By Lee Billings
Not all planets have a home. For decades, astronomers and science fiction authors alike have speculated about orphaned orbs cast adrift from their home stars, endlessly wandering the boundless reaches of interstellar space. Most theorists hold that such ejections should be quite common during the chaotic tumult of a planetary system’s early days, when closely-packed worlds whirling around a star can scatter off each other like billiard balls in a break shot. Studying the properties of these far-flung planetary nomads—their numbers, masses and trajectories—could allow scientists to reconstruct these bodies’ murky origins and peer into a crucial formative stage of planetary systems that is otherwise largely hidden to us.
Hard evidence for this population of planetary nomads has proved elusive—floating cold and lightless in the void, these dark worlds cannot be directly observed by any conceivable telescope. Very rarely, however, one might pass in front of a far-distant background star, creating a detectable blip of light as the planet’s gravitational field acts as a magnifying lens. The duration and strength of such a “gravitational microlensing” event could reveal not only a rogue planet’s existence but also its mass, as bigger worlds tend to create longer, stronger amplifications of a background star’s light. A typical free-floating Jupiter-mass planet, for instance, is estimated to create an amplification lasting one to several days. A smaller, Earth-sized object might only amplify a star for a few hours.
It takes intensive calculations and a complicated series of assumptions to extract a rogue planet’s basic details from the deceptively simple brightening of faraway stars. But experts broadly agree that it can be done—so a handful of telescopic surveys around the world now monitor hundreds of millions of suns night after night to seek these objects, gradually taking a bulk census of the Milky Way’s loneliest worlds from the telltale twinkles of chance cosmic alignments.
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