The Goldilocks zone for life may be rarer than we thought
Many icy planets will never be capable of hosting life, instead moving quickly to a state too hot to sustain organisms as we know them
Some planets thought to be likely candidates for developing a “Goldilocks phase” – a period during which a climate that is “just right” for the emergence of life evolves and persists – might in fact be locked into icy or super-hot futures.
That’s the central finding of a report published in the journal Nature Geoscience, and written by a team headed by Jun Yang from the Department of Atmospheric and Oceanic Sciences at Peking University’s School of Physics in Beijing, China.
Working with researchers at the Carl Sagan Institute at Cornell University, the Department of the Geophysical Sciences at the University of Chicago, and physicists at Canada’s University of Toronto, Yang’s team constructed global climate models for several candidate exoplanets, as well as Jupiter’s moon Europa and Saturn’s moon Enceladus.
The results confounded predictions that the planets would eventually enter a period in which much of the ice would melt, transforming into bio-available liquid, sustained by a stable atmosphere that maintains densities and temperatures compatible with life.
Yang and colleagues calculated that the target planets all had inactive carbonate–silicate cycles – that is, the geological process by which rocks cycle between carbonate and silicate stages, through, variously, weathering, sedimentation and volcanic activity.
Carbonate–silicate geochemistry is a major cause of the release of carbon, and thus its contribution to the development of atmosphere.
As a result, the target planets all have very thin atmospheres, and the scientists discovered this has serious implications for the evolution of Goldilocks zones.
In particular, the flimsy layers of gas will never grow warm enough to trigger the melting of the ice without the input of very large amounts of heat from outside the planet’s own system – for instance, in the form of a solar flare or a brightening star.
And that pretty much rules out any life-friendly results.
“We find that the stellar fluxes that are required to overcome a planet’s initial snowball state are so large that they lead to significant water loss and preclude a habitable planet,” the scientists write.
In this scenario, the models predicted two possible end-points, neither of them helpful.
The first was the busting of the “moist greenhouse limit”, which would see water vapour accumulating in the upper reaches of the atmosphere and then escaping into space.
The second was breaking through the “runaway greenhouse limit”, in which atmospheric warming continues unchecked, resulting in all liquid boiling away.
“We suggest that some icy planetary bodies may transition directly to a moist or runaway greenhouse without passing through a habitable Earth-like state,” the team concludes.