An international team of scientists, led by Polish astronomers, recently discovered an earth-sized rogue planet in a milky way. This discovery indicates that our milky way may be teeming with rogue planets.
Free-floating planets emit virtually no radiation and—by definition—they do not orbit any host star. Thus, they cannot be discovered using traditional methods of astrophysical detection.
In any case, rogue planets can be spotted utilizing an astronomical phenomenon called gravitational microlensing. Microlensing results from Einstein’s theory of general relativity—a massive object (the lens) may bend the light of a bright background object (the source). The lens’s gravity acts as a huge magnifying glass that bends and magnifies distant stars’ light.
Dr. Przemek Mroz, a postdoctoral scholar at the California Institute of Technology and a lead author of the study, said, “If a massive object (a star or a planet) passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source. The observer will measure a short, brightening of the source star. Chances of observing microlensing are extremely slim because three objects—source, lens, and observer—must be nearly perfectly aligned. If we observed only one source star, we would have to wait almost a million years to see the source being microlensed.”
That’s the reason modern surveys hunting for gravitational microlensing events are monitoring hundreds of millions of stars in the Milky Way center, where the chances of microlensing are highest.
As the technique gravitational microlensing doesn’t rely on the lens’s brightness, it enables the study of faint or dark objects such as planets. Time taken by events depends on the lensing object’s mass—the less massive the lens, the shorter the microlensing event.
The scientists announced the discovery of the shortest-timescale microlensing event ever found, called OGLE-2016-BLG-1928, which has a timescale of just 42 minutes.
Dr. Radoslaw Poleski from the Astronomical Observatory of the University of Warsaw, a co-author of the study, said, “When we first spotted this event, it was clear that an extremely tiny object must have caused it.”
“Indeed, models of the event indicate that the lens must have been less massive than Earth; it was probably a Mars-mass object. Moreover, the lens is likely a rogue planet. “If the lens were orbiting a star, we would detect its presence in the light curve of the event. We can rule out the planet having a star within about eight astronomical units (the astronomical unit is the distance between the Earth and the sun).”
A few years ago, Polish astronomers from the OGLE team from the Astronomical Observatory of the University of Warsaw provided the first evidence for such planets in the Milky Way. However, the newly-detected planet is the smallest rogue world ever found.
Prof. Andrzej Udalski, the PI of the OGLE project, said, “Our discovery demonstrates that low-mass free-floating planets can be detected and characterized using ground-based telescopes.”
“Theories of planet formation predict that the ejected planets should be typically smaller than Earth. Thus, studying free-floating planets enables us to understand the turbulent past of young planetary systems, such as the solar system.”
- Przemek, Mróz et al. A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event, The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/abbfad