In a recent paper published in the journal Astrobiology Jack O’Malley-James from the University of St. Andrews in Scotland and co-authors ask whether there might be any old-Earth-analog planets in our galactic neighborhood. If there are, we could learn something about the evolution of biospheres on exoplanets, and possibly about the future of our own world.
Environmental requirements for an old-Earth-analog planet are even stricter than for Earth—that is, if the planet is to remain continuously habitable. According to current projections, our own world may warm up so much in the next billion years that complex life will start to disappear. After another 3 billion years, our sun will become a red giant, and no life will be able to exist. The last organisms standing would likely be some hardy microbes and possibly some insects.
However, other planets around main-sequence G stars may have better chances. O’Malley-James and his colleagues used climate models to assess whether such old-Earth-analog planets could stay continuously habitable, and whether they’d have detectable signatures of life. The researchers found that only a small fraction (0.36 %) of G stars like our Sun are likely to host an old-Earth-analog planet. Since we don’t know the odds of life originating and developing a global biosphere on other planets, the probability of finding an inhabited old-Earth-analog planet is probably much rarer still. The authors estimated that we will only find a few thousand such planets in the whole galaxy, meaning that worlds with life, or at least advanced life, may be few and far between, since the average distance between these inhabited Earth-analog planets would be more than 300 light years.
O’Malley-James and his co-authors suggest that the most likely G star in our galactic neighborhood to have formed exoplanets with a long-lived habitable zone would be 61 Virginis, 28 light years from Earth. 61 Vir is a G type main-sequence star, and is very similar to our Sun, although slightly less massive.
Some exoplanets already have been found orbiting 61 Virginis, but none of them seems to be habitable. One is a so-called Super-Earth, about 5 times as massive as Earth, but its orbit is much too close to the star to be habitable. The other two (one of them not confirmed yet) are most likely gas giants. However, this does not exclude the possibility of another Earth-like planet still undetected, or of a habitable moon around one of the gas giants. And of course, the next habitable planet, even a very old planet, may orbit an M-class main-sequence star instead. An M star has a lower mass than a G star, which partly explains why it does not convert into a red giant, and thus has a stable radiation output for a much longer time—up to 50 billion years or more.