The galaxy is awash in potentially habitable planets, and next-generation telescopes are gearing up to scan the atmospheres of these alien worlds, looking for hints of life-friendly conditions. But in a twist, a team of scientists used computer simulations to find out what might kill off some of these promising planets, and the results show that not every whiff of life will be a sure-fire hit.
Scientists in Germany started with a model of an Earth-like world entirely covered by oceans. The team then used global climate models to see what happens when the amount of carbon dioxide in the air rises.
The simulations showed that at a certain point, the planet's climate becomes unstable and shifts to a state called a moist greenhouse, with temperatures above 134 degrees Fahrenheit.
Like a dehydrated human in a steam bath, one of the consequences of this sweltering state is water loss. To start, the heat triggers changes in the atmospheric layers that allow water vapor to mix higher up. That means more ultraviolet light from the sun can hit the water molecules, breaking them up into hydrogen and oxygen. The oxygen atoms recombine, while the hydrogen escapes to space.
"At that point, you will be in a state where you start losing water at a quick rate," says study leader Max Popp of the Max Planck Institute for Meteorology.
After several million years, all the water on the planet would evaporate away, the team reports this week in Nature Communications. If the water world started out with an atmosphere like Earth's—mostly nitrogen with a smaller portion of oxygen and trace gases—the end result would be a dry world with a mostly nitrogen atmosphere.
The study suggests that finding water—or even oxygen—in a far-flung planet's atmosphere doesn't necessarily mean it is hospitable to life. For example, a planet in a moist greenhouse state might generate a lot of oxygen as the water vapor breaks apart, not because of any living things producing the gas, says James Kasting, a professor of planetary science at Penn State University who reviewed the paper for publication.
The model also showed that CO2 is a really efficient greenhouse gas, more so than many scientists had assumed, Popp says. Once a planet gets into a moist greenhouse state, it's hard to go back. Even cutting the CO2 concentration in half doesn't cool the planet much once the steamy conditions have taken over.
The reason is the clouds. Scientists had thought that water vapor would retain heat more efficiently than CO2, but clouds alter this situation and allow CO2 to be the better heat-trapper.
While this all sounds dire in an age of rising CO2 levels on Earth, Popp stresses that these simulations don't apply to our planet. The initial global average temperature used for this study was 10.8 degrees Fahrenheit warmer than Earth today. To get to that temperature, you'd have to push the concentration of carbon dioxide roughly four times higher than it is now, perhaps more.
The simulations also weren't done with a truly realistic planet. The idealized model assumes that this planet is in a perfectly circular orbit, that it lies at the same distance Earth is from the sun and that it spins at about the same rate but is not tilted on its axis. The researchers assumed there were no ocean currents, no continents and no ice caps, and their global ocean is just 164 feet deep.
This is in part because of the computing power required, but also so the team could more clearly see the dynamics and feedbacks involved. They did include the effects of clouds and the pressure of water vapor in the air, and they treated water as a major constituent of the atmosphere, something previous studies left out, Kasting says.
The work offers some insight into Earth's sister planet, Venus, which started with roughly the same raw materials but lost its water early on. One key difference, though, is that early Venus was likely even hotter than their virtual starter world. "Venus had 35 or 40 percent higher solar radiation than Earth does now," Popp says. The planet might have been a moist greenhouse, but not for long, he says, and it may well never have had oceans.
Kasting agrees, adding that over the last decade or so the consensus has settled around the theory that Venus was still covered in a largely molten surface when the planet started losing its water.
One thing this study does, Kasting says, is help define the inner edge of the habitable zone, the region around a star where a planet should be able to host liquid water on its surface. Simulations like this help define how big a role atmospheric composition may play and show what the possibilities are.
"Do you go directly to runaway greenhouse or end up in a moist greenhouse?" he says. Direct imaging of exoplanets—something that is still in the future for Earth-sized worlds—might one day help answer this question with hard data on a real planet's steamy qualities.