Rogue Planet Found Using Gravitational Microlensing | TechNews

Astronomers used survey observations of the gravitational microlensing effect to discover that there are many drifting stars in the Milky Way that are not bound by the gravity of stars. However, the observed gravitational microlensing effect will weaken rapidly as it gets further away from the Earth. Therefore, so far, it is still impossible to directly measure the mass of drifting stars. It can only be inferred from the statistical model of the population that the mass of most drifting stars is smaller than Jupiter. Recently, astronomers discovered an unprecedented microlensing observation data. This event originated from an exoplanet drifting star with a mass of approximately 21.9% that of Jupiter, approximately 9,785 light-years from the center of the Milky Way.

Astronomers once believed that planets mostly orbited stars. However, more and more observational evidence shows that some planets will be thrown away from the planetary system after their formation and become drifting objects wandering alone in the Milky Way. Such planets that do not orbit around a star are called drifting planets. Because the radiation it emits is extremely weak, it can only be observed and discovered through the extremely small gravitational lensing effect caused by the light emitted by the background star passing through the vicinity of the drifting star, known as the “gravitational microlensing effect.”

The main observational limitation of this method is that it is difficult to independently determine the distance between the celestial body that produces the microlensing effect and the Earth, which makes the calculated mass less accurate, causing the study of the properties of drifting stars to remain in the speculative stage for a long time. In this study, the team captured a very brief microlensing event of a free-drifting planet. What is different from the past is that this event was observed simultaneously by multiple large ground survey telescopes and the Gaia space telescope.

Due to the distance difference between ground telescopes and space telescopes, there are very subtle differences in the time changes in the light of background stars. This allowed the research team to measure the parallax of the microlensing effect. The results were then analyzed using finite light sources and point lens models, and the mass of the drifting planet and its position in the Milky Way were successfully calculated simultaneously.

The researchers pointed out that comparing this observational result with the statistical properties and numerical simulations of other microlensing events, it can be inferred that the object was more likely to be formed in a protoplanetary disk and born like a normal planet, rather than independently like a brown dwarf. Later, it was ejected out of the original planetary disk through the gravitational interaction between the original planets and became a drifting planet.

(This article is reprinted with permission from the Taipei Planetarium; first image source: NASA)