February 24, 2005
by Seth Shostak, Senior Astronomer

Try to imagine this, a scene unwitnessed by any thinking being, although it could play out every few weeks somewhere in the Milky Way:

You are on the curdled, hot surface of a new-born world; an unknown cousin of Earth only a few millions of years old.  The landscape is a sweltering, fulminating jumble of soft rock, as sterile as space itself. 

If anyone could gaze at the night sky, they would see a dark bowl riddled with hard, bright dots. The dots are sibling worlds – new planets in various stages of gestation, careening through the viscous disk of gas and lumpy dust that has given them substance and form.

 

Suddenly, and by chance, a not-so-improbable encounter takes place.  The trajectory of another object, a litter mate, crosses nearby.  For several days, the second planet sails large across the sky: silent and dangerous.

 

There is no actual collision, no cataclysmic shattering of nascent worlds.  But gravitational interaction during this brief encounter changes the motion of both objects; speeding up one, and slowing the other.  And now a dispiriting event unfolds, although this world has no eyes to see it.  Ejected by chance from the solar system of its birth, the planet sideslips into deep space.  Every hour, the sun that had promised to warm its surface for billions of years recedes by another fifty thousand miles.  In a mere decade, the home star shrinks to a point of light, eventually indistinguishable from other stars of the sky.  The planet’s surface cools, its atmosphere condenses, falls, and piles up in frozen drifts.  This is an orphan world, wandering without destination in the numbing, frigid desert of deep space.

 

Although it was kicked from the litter by accident, this planet’s involuntary exile may be a frequent fate for newborn worlds.  Doug Lin, an astronomer at the University of California at Santa Cruz, says “my sense is that orphan planets could be numerous. There’s already indirect evidence that Jupiter-sized worlds have been ejected from some of the extrasolar planetary systems we’ve discovered in the last decade.  The clue is that large planets in these systems often have highly elliptical orbits.”  Giant worlds in egg-shaped orbits are, presumably, the objects that were left behind when a planetary fender-bender took place.

 

“You don’t have that situation in our solar system.  We’re lucky because Jupiter – which had a low-eccentricity orbit to begin with – has nudged the other planets into similar near-circular orbits, where they don’t get in one another’s way,” notes Lin.

 

But imagine a system in which no Jupiter-sized planet forms, simply because of a shortage of raw material.  Even in this case, there still might be sufficient resources to construct Earth-size and smaller worlds.  Some of these will inevitably form at distances between 0.5 to 1 billion miles from the star (the range of orbits staked out by Jupiter and Saturn in our own solar system).  At those distances, the orbital velocities are roughly 10 miles per second.  That’s slow enough that a gravitational encounter can easily add the 4 additional miles per second that would firmly evict a relatively heavy object from the star system.  (For the numerically minded, escape velocities are 41% higher than orbital velocities.)

 

How often does this happen?  “I don’t know what fraction of planets will be tossed out,” Lin admits. “But I would imagine the fraction is probably pretty high; in fact I wouldn’t be surprised if it were 50%.” 

 

If that’s the case, then orphan planets could be more numerous than stars!  In our own galaxy alone, there would be hundreds of billions of these wandering worlds.

 

That’s a lot of errant real estate, and so the question naturally arises whether there could be life on these Bedouin bodies, given that they might constitute a large fraction of all planetary acreage.  At first glance, you might assume there’s not much hope.  In deep space, precious little energy is available to either warm an ocean or provide the calories required for metabolism. On Earth, sunlight ultimately fuels most life.  Nearly a kilowatt of power impinges on each square yard of the landscape on a sunny day.  In interstellar space, where the stars are distant and faint, the energy flux is a billion times less.

 

Facts are, any surface oceans on an orphan planet will freeze harder than a granite countertop, and photosynthesis will be a non-starter.  But bear in mind that life on Earth may have originated from – and certainly still exists in – the superheated waters of deep ocean vents.  The muddy water that spews from these sizzling fissures is brought to a boil by heat from the interior – heat that is mostly left over from Earth’s birth.  After more than four billion years, our planet is still warm inside.  Mars, a smaller world, has largely cooled.  There are no moving plates or active volcanoes on the Red Planet.  It’s chilled out.

 

But clearly, for the bulkier orphan planets – those that might be Earth-sized or larger – underground energy for supporting life would last for billions of years.  So you can well imagine that while the surface of any oceans on these worlds would be solid ice, biology might still thrive in liquid water below.  And after all, you don’t need a lot of warmth.  Magma’s not required, merely hot water.  As Lin comments, “Melting rocks takes a lot of heat.  But for life, all you need do is boil water.”

 

Life on orphan worlds is possible, in other words.  But could complex, or even intelligent life arise there?  That’s clearly more of a long shot.  Frankly, it seems unlikely that life dependent on leakage heat from a planet’s interior would ever become elaborate enough to understand the universe.

Nonetheless, this is a type of habitat that, while possibly quite common, is very alien to our own experience.  We cannot say for certain that intelligent life is impossible on these worlds.  If such an extravagant development occurs, it would be interesting to know what the inhabitants think about the possibility of life on a planet orbiting a star.  Indeed, from their point of view, our home could seem thoroughly unattractive.  They might doubt the desirability of living desperately close to a roiling ball of incandescent gas, one that routinely spouts deadly radiation from its stormy countenance.  Maybe it’s better to be safely ensconced on a world where the sun never shines.