A paper just published in the journal Astrobiology has sparked a debate over whether planets might exist that are even more suitable for life than Earth—maybe even in our own stellar neighborhood.
Authors René Heller of McMaster University and John Armstrong of Weber State use the term “superhabitable” for theoretical worlds with life-friendly properties. Higher gravity, for example—say two to three times that on Earth—might flatten a planet’s topography, producing more shallow seas and a greater diversity of organisms. A sun slightly smaller than our own would be beneficial because it would keep planets in the habitable zone (where liquid water is stable on the surface) for a longer period and allow more time for the origin and diversification of life. Based on these arguments, Heller and Armstrong say that an exoplanet around Alpha Centauri B, a so-called K dwarf star or orange sun, could, in theory, be superhabitable.
Unfortunately, there are reasons to doubt that such a hospitable world exists in the closest solar system to our own. First, Alpha Centauri is actually a triple star system, and whether a planet could exist far enough from Alpha Centauri B to be habitable and still remain stable for billions of years is questionable (a planet has in fact been detected around the star, but too close in to support life). Also, even though Alpha Centauri B is a K star with about 90 percent of the Sun’s mass and 45 percent of its luminosity, nearby Alpha Centauri A has 110 % of the Sun’s mass and is 150 percent as bright.
Undoubtedly, there should be planets in our galaxy more habitable than our own. However, we shouldn’t too easily dismiss all the factors Earth has going for it. Yes, a planet with lots of shallow seas would be well-suited for life, but would it necessarily have greater biodiversity? It should have more total biomass, and probably has more biodiversity within the shallow seas, but not planet-wide. There were also periods in Earth’s past when its surface was dominated by shallow seas, just as there were times when all the major land masses were lumped together in one giant continent. These changes in habitability factors have been a good thing for the evolution of complex life and biodiversity. Only if life is constantly challenged, for example in having to adapt from dry deserts to lush tropical jungles, can it develop new tools for coping with future changes.
In fact, such a varied tool set has allowed life on Earth to continue and repopulate the planet following large meteorite impacts that killed more than 90% of species on the planet. Given that all our SETI efforts have been unsuccessful so far, it seems unlikely that technologically advanced life is present in our stellar neighborhood. Thus, in its ability to promote evolution, Earth may be as nearly perfect a planet as there is.