If aliens exist, where are they? Many astronomers look to the nearest stars, in the hope that they harbor a warm, wet planet like Earth. But now a pair of researchers believe extraterrestrial life could exist on a rogue planet that has been ejected from its birthplace.
Astronomers have never spotted a rogue planet with certainty, but computer simulations  suggest that our galaxy could be teeming with them. Slingshotted out of their planetary system by the gravity of a bigger planet, these lone worlds zoom far from their parent suns, slowly freezing in the cold of outer space. Any water fit for life would freeze, too. Yet in a paper submitted to The Astrophysical Journal Letters, planetary scientists Dorian Abbot and Eric Switzer of the University of Chicago in Illinois suggest that a rogue planet could support a hidden ocean under its blanket of ice, kept warm by geothermal activity.
They call such a world a Steppenwolf planet after a novel by the German-Swiss author Hermann Hesse, because "any life ... would exist like a lone wolf wandering the galactic steppe." If Steppenwolf planets do exist, there's a chance that some of them could be lurking in space between Earth and nearby stars. If so, they might be a more realistic human destination for the search of alien life than another planetary system, which would be at least several light-years away. There is even a chance—albeit very small—that a Steppenwolf planet crashing into our solar system billions of years ago was the origin of life on Earth.
Abbot and Switzer came to their conclusion by simulating an isolated planet between 1/10th and 10 times the size of Earth. By comparing the rate at which heat would be lost through an ice shell with the rate at which heat would be produced by geothermal activity, they calculated that a planet with Earth's composition of rock and water but three times as big would generate enough heat to maintain a hidden ocean. If the planet had much more water than Earth, say Abbot and Switzer, it would need to be only about a third as big as our planet. "Several kilometers of water ice make an excellent blanket that could be sufficient to support liquid water at its base," says Switzer.
The Chicago researchers are not the first to consider the possibility of liquid water on rogue planets. In 1999, planetary scientist David Stevenson of the California Institute of Technology in Pasadena, calculated that liquid water could exist if a planet had a dense atmosphere of hydrogen—so dense that a greenhouse effect would trap warmth on the surface without the need for ice. But Abbot thinks the new result is more surprising because they are considering a more generic planet, without an extraordinary atmosphere.
"This is certainly an interesting study regarding the extent of the possible locations where life could arise, or be sustained, in the universe," says David Ehrenreich, a planetary scientist at the Joseph Fourier University in Grenoble, France. "However, it will certainly be very difficult to actually detect life on such a world, since it would be buried under an ice shell."
Switzer admits detection would be difficult. An astronomer would need to spot a Steppenwolf planet by looking for its infrared emission to see if it is as warm as he and Abbot predict. But at present, even the best observatories could detect rogue planets only within about 100 billion miles of Earth—not a huge distance in astronomical terms—and Switzer says the probability of a Steppenwolf planet existing in this range is just one in a billion.
Still, as planetary scientist Gaetano Di Achille of the University of Colorado, Boulder, points out, that might mean that the first occupied planet humans set foot on is not in another planetary system, but in the lonely depths of outer space. "If the hypothesis of oceans on rogue planets is correct, we will certainly have to expand the inventory of places with a high potential for life," he says.