The conditions for life on planets entirely covered in water are more fluid than previously thought, which opens up the possibility of water worlds being habitable, according to a new study.
The scientific community has largely assumed that planets covered in a deep ocean would not support the cycling of minerals and gases that keeps the climate stable on Earth, and thus would not be friendly to life.
But the study, published in The Astrophysical Journal, found that ocean planets could stay in the "sweet spot" for habitability much longer than previously assumed.
"This really pushes back against the idea you need an Earth clone, that is, a planet with some land and a shallow ocean," said lead author Edwin Kite, Assistant Professor at University of Chicago.
Because life needs an extended period to evolve, and because the light and heat on planets can change as their stars age, scientists usually look for planets that have both some water and some way to keep their climates stable over time. The primary method we know of is how Earth does it.
Over long timescales, Earth cools itself by drawing down greenhouse gases into minerals and warms itself up by releasing them via volcanoes.
But this model does not work on a water world, with deep water covering the rock and suppressing volcanoes, the researchers said.
To find if there was another way, they set up a simulation with thousands of randomly generated planets, and tracked the evolution of their climates over billions of years.
"The surprise was that many of them stay stable for more than a billion years, just by luck of the draw," Kite said. "Our best guess is that it's on the order of 10 per cent of them."
These lucky planets sit in the right location around their stars. They happened to have the right amount of carbon present, and they do not have too many minerals and elements from the crust dissolved in the oceans that would pull carbon out of the atmosphere.
They have enough water from the start, and they cycle carbon between the atmosphere and ocean only, which in the right concentrations is sufficient to keep things stable.
"How much time a planet has is basically dependent on carbon dioxide and how it's partitioned between the ocean, atmosphere and rocks in its early years," Kite noted. "It does seem there is a way to keep a planet habitable for a long-term without the geochemical cycling we see on Earth."
The simulations assumed stars that are like our own, but the results are optimistic for red dwarf stars too, he said.
Planets in red dwarf systems are thought to be promising candidates for fostering life because these stars get brighter much more slowly than our sun, giving life a much longer time period to get started.
The same conditions modelled in the study could be applied to planets around red dwarfs, Kate said, adding that theoretically, all you would need is the steady light of a star.