Finding Life on Other Planets May Be Less Likely Than Scientists Thought

A hand out image made available by the European Southern Observatory on August 24 2016, shows an artist's impression of a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Scientists on August 24, 2016 announced the discovery of an Earth-sized planet orbiting the star nearest our Sun, opening up the glittering prospect of a habitable world that may one day be explored by robots. Named Proxima b, the planet is in a 'temperate' zone compatible with the presence of liquid water -- a key ingredient for life.

Scientists dealt a blow Monday to the quest for organisms inhabiting worlds besides Earth, saying our planet was unusual in its ability to host liquid water — the key ingredient for life.

It was thought likely that distant worlds orbiting stars similar to our Sun would go through water-rich phases.

This would happen when the young, dim star of an icy, lifeless planet — such as early Earth — starts warming, becomes Sun-like, and melts the ice on planets orbiting it at just the right distance — the so-called "Goldilocks" zone.

Icy orbs in our own Solar System, including Jupiter's moon Europa and Saturn's Enceladus, or "exoplanets" in other star systems, may become habitable in this way, the theory goes.

But a team wrote in the journal Nature Geoscience on Monday that this was unlikelier than had been imagined.

Jun Yang of Peking University in China and a team used climate models to simulate the evolution of icy planets.

Without atmospheric greenhouse gases — a feature of Earth — the energy required to thaw an icy planet would be so high that it would transit from frozen to inferno without an intermediate, liveable phase, they found.

"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 team wrote.

Some icy bodies, they suggested, may therefore never pass through a habitable Earth-like state.

Among these, Europa and Enceladus will likely morph from iceballs into fireballs by the time the Sun reaches it's super-hot red giant phase heat in billions of years from now, said the team.

Earth was an example of an icy world that thawed just enough, some 600-800 million years ago, thanks to planet-warming atmospheric greenhouse gases emitted by volcanic eruptions during its snowball phase, the team said.

This meant that less solar heat would have been required for the ice to melt, enabling our planet to achieve a temperate middle ground.

Greenhouse gases, which are naturally present in the atmosphere but also released by humans burning coal, oil, and gas, are what has kept our planet warm enough for humans, animals, and plants to inhabit.

Read more at Seeker