Source: Unexplained Mysteries

Up until now, the habitable (or ‘Goldilocks’) zone of a star has been defined as the region in that star’s orbit in which the temperature is sufficient for liquid water to exist on a planet’s surface.

In practice however, while this might be sufficient for single-celled microbes, the actual conditions required for complex life forms to develop are actually much more specific and harder to come by.

The main problem is that many ‘habitable’ worlds have high levels of toxic gas, especially those that exist nearer to the edge of the zone traditionally considered to support life.

“To sustain liquid water at the outer edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today,” said study lead researcher Edward Schwieterman.

“That’s far beyond the levels known to be toxic to human and animal life on Earth.”

Taking this in to account, the size of the habitable zone around a star shrinks to half the size for simple animal life and a mere third of the size for complex life forms such as humans.

Based on these new figures, potentially habitable worlds around some of our nearest stellar neighbors such as Proxima Centauri can no longer be considered capable of hosting complex life.