Planets can sometimes be cast loose during the star formation process, left to wander the galaxy cold and alone. However, a new study finds we probably greatly overestimated how often this happens, at least for objects the size of Jupiter or larger.
Since planets don't produce enough of their own heat or light to be detected directly (unless they are newly formed), it is hard to find such objects when there is no nearby star to illuminate them. However, sometimes such an object will pass in front of a star and its gravity will focus the light of the more distant object – a process known as microlensing. An early attempt to extrapolate from the number of such events suggested that these unbound planets are astonishingly common, with twice as many objects with masses greater than or equal to Jupiter as there are stars.
However, a paper in Nature concludes that the true figure is at least 10 times lower. The Optical Gravitational Lensing Experiment (OGLE) tracked 50 million stars in dense star fields towards the galactic center, looking for the temporary brightening characteristic of microlensing. Between 2010 and 2015, there were 2,617 events seen, but most of these appear to have been caused by faint foreground stars, not planets.
Even allowing for the greater difficulty in detecting smaller microlensing events, first author PhD student Przemek Mróz and colleagues at the Warsaw University Observatory concluded that the numbers observed were consistent with a galaxy containing 0.25 Jupiter-sized planets or greater for each ordinary star. Moreover, while some of these are probably genuinely free-floating, others could be true planets that orbit far from their stars.
Objects of Earth mass are much harder to detect in this way, but six possibilities were found. If all are real, the low chance of catching such events suggests they are actually common, possibly indicating five such lonesome Earths for every star. However, the uncertainties are huge.
The new study produces more intuitive conclusions than the previous work. Stellar systems with two high-mass objects can be unsafe places for lighter entities, which can often be tossed out by gravitational interactions between the dominant partners. Consequently, some planets will be expelled from close binary star systems and some smaller planets thrown loose by the combination of a large planet and the parental star. However, it seems improbable for every star system, on average, to produce two planets at least as large as Jupiter, and then cast them out.
It is thought that some planets form entirely in the absence of a star. This happens when a patch of gas with insufficient mass to ever sustain nuclear fusion condenses into a planet-sized object. Nevertheless, the frequency of this is expected to be lower than the gravitational exclusion route.