A new study suggests that a layer of mysterious haze could house signs of life in the depths of Venus' hellish atmosphere.

Sara Seager and her colleagues at MIT have written a paper suggesting a way that microbial life could permanently thrive on the planet Venus' lower atmosphere. The theory that microbes could reside in the Venusian clouds, at an altitude of 50 to 65 kilometers above ground level, was suggested by the late Carl Sagan more than 50 years ago and has been supported by several other authors since.

At first the idea of living in such an unusual location could sound unbelievable. But note that Venus used to be in our Solar System's habitable region, and that "habitability" has to understand the whole life cycle of a planet. In truth, one theory suggests that early in the history of the Solar System, three of the four inner planets - Venus, Earth , and Mars - harbored surface life.

Venus eventually becomes too warm, so life can now just stay in the atmosphere. Mars is too cold so today life can only occur underground. Only Earth is tolerant enough to hold a biosphere which is abundant and humming.

Since the concept has been around for years, what's new about the work of Seager? For one thing, earlier papers did not expand about what cloud existence entails, and how it could communicate with the environment.

The exception was a 2004 paper by Professor Dirk Schulze-Makuch of the Technical University of Berlin, Germany, in which he and his fellow researchers out that microbes could use sulfur as a UV sunscreen and as a way of transforming ultraviolet light into other wavelengths of light that could be used for photosynthesis.

The concern with this idea was that all of those bacteria would slip through the lower haze layer through the clouds and reach a fiery end. But it was predicted that they would only remain in the clouds for around a month, and that their atmospheric reproductive rates would offset the loss of microbes coming out of the clouds.

Seager and her team had a far more effective approach to this. They propose that the droplet environment in which the microbes live will expand inexorably, and be compelled by gravity to settle under the Venusian clouds in the cooler, uninhabitable atmosphere.

The bacteria would dry out while the droplets evaporate after settling, and the lower haze layer would become a dump for desiccated, inactive life. Yet upward drafts would bring the latent microbes periodically back into the atmosphere, where they would be rehydrated and triggered again.

There would still be some of those bacteria missing. But this kind of life cycle would increase the chances of an aerial biosphere being able to last for many million years, and may still be present today.

The researchers detailed their findings online Aug. 13 in the journal Astrobiology.