Planet earth: not long to live
Only about 1750000000 years of life left on earth, according to a paper by Andrew Rushby and friends in the journal Astrobiology:
The HZ [habitable zone] lifetime for Earth ranges between 6.29 and 7.79×109 years (Gyr). The 7 exoplanets fall in a range between 1 and 54.72 Gyr, while the 27 Kepler candidate planets’ HZ lifetimes range between 0.43 and 18.8 Gyr. Our results show that exoplanet HD 85512b is no longer within the HZ, assuming it has an Earth analog atmosphere. The HZ lifetime should be considered in future models of planetary habitability as setting an upper limit on the lifetime of any potential exoplanetary biosphere, and also for identifying planets of high astrobiological potential for continued observational or modeling campaigns.
A news item in Nature explains:
Earth will be able to host life for just another 1.75 billion years or so, according to a study published on 18 September in Astrobiology. The method used to make the calculation can also identify planets outside the Solar System with long ‘habitable periods’, which might be the best places to look for life.
The habitable zone around a star is the area in which an orbiting planet can support liquid water, the perfect solvent for the chemical reactions at the heart of life. Too far from a star and a planet’s water turns to permanent ice and its carbon dioxide condenses; too close, and the heat turns water into vapour that escapes into space.
Habitable zones are not static. The luminosity of a typical star increases as its composition and chemical reactions evolve over billions of years, pushing the habitable zone outward. Researchers reported in March that Earth is closer to the inner edge of the Sun’s habitable zone than previously thought.
The inner edge of the Sun’s habitable zone is moving outwards at a rate of about 1 metre per year. The latest model predicts a total habitable zone lifetime for Earth of 6.3 billion–7.8 billion years, suggesting that life on the planet is already about 70% of the way through its run. Other planets — especially those that form near the outer boundary of a star’s habitable zone or orbit long-lived, low-mass stars — may have habitable-zone lifetimes of 42 billion years or longer.