Our Sun will evolve into a red giant star billions of years from now. The increased heat from the expanding Sun will scorch the Earth with dire effects to life. Climate models can be used to predict how this will happen, but, of course, this cannot be tested out on Earth.

Jack O'Malley-James of the Institute for Pale Blue Dots at Cornell University, along with colleagues, have been calculating the chances of discovering an old-Earth analog approaching the end of its habitable lifetime. This follows his work on Swansong Biospheres in which the potential bio-signatures of a dying world were assessed.

The new paper, "In Search of Future Earths: Assessing the possibility of finding Earth analogues in the later stages of their habitable lifetimes,"has been accepted for publication in the journal Astrobiology and is available in preprint.

The far future Earth
Searches are already in place to find Earth's twin, a planet with a similar mass and radius as the Earth and orbiting at the same distance as the Earth does from the Sun. However, finding an equivalent of Earth's much older cousin involves a different set of criteria.

The "habitable zone" is defined as the region where liquid water can exist on the surface of a planet. Habitable zones move outwards as a star ages, so a planet that was in the zone when the star was younger may not necessarily remain there.

An old Earth analogue is one that has been in the star's habitable zone for the entire main sequence lifetime of the star, known as the continuously habitable zone. As the purpose is to study planets in the final stages of habitability, a far future Earth would also have to be approaching the inner edge of the habitable zone.

As up to one-third of main sequence solar-like stars are thought to be in the later stages of their evolution, it is feasible that old Earth analogues could be detected. If any of these planets exist in the solar neighborhood, then they would be excellent candidates for future space telescopes with the capability to characterize a planet's atmosphere from its spectrum.

Searching nearby
There are six solar-like stars within 10 parsecs of the Sun that are old enough to harbor an old Earth analogue. A parsec is the equivalent of 3.26 light years. O'Malley-James calculated the location of the habitable zone for each star over its entire lifetime.

He then placed hypothetical planets in each system at a distance where the planet could remain habitable for billions of years. The temperature changes on the planet over the main sequence lifetime of the star can be modeled by comparing the predicted incoming and outgoing radiation.

The paper concludes that if Earth-like planets existed around these stars, then the one around 61 Vir would be at the right stage of its lifetime to be considered a far future Earth. Such a planet might be home to a declining microbial population, assuming that life evolved there in a similar manner to the Earth.

This hypothetical planet would be akin to the stage in future Earth's lifetime when the temperature has risen too high for complex life to survive, and microbes are the last lifeforms to cling to existence. Other stars could host planets similar to future Earth where only extremophile microbial life remains in a few select niches, however these biosignatures would be much more difficult to detect than the declining microbial biosphere.

A Galaxy Teeming with Earth-like Planets?
If an Earth-like planet existed around 61 Vir, it would provide a good opportunity to study the far future Earth. But what are the actual chances of such planets existing?

O'Malley-James used previous studies by other scientists in order to find out. One study, based on the number of planets found by NASA's Kepler mission, predicts that 8.6 percent of solar-like stars could harbor an Earth-like planet orbiting in the habitable zone.

A solar-like star is one that is of a similar temperature and mass as our own Star. There are 276 stars like our Sun within 100 parsecs, around half of which are older than six billion years. This means that there should be 11 potential targets.

However, another study showed that terrestrial planets are more likely to form less than one astronomical unit (AU, the distance between the Sun and the Earth) from the star. From the six example stars that O'Malley-James studied, the continuously habitable zone is located slightly further from the star than this. Combining these results indicates that there would actually only be one potential old-Earth analogue within the solar neighborhood.

"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," said O'Malley-James. "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."

Yet while there may only be one potential old-Earth analogue close enough to be studied in detail, there could still be thousands more in the distant reaches of our Galaxy.