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Tiny, lonely rogue planet is smallest ever discovered

Out in the depths of space, in the huge swaths of blackness between star systems, you can find lonely, wandering rogue planets. These planets don’t orbit around a star but meander through the cosmos alone. And now, astronomers from the OGLE (Optical Gravitational Lensing Experiment) project have discovered the smallest rogue planet ever spotted.

Most rogue planets discovered so far have been several times the mass of Jupiter, but the new lonely planet is more like the size of Earth or Mars. It’s hard to spot such relatively tiny planets, so the researchers use a technique called gravitational microlensing.

This method works by looking at the light emitted by distant background stars. When an object passes between us and the background star, the light from the star is very slightly focused by the gravity of the object, making the star appear brighter for a short time. The problem is that background stars and foreground objects need to be exactly aligned for this effect to happen, which only occurs very rarely.

“If we observed only one source star, we would have to wait almost a million years to see the source being microlensed,” lead author Dr. Przemek Mroz explained in a statement. So they look at multiple source stars instead.

This allowed them to spot the tiny object which they believe to be a rogue planet, in the shortest-ever microlensing event which lasted just over 40 minutes. Typically, microlensing events caused by stars last for several days, and for exoplanets last several hours. The fact this event was so brief indicates that the object causing it, the rogue planet, is much smaller than previous detections.

An artist’s impression of a gravitational microlensing event by a free-floating exoplanet.
An artist’s impression of a gravitational microlensing event by a free-floating exoplanet. Jan Skowron / Astronomical Observatory, University of Warsaw.

“When we first spotted this event, it was clear that it must have been caused by an extremely tiny object,” co-author Dr. Radoslaw Poleski, an astronomer with the Astronomical Observatory at the University of Warsaw, said in the statement. “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.”

The research not only identified this curious rogue exoplanet but also demonstrated how microlensing can be used to spot even relatively tiny objects in space. This technique could be used to find many more of these wandering planets in the future.

The research is published in the journal Astrophysical Journal Letters.

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