Researchers at Cornell University are taking a new approach to the search for alien life: looking for habitable planets older than Earth, “old Earth analogs,” which may be nearing the end of their habitable lifetimes. Astronomers would search for biosignatures from worlds much older than Earth, where lifeforms are dying off due to circumstances such as the planet’s star expanding in its old age, gradually heating the planet to a point where life is no longer possible.
Billions of years from now, our own Sun will meet this fate, expanding into a red giant star and scorching, or even engulfing, the Earth. In that sense they are searching for “future Earths”—planets already experiencing what Earth will far in the future.
The research, conducted by Jack O’Malley-James, of the Institute for Pale Blue Dots at Cornell University, and his colleagues, calculates the chances of discovering an old Earth analog nearing the end of its habitable lifetime. The new paper, “In Search of Future Earths: Assessing the possibility of finding Earth analogues in the later stages of their habitable lifetimes,” will be published in the journal Astrobiology.
What are the chances of finding any old Earth analogs, if they exist? Smaller rocky planets are now thought to be quite common in the galaxy, thanks to discoveries from the Kepler Space Telescope and other telescopes. Plus, up to one-third of main-sequence stars are thought to be in the latter stages of their evolution, increasing the odds that some of them would have “Earths” which are also much older than our own planet.
Of course, the closer one of these planets might be, the easier it would be to study it and learn about its composition, atmosphere, and more. There are six stars similar to the Sun within 10 parsecs of Earth (1 parsec = 3.26 light-years). According to the new paper, the star 61 Vir would be the most likely to have an older version of Earth orbiting it. The postulated planet would be at a similar stage in Earth’s future, where temperatures have increased too much for anything to survive except microbes. The only life left on this world would likely be a declining microbe population.
To come to these conclusions, O’Malley-James calculated the location of the habitable zone for each of the six stars over their lifetimes. Then, the hypothetical planets in each system were placed at a distance where the planet could remain habitable for billions of years. By comparing the predicted incoming and outgoing radiation from the star(s), the temperature changes on the planets over the main sequence lifetime of the star could then be modeled.
A bit farther out, there are 276 stars similar to the Sun within 100 parsecs. About half of those are estimated to be more than six billion years old. By O’Malley-James’ calculations, there should then be approximately 11 old Earths within that distance from Earth.
But it may not be that simple; according to results from another study, smaller rocky planets like Earth are most likely to form less than one astronomical unit (AU), the distance between the Sun and the Earth, from their stars. But the continuously habitable zone of the six stars that O’Malley-James and his colleagues studied tended to be farther out from the stars than that, meaning that there might be just one old Earth in the near solar neighborhood, or maybe even none at all.
“It turned out that these planets are probably not that common at all, so in reality any habitable planets in the 61 Vir system will probably not resemble an older version of Earth,” O’Malley-James said. “This study highlights that finding replicas of our own world, in terms of the diversity and complexity of life, is going to be a much harder task than simply finding life.”
It should be remembered, however, that the close solar neighborhood examined by O’Malley-James and his team are a very tiny percentage of all of the billions of stars in the galaxy overall. There could be many old Earths out there, but most might simply be too far away to study in any detail.
It is also thought that some “super-Earths,” rocky exoplanets larger than Earth but smaller than ice giants like Neptune, might be able to support some form of life. Current studies suggest that they might be even more numerous than smaller rocky planets. Computer simulations indicate that some of them might be true water worlds, completely covered by oceans.
Future space telescopes, such as TESS, JWST, WFIRST, and PLATO, could help to find some of these ancient worlds, however, and characterize their compositions and atmospheres, as well as look for potential biosignatures.
As previously reported by AmericaSpace, the oldest known exoplanetary system found so far is Kepler-444, with five known planets, all smaller than Earth, and estimated to be 11 billion years old. Unfortunately, these worlds are most likely uninhabitable, already being scorched by their star in their small, tight orbits. The discovery does show, however, that very ancient Earth-sized worlds are out there. Any habitable exoplanets that old would have had much more time than even Earth to possibly develop a wide variety of life forms. There might even be worlds which are more biologically active than our home planet. What would life look like on a planet more than twice as old as Earth?
Indeed, astronomers now estimate that there are billions of smaller, rocky planets like Earth in our galaxy alone. Some will have conditions favorable for possible life, and some will not, but it increases the chances that there are indeed some older versions of Earth waiting to be discovered—worlds where life may have flourished for billions of years, but is now dying out. On some others, though, life might be thriving still on these older cousins of Earth, ones which have not yet been eradicated by their aging, swelling stars.