Gravitational microlensing to the rescue. We now have evidence for the existence of the rogue planets — interstellar wanderers moving through space unattached to any star system — that we talked about just the other day. It’s been assumed that such planets existed, because early solar systems are turbulent and unstable, with planetary migrations like those that lead to ‘hot Jupiters’ in the inner system. Moving gas giants into orbits closer to their star would cause serious gravitational consequences for other worlds in the system, ejecting some entirely.
But while we’ve been thinking in terms of detecting such worlds through auroral emissions like those produced by Jupiter, researchers at two microlensing projects have made a series of detections by using gravity’s effects upon spacetime. Specifically, a stellar system passing in front of a far more distant background star will warp the light of the background object. The resulting magnification and brightening flags the presence of the intermediate object, and surveys like Microlensing Observations in Astrophysics (MOA), based in New Zealand, have developed the necessary expertise to distinguish between intermediate stars and planets.
Both MOA, which scans the galactic center for these microlensing events, and the Optical Gravitational Lensing Experiment (OGLE), using a 1.3 meter telescope in Chile, have studied and built a case for the existence of up to 10 rogue planets of roughly Jupiter mass. Microlensing because of its nature picks up objects a long way from our stellar neighborhood — these average between 10,000 and 20,000 light years from Earth. Extrapolating from the lensing probabilities, the efficiency of their equipment and the rate of lensing, the researchers now conclude that there could be as many as 400 billion rogue planets in the Milky Way.
How Do Rogue Planets Form?
That’s a big number, but at this point we’re still shooting in the dark. After all, lower-mass planets should be ejected from young solar systems more frequently than the gas giants this work has detected, which is why planet hunter Debra Fischer (Yale University) told Nature News in a related story that lighter planets “…might be littering the galaxy.” What a scenario, particularly given the possibility that a hydrogen atmosphere could trap enough heat to allow the presence of liquid oceans. Unfortunately, the current survey was not sensitive enough to detect planets smaller than Saturn.
This work is getting huge play in the press, but I think it raises as many questions as it answers, for the scenario shifts depending on how these wandering worlds were formed. The current work draws on the idea that they were the result of ejection from solar systems. In fact, David Bennett (University of Notre Dame), a co-author of the study in Nature, assumes ejection as the primary mechanism:
“If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10,” Bennett said. “Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth.”
And if ejection is the driver here, then we should assume a huge population of low-mass planets moving through space without any star, just like these gas giants. But if there is another formation mechanism at work (Greg Laughlin speculates about this in the Nature News article I linked to above), then the low-mass wanderers are much less prevalent. Right now we just don’t know, because we would need a formation mechanism that would account for objects not much larger than Jupiter, “…something more similar to that of a tiny star than a giant planet,” Laughlin adds. Whether or not ejection is the mechanism thus becomes crucial for any hypothesis about rogue ‘Earths.’
Outer Orbits and Unseen Hosts
Also in play is the question of whether the ten detections could be of gas giants in planetary orbits around stars that were simply not detected. The study sees no host stars within 10 AU, a figure that remains relatively close to any potential host. We don’t have a firm answer, and I see that Alan Boss (Carnegie Institution) told the New York Times’ Dennis Overbye that this scenario is the more likely one. If that’s the case, then we should look with even greater interest at data from the WISE (Wide-Field Infrared Survey Explorer) mission, which should have been able to spot any gas giant lurking in the distant regions of the Oort Cloud. Ten detections like this would imply such outer orbits may be common around stars.
Where we go from here seems obvious: We need to confirm there are no host stars. If we do, then the presence of twice as many rogue gas giants as there are stars in the galaxy is enough to take the breath away, whether or not they’re in the company of rogue ‘Earths.’ The planned Wide-Field Infrared Survey Telescope (WFIRST) might be able to make a detection of such Earth-mass rogue planets and give us some constraints on their numbers. We’ll also learn, as we continue the study of galactic wanderers, to tighten up our theories of planet formation and migration to account for the suddenly increased population of sub-stellar objects among the stars.
The paper is Sumi et al., “Unbound or distant planetary mass population detected by gravitational microlensing,” Nature 473 (19 May 2011), 349-352 (abstract).