Astronomers from the OGLE (Optical Gravitational Lensing Experiment) project have discovered a Mars- to Earth-mass free-floating, or rogue, planet through a technique called gravitational microlensing.

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

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

Gravitational microlensing is a technique that facilitates the discovery of distant objects by using background stars as flashlights.

When a star crosses precisely in front of a bright star in the background, the gravity of the foreground star focuses the light of the background star, making it appear brighter.

An exoplanet orbiting the foreground object may cause an additional blip in the star’s brightness.

This technique has found the most distant known planets from Earth, and can detect free-floating or wide-orbit planetary objects.

“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,” said lead author Dr. Przemek Mroz, a postdoctoral researcher at Caltech.

“An 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,” he added.

“If we observed only one source star, we would have to wait almost a million year to see the source being microlensed.”

The newly-detected microlensing event, designated OGLE-2016-BLG-1928, lasted only 41.5 minutes.

“Duration of microlensing events depends on the mass of the lensing object — the less massive the lens, the shorter the microlensing event,” the astronomers explained.

“Most of the observed events, which typically last several days, are caused by stars.”

“Microlensing events attributed to free-floating planets have timescales of barely a few hours.”

“By measuring the duration of a microlensing event and shape of its light curve, we can estimate the mass of the lensing object.”

OGLE-2016-BLG-1928 is the shortest-timescale microlensing event ever found.

The lens in this event, OGLE-2016-BLG-1928b, is likely a sub-Earth-mass object, one of the lowest-mass objects ever found by microlensing.

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

“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 discovery of OGLE-2016-BLG-1928 demonstrates that current microlensing surveys are capable of finding extremely-short-timescale events,” the researchers said.

“Although the mass of the lens cannot be unambiguously measured, properties of the event are consistent with the lens being a sub-Earth-mass object with no stellar companion up to the projected distance of 8 AU (astronomical units) from the planet.”

“Thus, the lens is one of the best candidates for a terrestrial-mass rogue planet detected to date.”

“This population of low-mass free-floating or wide-orbit planets may be further explored by the upcoming microlensing experiments.”

The team’s work was published in the Astrophysical Journal Letters.


Przemek Mróz et al. 2020. A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event. ApJL 903, L11; doi: 10.3847/2041-8213/abbfad