Rogue Planets May Be Even Lonelier Than We Thought

By Rae Paoletta on at

Rogue planets sound adventurous, like pirates of the final frontier minus the scurvy. The reality is much more depressing: these bodies are untethered to a star, so they’re doomed to dance around the void solo. To make things even sadder, new research suggests certain rogue planets, namely the Jupiter-sized ones, are far lonelier than previously suggested.

A leading idea about the origin of rogue planets suggests they were kicked out of their parent systems when two planetary bodies got in a gravitational tussle. Over the last few years, various surveys have attempted to pin down how common these cosmic nomads are—for example, in 2011, a group of astronomers suggested Jupiter-sized rogue planets could be more common than main sequence stars in the Milky Way.

But recently, a team of researchers at Warsaw University Observatory in Poland analysed 2,600 microlensing events detected by the Optical Gravitational Lensing Experiment (OGLE-IV) between 2010 and 2015, and found that Jupiter-mass rogue planets could be at least ten times less common than this 2011 estimate. Microlensing is a technique used to study objects that are extremely far away from Earth, that relies on examining how light from one source (a star) bends from the gravitational field of another lens (in this case, a planet). The technique is useful because it doesn’t rely on how bright the object of interest is, and rogue exoplanets are dark.

This study analysed a much larger sample of events than the 2011 study, which observed only 474 microlensing events, which could help explain the discrepancy in the two paper’s findings. Through their calculations, the group at Warsaw University suggests there are no more than 0.25 rogue Jupiter-mass planets per Milky Way main-sequence star. At the same time, the team posits rogue planets similar to Earth’s mass could be pretty abundant—possibly as many as two rogue Earth-sized planets per main sequence star.

“A previous analysis of 474 microlensing events found an excess of ten very short events (1–2 days)...indicating the existence of a large population of unbound or wide-orbit Jupiter-mass planets,” the team wrote. “These results, however, do not match predictions of planet-formation theories and surveys of young clusters.”

There’s still much to learn about these nomadic worlds. Some scientists, for example, think the hypothetical object known as Planet 9 could have been a rogue planet in its early life. Others posit our solar system might have had a fifth gas giant that was ejected and left to wander the cosmos.

The argument over the abundance of rogue planets isn’t going away any time soon, and more survey data will surely help us to better pin down the abundance of these cosmic nomads. Still, one thing’s for sure—these wandering planets sure could use a friend. [Nature]

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