, estimates Earth’s habitable lifetime based on our distance from the Sun and the temperature range allowing for liquid water. The researchers looked to the stars for inspiration: using recently found planets outside our solar system, or exoplanets, as examples, they investigated the planets’ potential to host life. “We used stellar evolution models to estimate the end of a planet’s habitable lifetime by determining when it will no longer be in the habitable zone,” Rushby said. “We estimate that Earth will cease to be habitable somewhere between 1.75 and 3.25 billion years from now. After this point, Earth will be in the ‘hot zone’ of the sun, with temperatures so high that the seas would evaporate. We would see a catastrophic and terminal extinction event for all life.” Complex life forms will be more vulnerable and die off sooner than our fast-reproducing, adaptable and incredibly numerous microbial sisters and brothers, he added. Our earlier demise “is being accelerated by anthropogenic [human-caused] climate change,” he explained. “Humans would be in trouble with even a small increase in temperature, and near the end only microbes in niche environments would be able to endure the heat. “Looking back a similar amount of time, we know that there was cellular life on earth. We had insects 400 million years ago, dinosaurs 300 million years ago and flowering plants 130 million years ago. Anatomically modern humans have only been around for the last 200,000 years—so you can see it takes a really long time for intelligent life to develop. “The amount of habitable time on a planet is very important because it tells us about the potential for the evolution of complex life—which is likely to require a longer period of habitable conditions,” he went on. “Looking at habitability metrics is useful because it allows us to investigate the potential for other planets to host life, and understand the stage that life may be at elsewhere in the galaxy. “Of course, much of evolution is down to luck, so this isn’t concrete, but we know that complex, intelligent species like humans could not emerge after only a few million years because it took us 75 percent of the entire habitable lifetime of this planet to evolve. We think it will probably be a similar story elsewhere.” Astronomers have identified almost 1,000 planets outside our solar system. Rushby and colleagues looked at some of these as examples, and studied the evolving nature of planetary habitability over astronomical and geological time. “Interestingly, not many other predictions based on the habitable zone alone were available, which is why we decided to work on a method for this. Other scientists have used complex models to make estimates for the Earth alone, but these are not suitable for applying to other planets,” he said. “We compared Earth to eight planets which are currently in their habitable phase, including Mars. We found that planets orbiting smaller mass stars tend to have longer habitable zone lifetimes. “One of the planets that we applied our model to is Kepler 22b, which has a habitable lifetime of 4.3 to 6.1 billion years. Even more surprising is Gliese 581d, which has a massive habitable lifetime of between 42.4 to 54.7 billion years. This planet may be warm and pleasant for 10 times the entire time that our solar system has existed! “To date, no true Earth analogue planet has been detected. But it is possible that there will be a habitable, Earth-like planet within 10 light-years, which is very close in astronomical terms. However reaching it would take hundreds of thousands of years with our current technology.” A light-year is the distance light travels in a year. billion years