Why Earth Is Only 'Marginally Habitable,' and a Portrait of a Superhabitable Planet
Could there be planets that are more habitable than Earth in our own galaxy? According to Rene Heller, an astrophysicist at McMaster University, it's extremely likely, especially considering that Earth is only "marginally habitable."
Writing for Scientific American, Heller critiqued the tendency of astrobiologists to use Earth as a "gold standard" while searching habitable planets: "Because earthlings still know of just one living world-our own-it makes some sense to use Earth as a template in the search for life elsewhere, such as in the most Earth-like regions of Mars or Jupiter's watery moon Europa. Now, however, discoveries of potentially habitable [exoplanets] are challenging that geocentric approach."
According to Heller, who has experience modeling so-called "superhabitable" worlds that are optimized for the presence of life, Earth is neither the size nor the distance from the sun that is ideal for fostering life. The optimal planet would be a little further from its star than Earth is from our Sun, and would orbit around a star that is much smaller and more efficient than the Sun, which is only expected to last for a total of 10 billion years before it uses all of its hydrogen fuel and dies.
This ideal planet would also be approximately twice Earth's size, which would lead to higher surface gravity. This would, in turn, lead to a thicker atmosphere as well as more "erosive" weather. As a result, it would be an "archipelago planet," with a much flatter topography than Earth and many more islands on the oceans, rather than just a few continents. Since archipelagos on Earth are host to some of the most biologically rich and diverse environments, it stands to reason that this hypothetical world would be much more habitable than Earth as a whole.
Earth is not particularly habitable in the grand scheme of things, according to Heller. Our deserts and poles are hostile to most forms of life, and and our oxygen is depleting over time. "All things considered," he writes, "our planet is only marginally habitable."
While many of these facts were already known to the scientific community, the call for an attitude shift in the search for habitable exoplanets is a necessary one. If Earth is just barely habitable, then we should no longer hail the discovery of "Earth-like" planets, but rather turn the search towards planets that represent Heller's "superhabitable" world.