Earth may be a nice and cozy place for life as we know it to evolve, but is it really the best place for life to thrive? Probably not, say two researchers. In fact, Earth may be one of the more extreme examples of a "habitable" world where life was lucky to survive.
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The search for extraterrestrial life is fraught with uncertainly. Faced with a seemingly infinite Universe and an assumption that life beyond our planet is an inevitability, we focus on nooks and crannies that have similar habitable environments to Earth and exoplanets that resemble our world orbiting stars that resemble our sun. But finding these special places feels like we're looking for a very specific needle in a very big haystack. What if our definition of "habitable" is just not all that, well, habitable?
"The Earth just scrapes the inner edge of the solar system's habitable zone - the area in which temperatures allow Earth-like planets to have liquid surface water," said René Heller of McMaster University, Ontario, Canada. "So from this perspective, Earth is only marginally habitable. That led us to ask: could there be more hospitable environments for life on terrestrial planets?"
Looking at the history of our planet, it may seem amazing that life was ever able to gain a foothold. Between the asteroid and comet bombardments, intense volcanic activity, frigid ice ages and often poisonous atmospheres, how were the conditions ever ripe for single celled lifeforms to form? Perhaps life was just really lucky to have found its way in such an inhospitable place.
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Heller and colleague John Armstrong of Weber State University think that we may be looking for life in all the wrong places and suggest that we should be looking not for "second-Earths" but a class of planet that is superhabitable.
In an article published in Astrobiology in January, the researchers describe some of the characteristics a superhabitable planet may have. Some of the features - such as the necessity for a global magnetic field to protect life from ionizing solar wind particles - sound very familiar. But Heller and Armstrong highlight the need for a more efficient global "thermostat" that would avoid damaging ice ages. Also, a more massive planet with shallower oceans may be a more habitable solution.
In their research, they highlight the nearby star Alpha Centauri B as an ideal candidate that could support a superhabitable world. Slightly smaller than our sun, Alpha Centauri B would be able to incubate hypothetical lifeforms on a superhabitable world for much longer owing to its longer lifespan. "You want to have a host star that can keep a planet in the habitable zone for 7 to 10 billion years," which would allow enough time for life to evolve and ecosystems to flourish, Heller told New Scientist.
"We propose a shift in focus," said Heller. "We want to prioritize future searches for inhabited planets. We're saying ‘Don't just focus on the most Earth-like planets if you really want to find life.'"
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The researchers believe our hunt for extraterrestrial life is too set in its ways and blinkered toward worlds that we consider to be habitable because they resemble Earth. It's a grand observational bias that may ultimately mean that while looking for second-Earths, we overlook far more habitable planets.
While discussions like this are valuable, I think they can be counterproductive.
Although it is very important to have "out of the box" thinking when it comes to the search for extraterrestrial life (whether that life be single-celled organisms hidden deep inside Mars' crust or highly intelligent alien civilizations), using life as we know it as a template is no bad thing. After all, we are the only lifeforms we know of in the entire Universe, why would we ever consider Earth "barely habitable"?
How life is sparked remains one of the biggest questions hanging over modern science and the search for extraterrestrial life is an offshoot from that. Perhaps Earth, in its barely habitable state, is actually the perfect environment for life to gain a foothold?
Lacking evidence to the contrary, seeking out second-Earths probably isn't such a bad idea.