Original URL: http://www.theregister.co.uk/2013/09/20/habitable_zone_study_determines_earths_lifetime_exoplanet_intelligence_candidates/

Boffins: Earth will be habitable for only 1.75 BEEELLION more years

UK team floats method for picking exoplanets to study in search for the lair of ET

By Rik Myslewski

Posted in Science, 20th September 2013 00:22 GMT

Unless we meddlesome humans – or our follow-on Earth inhabitants – muck up our planet with a nuclear holocaust, runaway greenhouse emissions, or some other ecological disaster, our 4.54-billion-year-old home should be habitable for at least the next 1.75 billion years.

Well, there's always the possibility of chance encounter with some wandering asteroid or a visit by hungry Kanamits bearing a copy of To Serve Man, but if habitability due to the sun's output and expansion are the only considerations, we've got plenty of time left on terra firma.

"The Earth seems to be habitable for perhaps 6.29 billion years (Gyr), but this is excluding the influence of humans and our pesky habit of pumping extra CO2 into the atmosphere," blogs Andrew Rushby of the University of East Anglia, lead author of a paper (PDF) published on Thursday in the journal Astrobiology entitled "Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars".

Rushby takes pains in his blog post to explain that the true motivation and purpose of his paper was to explore methodologies for determining the habitability of exoplanets that share earthlike characteristics, and not to make "a definitive statement" about our check-out time from our comfortable digs on Hotel Earth.

"We are not" making such a statement, he writes. "Earth is [our] test, our standard, our control. In reality we only validated the model against other more complex models for the Earth, and came to similar conclusion."

The Reg gives a respectful nod to Rushby's intent – and to his observation that "the press machine is a rapid, yet inefficient beast" – but setting a terrestrial deadline is too attractive a headline to let pass unexploited. So sue us.

But to be fair, Rushby's paper is far richer than mere Earth lifecycle dating. In it, he takes into account a broad range of exoplanetary variables, and concludes that a number of candidates may have habitable lifetimes that far exceed that of Earth. For example, he notes that Gliese 581d "might be clement for 42 Gyr! A huge amount of time."

A planet with such a long habitable period would have plenty of time to evolve advanced life forms, Rushby reasons, "if we make the assumption that evolution by natural selection is a universal constant, operating in a similar way in potential exobiological systems as it does on Earth."

Rushby suggests that future exoplanetary investigations – "or SETI campaigns" – might do well to focus on such planets, since the evolution of intelligent life is likely not a simple, few-billion-year affair. In his paper, however, he rightly cautions that "This tendency to view evolution as a progression toward increasing biological complexity and intelligence likely stems from an anthropocentric bias."

That said, he also reasons that if we want to search for extraterrestrial intelligence, it would be wise to rigorously analyze planets that have been around for awhile for evidence of, for example, organisms that have altered their planet's biosignatures to such an extent that they could be detected across interstellar space, since those life forms would "undoubtedly require some level of complexity beyond that of simple replicating molecules."

So if we're going to study exoplanets in some detail, we could do worse than to use Rushby's habitable-zone lifetime filter. After all, we only have about 1.75 billion years left to finish the job. ®