From our understanding about how life forms, and thrives, astronomers know that our planet exists within a solar system "sweet spot" where the distance from the sun is not too cold and not too hot. Central to our sun's "habitable zone" is that liquid water can persist on the surface of the Earth - and possibly Mars, in the past, when its atmosphere was a tad thicker.
As we discover more and more exoplanets orbiting their host stars, each star with their specific habitable zone (depending on how much energy they pump out), we can hypothesize that alien life may be able to survive, assuming the presence of liquid water, if the exoplanets orbit within their specific habitable zones.
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But what if an exoplanet has an atmosphere that could trap the heat from a star, keeping water in a liquid form, beyond the habitable zone?
According to Raymond Pierrehumbert, of the University of Chicago, and Eric Gaidos of the University of Hawaii in Honolulu, a hydrogen atmosphere blanketing an exoplanet may be very efficient at boosting the greenhouse effect of that world, thus storing the meagre heat from a host star at distances beyond that star's habitable zone.
The greenhouse effect is very important for our ecosystem in that it keeps the Earth's surface nice and cosy for water to remain a liquid. However, if we transplanted Earth to the orbit of Saturn, say, the greenhouse gases - such as carbon dioxide and water vapor - would "freeze out" of the atmosphere, freezing surface water, turning Earth into a barren snowball.
Hydrogen, on the other hand, remains a gas to very low temperatures, and at high pressures the gas is also a very good insulator, helping the greenhouse effect maintain surface temperatures to habitable levels.
According to Ken Croswell over at New Scientist, the researchers calculated that if an exoplanet has a hydrogen-rich atmosphere "a few dozen times thicker than our nitrogen-oxygen one, such a planet could keep warm at up to 15 times Earth's distance from the sun."
Usually, 15 times the sun-Earth distance wouldn't provide sufficient sunlight, but Pierrehumbert and Gaidos have calculated that a distant planet should still receive enough energy to foster photosynthesis in plant life.
Sadly, there's two problems with this hypothesis. Firstly, how could such a hydrogen atmosphere form? As pointed out by James Kasting of Pennsylvania State University in University Park, it seems a stretch to think that so much hydrogen could be trapped by the exoplanet from its parent star.
Secondly, some terrestrial bacteria consume hydrogen and carbon dioxide, so if bacteria with similar metabolisms were nurtured on these kinds of worlds, they'd deplete the gas that helped them thrive. The planet would then enter a terminal phase of cooling, entombing the poor bacteria in ice.
But, as we're talking about aliens here, perhaps other primitive lifeforms might be able to avoid the same fate. Which begs the question, if these alien life forms are so different, would they even need liquid water to survive? In which case, the whole concept of a habitable zone would be thrown into a spin.
But, until we find another life form different from our own, we only have "Earth-Brand™" life to use as a template and the "habitable zone" is the place to be.
Image: NASA Source: New Scientist