Image courtesy NASA/ESA
Published August 5, 2011
The theory offers an explanation for the handful of free-roaming planets found so far, and it could mean many more such rogue worlds exist across the Milky Way.
"Because every star dies, and many of these stars are massive enough to trigger planetary ejection, there is ample opportunity throughout the galaxy for stellar deaths to contribute to the free-floating population," said study leader Dimitri Veras, an astronomer with the U.K.'s University of Cambridge.
"We don't know yet how common these planets are, but the observational evidence suggests that there could be more planets floating in between stars than orbiting them," he added.
(Related: "Alien Planets Outnumber Stars, Study Says.")
The new model also hints that—in very rare cases—some survivor planets may remain bound to the supernova remnants, finding new orbits around the neutron stars or black holes left behind by the explosions. (See pictures of supernova remnants.)
A Wrinkle in the Two-Body Problem
The new theory is based on a complex set of computer models that take into account something known in physics as the two-body problem.
This formula in classical mechanics helps determine the orbits of two interacting bodies, from an electron around an atomic nucleus to a planet circling a star.
As long as astronomers can measure the masses, positions, and velocities of the two objects, they can determine their past, present, and future orbits using simple mathematical equations.
The new study, however, applies a wrinkle to the two-body problem, Veras said, because the stars are losing mass as they go through their death throes.
"In this situation, we don't know if a complete solution by simple equations alone exists, so in many cases we have to use computers to simulate their orbits," he said.
According to the model, when stars at least 7 to 10 times the mass of our sun go supernova, the blast swallows any inner planets, those orbiting out to several times the distance between Earth and the sun.
Planets orbiting hundreds of times the Earth-sun distance will instead have their orbits disrupted and elongated in such a way that they're eventually flung into interstellar space.
In some cases, the disrupted planets will be pushed into more distant but stable orbits around the supernova remnants.
These planets may continue orbiting stellar remnants such as pulsars and even black holes far enough away that they would not be swallowed by their intense gravitational pull—but the survivors would be charred, dead worlds.
What's more, these worlds on the edge can easily be stripped off the system and turned rogue by the gravitational pulls of other nearby stars.
Billions of Rogue Planets Out There?
In May 2011 a different team of astronomers described observational evidence of up to ten planets that appear to be wandering independently through interstellar space.
The new expulsion mechanism may account for how those newfound planets escaped their star systems, said Steinn Sigurdsson, an astronomer at Pennsylvania State University not connected to the study. (Read a commentary on the paper on Sigurdsson's blog Dynamics of Cats.)
But Sigurdsson cautions that there may be another method at play for how planets get kicked out of their star systems.
"The other possibly dominant mechanism is planet-planet scattering, where massive planets knock other planets out of their orbits, and some fraction are thrown into outer space as unbound rogues," Sigurdsson said.
It's even possible the two mechanisms are working together, Sigurdsson added, with the scattering effect putting planets in wider orbits and the supernova then ejecting them from the system.
Either way, he believes the possibilities add up to a lot of free-floating planets.
"The numbers [with both ejection mechanisms] are broadly consistent, and both imply billions of rogue planets in total," he added.
Ejected Worlds May Host Underground Life
One big question raised by the new study is whether any life could survive expulsion from its planetary clan.
The answer may be yes, if the planet had enough internal heat and already supported subsurface life, said John Debes, a planet hunter not affiliated with Veras' team.
"Moons like Europa, which is tidally heated by Jupiter because of constant interactions with other moons, would probably be the best bets for life surviving an ejection," said Debes, a postdoctoral fellow at NASA's Goddard Space Flight Center in Maryland.
"An Earth might remain habitable in certain conditions, if it had a moon survive the ejection as well," since that moon would provide tidal heating.
Life could also find a way to take hold on planets without host stars under the right conditions, Debes added. (See "Earth-Size 'Lone Wolf' Planets May Host Life.")
There have been studies showing that climatological, geodynamical, and biogeochemical processes might be able to sustain life on starless planets, specifically those with underground oceans with thermal vents, where free-energy flow may still be enough to run a biosphere.
Actual Planet Ejection Hard to Spot
The ultimate goal for Veras and his team would be to actually catch a star in the act of kicking out its planets—which may prove difficult if not impossible.
That's because, with current technology, odds are low of finding planets around a dying star about to explode.
For now, there is observational evidence of what happens to stars when they die, of wide-orbit planets, and of free-floating planets, Veras said.
"All the ingredients are there," he said. "However, the time scale to actually observe a planet being ejected, in most cases, is longer than a human lifetime."
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