If a planet is ripped from the warm cradle of its solar system and plunged into the frigid depths of space, it could still hold on to a liquid ocean – and maybe life – beneath an icy crust.

Planet formation models suggest that small planets are regularly flung from their solar systems by close encounters with neighboring gas giants. The giants' gravitational fields create an interplanetary slingshot effect, sending smaller planets on unstable orbits that quickly leave their star behind.

Prior to ejection, some of those planets could conceivably be like Earth, with continents, oceans and biospheres. A new model suggests that submarine aliens on such a planet could have a chance at survival.

"We originally started with the question, 'What if you turned off the sun?'" said University of Chicago geophysicist Dorian Abbot, co-author of a paper submitted to Astrophysical Journal Letters and prepublished Feb. 5 on arXiv.org.

Along with fellow University of Chicago astrophysicist Eric Switzer, Abbot ran the numbers to see if an ocean could stay liquid without heat from a star. They called their rogue world a Steppenwolf planet, "since any life in this strange habitat would exist like a lone wolf wandering the galactic steppe."

The pair assumed the planet was between 0.1 and 10 times Earth's mass, with a similar amount of water and rock. Once the planet was flung its warm, nurturing star, the ocean would start to freeze. But leftover heat from the planet's formation and decaying radioactive elements in the rock could keep the ocean warm beneath a shell of ice. As long as the planet could keep the ice from freezing all the way to the core, the ocean should be safe.

Abbot and Switzer calculated that a planet 3.5 times the mass of Earth would be warm enough at the core to maintain a liquid ocean beneath an ice crust a few kilometers thick. The ocean could last for about 5 billion years.

"That's a non-ridiculously short timescale," said astrobiologist Cynthia Phillips of the SETI Institute, who was not involved in the new work. "It seems like this thick ocean could actually persist for longer than you might assume, without going through the numbers."

Phillips studies the possibility of life beneath the icy crust of Jupiter's moon Europa, a world superficially similar to the hypothetical Steppenwolf planet. But unlike the rogue world, most of Europa's heat comes from tides raised by Jupiter.

In a slightly more bizarre twist, Switzer and Abbot imagined the Steppenwolf planet with volcanoes spewing carbon dioxide into the atmosphere. The gas would freeze and fall as snow almost immediately, covering the world with an insulating blanket of dry ice. In that case, planets as small as 0.3 times the mass of Earth could keep a liquid ocean.

"That, I’m a bit more dubious about," Phillips said. With only decaying radioactive elements providing heat, "it seems unlikely that you’d have serious volcanic activity going on, without any other energy present."

Life on the planet could consist not only of organisms that survived the interstellar turmoil and adapted, but those that evolved later, around hydrothermal vents at ocean floors.

Abbot and Switzer declined to speculate what such life would look like, but they and Phillips agreed that it would almost certainly be microscopic.

"I would be very, very surprised if a planet like this could sustain big macroscopic life forms, just because the energy is so limited," Phillips said.

If these inhabited, free-floating planets exist, they could have been a vehicle for bringing the seeds of life to Earth. If the planet came within about 0.01 light-years of Earth, it could even be observed from the ground, Abbot and Switzer suggested.

But the odds of that happening about one in a billion at best, Switzer said. The researchers mostly meant to muse on the extreme possibilities for habitable worlds.

"If you can imagine life on such an object," Abbot said, "potentially there could be life in many sorts of weird situations that we haven’t thought of before."

Image: Jupiter's moon Europa, which could harbor life in a liquid ocean beneath an icy crust. (NASA)

See Also:

"The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space." Dorian S. Abbot and Eric R. Switzer. arXiv.org, Feb. 5, 2011.