Earth may be our home, but another planet even cosier for life could be orbiting the star next door. A detailed analysis of what might make planets suitable for life says that Alpha Centauri B, the star closest to our sun, would be the perfect star to host a "superhabitable" planet – a world of islands, shallow seas and gentle slopes, where the conditions needed to support a diverse array of life forms would persist for up to 10 billion years. But the near-paradise would come at a cost to visitors from Earth: the pull of gravity would be about one-quarter stronger than on our home turf.
We normally assume that the best places to look for alien life are Earth-sized planets orbiting sun-like stars. But our best models for habitability consider only a few criteria, such as the planet's size and distance from its star, seeking rocky worlds like Earth in similar orbits to our own.
"But no one had ever touched the question of whether other places may be even more benign environments than Earth provides," says René Heller of McMaster University in Hamilton, Ontario, Canada. So he and his colleagues analysed at a host of additional criteria, including a hypothetical planet's gravity, age and internal structure, to explore the possibilities.
Earth has hosted life for at least 3.5 billion years of its 4.6-billion-year existence. But it is near the inner edge of the habitable zone in our solar system, and the sun is getting warmer as it ages. Eventually, perhaps 1 or 2 billion years from now, Earth will become a hot, Venus-like rock enveloped in a deadly murk of carbon dioxide and sulphuric acid.
Instead, the team's analysis says that the choicest locale for life could be a world slightly bigger than Earth orbiting an orange star slightly smaller than the sun. "You want to have a host star that can keep a planet in the habitable zone for 7 to 10 billion years," says Heller. That is thought to be enough time for ecosystems to reach an optimal state for diverse life to flourish.
Slightly more massive planets will alsohold on to their internal heat and moving molten innards for longer. This should drive plate tectonics, which recycle water and nutrients, and create a stronger magnetic field, which shields a planet from damaging solar and cosmic radiation.
Not a peep
We already suspect that the orange star Alpha Centauri B hosts a rocky planet, although if it is confirmed, that world would be too close to its star to support life as we know it. But exoplanet hunters such as NASA's Kepler mission have shown us that planets are rarely born alone, and an as-yet undetected superhabitable world could be orbiting farther from the star.
To be superhabitable, such a world would be so massive that gravity would make its landscape flatter than Earth's, producing mostly shallow seas and archipelagos like Indonesia and the Bahamas. On Earth, such environments produce far more diversity than the deep ocean or the arid centres of large continents. While the team did not specifically address the potential life that might emerge on superhabitable planets, Heller speculates that the higher gravity would keep vegetation closer to the ground and would make the atmosphere thicker, so land animals would probably be short and stocky compared with their earthly counterparts, while the dense air would allow larger creatures to take wing.
The star Alpha Centauri B is about 6 billion years old, which means life on a superhabitable planet in orbit around it would have an evolutionary head-start on us, adds Heller. But would any of that life be intelligent? So far, searches for signs of nearby civilisations, such as the SETI programme, have not heard a peep from our potential neighbours.
"The paper nicely points out important aspects related to habitability," saysRavi Kopparapu of Pennsylvania State University in College Park, who was not involved in the research. But he cautions that, until we find a superhabitable world, we do not have enough evidence to say Earth isn't, in fact, the best place for life.