A Giant Planet Microlensing Event

Over 5,000 exoplanets have been detected to date, with more than 90% of them found by using the transit or radial velocity techniques. Of the other 10%, 105 were found using the microlensing method which takes advantage of the fact that the path of a light beam is bent by the presence of a massive body. The gravitational force of the body acts like a lens (a “gravitational lens”) to distort the image of an object seen behind it. When a massive object fortuitously passes in front of a star, it acts as a gravitational lens and thus its motion across the sky causes the background star to appear to brighten briefly. When the foreground object is a star hosting a planet, both bodies can produce brightening events as they pass in front of the star, and the flashes as seen from Earth can be modeled to determine their masses and separation.

Two significant advantages are offered by the microlensing method over more common exoplanet detection techniques. First, the brightness of the microlensing effect does not depend on the brightness of the moving body, only on its mass, which makes it possible to spot faint, low-mass M dwarf stars. The second advantage is that the microlensing planet may orbit its star at a large distance, even many astronomical units. (Since normal exoplanetAn exoplanet (or extrasolar planet) is a planet that is located outside our Solar System, orbiting around a star other than the Sun. The first suspected scientific detection of an exoplanet occurred in 1988, with the first confirmation of detection coming in 1992.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>exoplanet techniques, like transiting, require multiple detections over many orbital periods, exoplanets with large orbits take years to complete their cycle and so far the vast majority of all measured exoplanets have orbits smaller than one astronomical unit.) As a result of their large orbits, the detected giant planets around microlensing host stars are usually far enough away to reside beyond the “snow line,” the distance at which surface water would freeze.

Harvard-Smithsonian Center for Astrophysics (CfAThe Harvard-Smithsonian Center for Astrophysics (CfA) is a joint venture between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. Founded in 1973, the Harvard-Smithsonian Center for Astrophysics is comprised of six research divisions: Atomic and Molecular Physics; Optical and Infrared Astronomy; High Energy Astrophysics; Radio and Geoastronomy; Stellar, Solar, and Planetary Sciences; and Theoretical Astrophysics.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>CfA) astronomer Jennifer Yee collaborates with a team of astronomers from the OGLE project (Optical Gravitational Lensing Experiment), which discovered the microlensing event OGLE-2017-BLG-1049. The analysis was led by her colleagues in the Korea Microlensing Telescope Network.

They modeled the brightening events using some probable assumptions, and concluded that the host star is an M dwarf with a mass of about 0.55 solar masses; the planet has a mass of about 5.5 JupiterJupiter is the largest planet in the solar system and the fifth planet from the sun. It is a gas giant with a mass greater then all of the other planets combined. Its name comes from the Roman god Jupiter.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Jupiter-masses and orbits at a distance of 3.9 astronomical units. These results have direct implications for models of planet formation. Fifty-four of the known microlensed exoplanets are giants around M dwarfs, like this new one, suggesting that planets are common around M dwarfs.

In the core accretion model of planet formation, however, in which planets gradually assemble from smaller rocks, very few planets are expected to be found around M dwarf stars. The result appears instead to support the alternative disk instability model in which a rotating disk fragments into clumps that form planets, and it predicts that many planets exist around M dwarf stars.

Reference: “OGLE-2017-BLG-1049: Another Giant Planet Microlensing Event” Yun Hak Kim, Sun-Ju Chung, A. Udalski, Ian A. Bond, Youn Kil Jung, Andrew Gould, Michael D. Albrow, Cheongho Han, Kyu-Ha Hwang, Yoon-Hyun Ryu, In-Gu Shin, Yossi Shvartzvald, Jennifer C. Yee, Weicheng Zang, Sang-Mok Cha, Dong-Jin Kim, Hyoun-Woo Kim, Seung-Lee Kim, Chung-Uk Lee and Dong-Joo Lee, 31 December 2020, Journal of the Korean Astronomical Society.
DOI: 10.5303/JKAS.2020.53.6.161