Technology Review | The Physics arXiv Blog | kfc | 2011 Feb 09
Rogue Planets Could Harbor LifePlanets that have been ejected from their solar systems could still support subglacial oceans of liquid water, according to new calculations.
In recent years, computers have become powerful enough to simulate the formation and evolution of planetary systems over many billions of years.
One of the surprises to come out this work is that planets are regularly kicked out of these systems by slingshot effects. By some calculations, this fate may still await planets in our own Solar System.
One interesting question is whether these so-called "rogue planets" could ever support life in the cold dark reaches of interstellar space.
Today, Dorian Abbot and Eric Switzer at the University of Chicago give us an answer. The generally accepted criteria for life is the presence of liquid water. They calculate that an Earth-like rogue planet could support liquid oceans if the water were heated from below by the planet's core and insulated from above by a thick layer of ice.
Their reasoning is straightforward. They define an Earth-like planet to have dimensions within an order of magnitude of Earth's and having a similar composition. They then calculate the heat flux from the core and suggest that the thickness of the ice above would reach a steady state in about a million years. That's much shorter than the lifetime of a hot core.
Note that this is somewhat different from the mechanism that keeps the subglacial ocean on Europa liquid. Here tidal forces play an important role and this generates heat within the ocean itself. By contrast, all the heat must come from the core of a rogue planet and travel through the ocean,
One important unknown is the role that convection and conduction play in the less viscous regions of ice. Since convection carries heat much more quickly than conduction, this is an important factor and could potentially make the difference between the existence of liquid oceans or solid ice.
But with reasonable assumptions Abbot and Switzer say that a planet just 3.5 times the mass of Earth could maintain a liquid ocean. Even more surprising is their conclusion that a planet with a higher fraction of water need only be 0.3 times the size of Earth and still have a liquid ocean. That's smaller than Venus but bigger than Mars.
They call such a body a Steppenwolf planet "since any life in this strange habitat would exist like a lone wolf wandering the galactic steppe." It's not hard to imagine the possibility of life evolving around hydrothermal vents before the planet's ejection or even afterwards.
These are exciting calculations. Steppenwolf planets would provide one way for life to spread through the galaxy. And if any come within a 1000 AU of our Sun, the reflected sunlight from them ought to be visible in the far infrared to the next generation of telescopes.
That raises an interesting idea: the possibility of visiting such a place. Any passers by would certainly be easier to get to than planets orbiting other stars.
Time to get out the binoculars and lens cloths and start looking.
Wired Science | Lisa Grossman | 2011 Feb 10
Outcast Planets Could Support LifeIf 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.”
Science NOW | Jon Cartwright | 2011 Feb 11
A Living Planet Between the Stars?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.
Centauri Dreams | Paul Glister | 2011 Feb 10
The Steppenwolf: A proposal for a habitable planet in interstellar space - DS Abbot, ER Switzer
- arXiv.org > astro-ph > arXiv:1102.1108 > 05 Feb 2011
- Astrophysical Journal Letters 668(2) L167 (2007 Oct 20) DOI: 10.1086/523103
arXiv.org > astro-ph > arXiv:0709.0945 > 06 Sep 2007