By all counts, Earth is on a one way trip to oblivion. Our aging Sun will see to that. Within 500 to 900 million years from now, photosynthesis and plant life on Earth will reach a death-spiral tipping point as the Sun continues its normal expansion and increases in luminosity over time.
Trouble is, researchers are still unsure about all the grisly endgame details, and their models of such slow motion horrors are hard to test. But a team of researchers now say that finding and observing nearby aging Earth-analogues, undergoing the ravages of their own expanding sun-like stars, will help Earth scientists understand how the stellar evolution of our own sun will affect life here on Earth.
“[Within] 500 million years figure most plants become extinct, although some could potentially last up to 900 million years from now by employing more carbon-efficient photosynthetic pathways,” Jack O’Malley-James, an astrobiologist at the University of St. Andrews in the U.K. told Forbes. “At this point the biosphere as we know it on Earth will be dramatically different, but not necessarily completely dead.”
In a paper to appear in the journal Astrobiology, O’Malley-James, the lead author and colleagues, notes that as the Sun’s luminosity pushes the inner edge of our solar system’s current habitable zone at 0.99 AU (or one Earth-Sun distance) just too far out.
Even so, finding a planet that is a “near analogue to the far-future Earth (an old-Earth-analogue)” could provide a means to test these predictions; including declines in species diversity, extent of habitat and ocean loss, and changes in such planets’ geochemical cycles.
“If we did find such a planet, detailed long-term studies could give us an insight into its long-term carbon cycle, possible showing us whether carbon dioxide (CO2) levels really will plummet over the next billion years in the way we expect,” said O’Malley-James.
As he explains, in Earth’s own far-future, plant life will be extinct and the biosphere as we know it will have collapsed into an unfamiliar form. Thus, even if astronomers spot such a dying Earth, around an older sun-like star, could they recognize any remaining signs of life there?
“When it comes to positively identifying life on a distant planet, it is still very early days,” said O’Malley-James. “It would be very difficult to pin down any remotely observable signature that we could be 100 percent certain is caused by life on a distant planet. However, this doesn't make work such as this futile.”
The hope is that if astronomers can determine the star’s age with high enough accuracy, coupled with the fact that the planet has been in a circumstellar habitable zone for billions of years, but is now encroaching upon the very inner-edge of the habitable zone, then O’Malley-James says future observations of such planets could make the case that they had observed the dying gasp of the planet's biosphere.
The team ran simulations that placed hypothetical Earths around six aging G spectral type stars all within some 30 light years from Earth. O’Malley-James notes that in each case, his team used hypothetical examples of an aging Earth-like planet, all in the inner-hot edge of their respective habitable zones.
The closest was Alpha Centauri A, at just over 4 light years. “This star burns its fuel slightly faster than the Sun, so the habitable lifetime of our [simulated] planet is slightly shorter than Earth,” said O’Malley-James. “Any inner habitable zone planet would be very near the end of its habitable lifetime at present.”
The farthest was 61 Virginis, some 28 light years away in the constellation of Virgo. “This star burns fuel at a slightly lower rate than the sun, giving our [simulated] planet a longer habitable lifetime,” said O’Malley-James. “Carbon dioxide (CO2) levels decrease more slowly, so plant life was able to cling on for longer than it will on Earth.”
“Finding a planet that has reached this "extreme warming" phase in its evolution could help provide a data point to aid in future predictions [about Earth],” said O’Malley-James.
The researchers note that one assumption is that in Earth’s far future, its “core will have cooled to such an extent that tectonic activity eventually stops.”
If astronomers could actually find an Earth-analogue in the latter stages of its life, then O’Malley-James says the hope is to use spectroscopy to determine if rates of volcanism and potentially planetary tectonics decrease with time as predicted.
As temperatures rise and these planets start losing water, the researchers note that plate tectonics --- which aid in carbon recycling in our atmosphere here on Earth --- may slow or even end. Meanwhile, these planets’ oceans would begin evaporating, saturating the atmosphere with water vapor, a very potent greenhouse gas. That, in turn, could increase surface temperatures, ocean loss, and carbon dioxide (CO2) drawdown rates that could inhibit or stop all photosynthesis.
“Finding life on a planet with a small, weak biosphere could be a lot more difficult,” said O’Malley-James. “Any confident observations of bio-signatures of a dying biosphere may not come for a few more decades.”
Any Earth-like biospheres in the Milky Way are, on average, likely to be older than Earth's biosphere, says O’Malley-James. Thus, finding a "second Earth" might be harder than we think, he says. He is quick to point out, however, that finding an inhabited planet around a Red dwarf star should be easier; it is just less likely to either ‘have’ or ‘have had’ an Earth-like biosphere.
“When it comes to looking for Earth twins we should expect to be disappointed,” said O’Malley-James. “We can expect [only] a few thousand dead or dying Earths in the Milky Way.”