When the Kepler Space Observatory launched in 2009, it began to find planets around other stars at a rate that thrilled astronomers. This data led to the estimate that our galaxy must contain around 40 billion Earth-like planets orbiting in the habitable zones of sun-like stars and red dwarfs.
The habitable zone is the region around a star where liquid water can exist on the surface of a planet. It is important because the evidence on Earth suggests that liquid water is crucial for life. And if life starts easily on Earth-like worlds, the numbers from Kepler suggest that our galaxy must be teeming with life. So the race is on to find evidence of it. Various space-based telescopes are being designed to look for the unique spectroscopic signature that life must produce.
But a key challenge will be to find the best targets—planets where conditions seem most conducive to complex life. And astrobiologists have begun to point out that liquid water alone is not enough. If Earth is anything to go by, the proportion of other molecules is important too. For example, too much carbon dioxide or carbon monoxide kills complex life as we know it.
Today, Edward Schwieterman at the NASA Astrobiology Institute in Riverside, California, and a few colleagues have revised the definition of a habitable zone to take account of carbon monoxide and carbon dioxide levels. As a result, they say the habitable zone for complex life must be significantly smaller—about a quarter as wide as the previous definition allows. “Our results have a number of important implications for the search for exoplanet biosignatures and complex life beyond our solar system,” say Schwieterman and co.
First some background. The size of a habitable zone is tricky to calculate because surface temperatures depend on various feedback processes in the atmosphere, such as the greenhouse effect. The conventional definition of a habitable zone specifies an atmosphere containing nitrogen, carbon dioxide, and water, stabilized by the same carbonate-silicate feedback process that exists on Earth.
The carbonate-silicate cycle is a long-term process in which silicate rocks react with water and carbon dioxide to create carbonate rocks, which are then converted back into silicate rocks and carbon dioxide gas by high pressures and temperatures and by volcanism. This leads to a feedback loop that keeps the levels of carbon dioxide in the atmosphere relatively stable, allowing a greenhouse effect to increase surface temperatures.
At the inner edge of the habitable zone, relatively low levels of carbon dioxide can create temperatures high enough for liquid water. On Earth, the necessary carbon dioxide levels have varied throughout history from tens to hundreds of parts per million.
“But for the middle and outer regions of the habitable zone, atmospheric carbon dioxide concentrations need to be much higher to maintain temperatures conducive for surface liquid water,” say Schwieterman and co.
For example, one exoplanet often thought of as a good candidate for extraterrestrial life is Kepler-62f. This planet is about three times the mass of Earth and orbits its host start in the constellation of Lyra at about the same distance as Venus. But because the host is less bright than the sun, Kepler-62F receives about the same amount of sunlight as Mars, so it is on the outer edge of the habitable zone.
The greenhouse effect could easily make Kepler-62f warm enough for liquid water. But Schwieterman and co have calculated that it would require three to five bars of carbon dioxide to do the trick. That’s 1,000 times more than has ever existed on Earth during the history of complex life here.
The team points out that these levels of carbon dioxide are poisonous for most complex life on Earth today, and that increased levels in the past are thought to have been a significant factor in mass extinctions. The physiological limits to the level of carbon dioxide that life can tolerate need to be taken into account when defining habitable zones. Thus, Kepler-62f may not be such a good candidate after all.
Carbon monoxide also threatens complex life. Schwieterman and co calculate that planets orbiting cool stars are likely to have higher levels of carbon monoxide because photochemical conditions are more conducive to producing it. This places another constraint on habitable zones.
The team’s final calculation is to work out how these constraints change our current understanding of the size of the habitable zone. “One implication is that we may not expect to find signs of intelligent life or technosignatures on planets orbiting late M dwarfs or on potentially habitable planets near the outer edge of their habitable zones,” say Schwieterman and co.
That will have significant impact on future searches for biosignatures from other planets. Astronomers may well decide to focus on warmer, sun-like stars where the conditions for complex life are probably more favorable.
But the designers of future space telescopes needn’t fear for lack of targets. Even if the habitable zone is significantly smaller than previously thought, the likelihood is that there will be many hundreds of millions of candidates in our galaxy alone. That should be more than enough for the missions currently planned.
Ref: arxiv.org/abs/1902.04720 : A Limited Habitable Zone For Complex Life