The search for what we would deem to be "habitable" worlds orbiting other stars has reached a crossroads in recent years. As our technology and techniques improve, we've been able to identify small rocky worlds orbiting within other stars' "habitable zones."
So far, we can measure an exoplanet's mass, understand its physical size, even calculate its average density, revealing whether or not that planet may contain quantities of water, for example. Add these factors to our knowledge of that alien world's orbit and we are given tantalizing clues as to whether liquid water may persist on its surface.
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These apparently habitable worlds are quickly labeled "Earth-like," but they may not be like Earth at all. There's a lot more to our planet besides its mass and orbit that makes it habitable -- namely our thick atmosphere and protective magnetosphere, both of which safeguarded burgeoning terrestrial life around four billion years ago from the ravages of space weather.
Lacking sufficiently powerful observatories, we're not able to detect atmospheres, let alone magnetospheres, around distant small exoplanets, so the "Earth-like" moniker is premature at best.
But as we continue to discover all the weird, wonderful and extremely varied exoplanets our galaxy has on offer, we're finding the potential for worlds that may be considered "habitable" for short periods, hypothetically allowing life (as we know it) to be sparked, only for the habitable conditions in these worlds' atmospheres to dramatically change, snuffing out the immature lifeforms.
An often-overlooked factor in the science behind the evolution of life is time, and many of the exoplanets we've found may well be habitable during short periods of their existence, but not long enough to allow life to gain a foothold.
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In an interesting discussion published in the journal Astrophysics and Space Science, Claudius Gros from the Institute of Theoretical Physics at Goethe University Frankfurt investigates the possibility of giving life a helping hand by blasting it into interstellar space and flinging our ready-made biology toward exoplanets that may only have a very short window of habitability.
"It is therefore certain that we will discover a large number of exoplanets which are inhabitable intermittently but not permanently," Gros said in a statement. "Life would indeed be possible on these planets, but it would not have the time to grow and develop independently."
When basic life took hold on Earth, its very existence helped to shape the habitability of our atmosphere. For example, bacterial mats changed the chemistry of atmospheric gases, boosting its oxygen content and facilitating the evolution of complex eukaryotic cells, upon which all multicellular life is based.
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