A new study finds that rogue planets — those floating free in the galaxy, unbound to any star — are not as plentiful as we once thought.
Astronomers know that there are rogue, Jupiter-sized planets roaming the galaxy on their own, without a star to call home. But how many dark and lonely planets are out there?
Six years ago, a study found that they might be so abundant, they outnumbered the stars two to one. The result had implications not only for how these dark planets might form, but also for the general messiness of the early years of planet formation. Now, a new study using the exact same technique has contradicted that first result — turns out there aren't nearly as many of these dark, Jupiter-sized worlds as we once thought.
Rogue Planet Detection
Planet detection methods are notoriously biased — radial velocity searches, for example, tend to find big planets close to their stars, while direct imaging tends to find big planets far from their stars. But there's a way to detect planets no matter their distance from their host star, or if they have a host star at all: microlensing.
With the Optical Gravitational Lensing Experiment (OGLE), astronomers monitor almost 50 million of stars in our galaxy, most of them in the dense galactic bulge, to find the rare occasion when one of them flashes, its light briefly magnified due to the gravitational effect of an object passing in front of it. The object, whether star or planet, is typically so far away we can't see it directly. But if astronomers carefully study the background star and the way its light is brightens and fades again, they can learn a great deal — including the object's mass (the less massive the object, the shorter the background star's flash), and whether the object has a companion (a planetary companion to a star, for example, will imprint a bump on the background star's light curve).
Between 2010 and 2015, OGLE detected 2,617 of these microlensing events, improving on an analysis of 474 events published in 2011. The 2011 study had found that rogue, Jupiter-sized bodies floating the Milky Way might amount to 1 trillion, and another study estimated that number might be even higher if smaller planets were accounted for.
But the newest analysis, published in Nature Astronomy, contradicts those results, suggesting that our galaxy may have less one Jupiter-size rogue planet for every star, so at most 75 billion of them. Even that is likely a vast overestimate, as most and perhaps all of these detections could be attributed to planets on very wide orbits — that is, still bound to their host stars.
"The difference is primarily due to statistics," says Takahiro Sumi (Osaka University, Japan), author of the 2011 study. The new study has more than five times as many microlensing events, enabling a more accurate count of unbound Jupiter-mass planets.
Intriguingly, the study also found six microlensing events that were so short, they could be due to free-floating Earth-size or super-Earth-size planets. However, given the short timescales, these might be stellar flares instead of real microlensing events. We won't always be in the dark, though: upcoming missions such as WFIRST and Euclid will have the ability to look for microlensing events on an even finer scale than OGLE, enabling study of Earth-size microlensing events across the galaxy.