Hot super-Earths could host life after all

From New Scientist.

11 July 2008

Massive, rocky worlds called 'super-Earths' – even those orbiting searingly close to their stars – may provide the right conditions for life, new research suggests. At up to 15 times the mass of Earth, the rocky bodies are bigger and easier to spot than Earth-sized worlds, which have yet to be detected. In fact, technological advances recently led to the discovery of up to 45 new super-Earths, and astronomers say a third of all Sun-like stars may host the brawny planets.

But could they host life? "There's no reason why the different chemical cycles that are important for life on our planet wouldn't work on super-Earths," says Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics in Massachusetts, US.

Kaltenegger helped organise a recent conference session on the topic, and she says the consensus of attendees was similarly positive – even for those planets once dismissed as being too harsh for life. Super-Earths orbiting close to their stars, for example, experience gravitational tugs that keep them 'tidally locked' to their hosts. That means one side of such a planet always faces its star, the way the Moon always shows the same side to Earth.

Astronomers previously assumed such planets would be two-faced worlds of fire and ice, with one half molten and the other frozen. Early models suggested the atmospheres of such worlds would quickly vanish, as water vapour and other atmospheric molecules on the planet's dark side would turn to ice and plunge to the ground. "It was thought that after the atmosphere on the dark side was completely iced out, then it would suck atmosphere from the hot side, freezing that out as well," Kaltenegger told New Scientist.

But new models show that if a tidally locked super-Earth has an atmosphere at least as dense as Earth's, strong winds could transport heat from its hot side to its cold side. Similarly, if the planet has a global ocean, its currents could help spread the warmth.

This effect still wouldn't offset the intense heat the planets would experience at close distances to Sun-like stars. But it means super-Earths could potentially host life as close as 0.05 astronomical units away from dim stars known as red dwarfs, which make up about 85% of the stars in the galaxy (for comparison, Mercury lies 0.38 AU away from the Sun).

And in some ways, super-Earths might even be more likely to support life than their Earth-sized cousins, scientists say. Recent research suggests that super-Earths will experience more plate tectonic activity than smaller rocky worlds. On Earth, plate tectonics – the shifting and colliding of continental plates – is necessary for life.

It plays a crucial role in the carbon-silicate cycle, which releases carbon dioxide into the atmosphere, warming the planet. Plate tectonics also locks the greenhouse gas in surface rocks and sequesters it in Earth's interior so that the planet doesn't heat up too much.

"The way we have experienced life on Earth is enabled by plate tectonics," says Diana Valencia, a graduate student at Harvard. Super-Earths should have larger molten cores and should generate more heat than Earth-sized worlds, Valencia told New Scientist. This could cause more vigorous convection in the planets' mantles and create thinner plates that slip and slide more easily.

Eventually, missions such as NASA's upcoming Kepler space telescope could find an Earth-sized planet in our galaxy. "But there's going to be a lot of super-Earths discovered before that," Valencia says. "If we're concerned about finding life, those are the planets we should be investigating right now."

[Well, it looks like the range of possible life-bearing planets just got a whole lot bigger! I’m hoping that they find something in the next 50 years. I think that it will have a profound impact on the way we see ourselves. Not being alone anymore will give some heart and make others afraid. I doubt if anyone will be unaffected by the idea.]