Gravitational Microlensing for Detecting Exoplanets

The Polish 1.3m telescope used by OGLE in Las Campanas Observatory in Chile, one of the telescopes working on gravitational microlensing.

After astronomers, Aleksander Wolszczan and Dale Frail announced (also published) the discovery of two planets orbiting the pulsar PSR 1257+12 for the first time in 1992, the search for the exoplanets in the vast universe boosted and continued to today’s astronomical study as well. To date (July 1, 2020) NASA has confirmed the detection of 4,171 exoplanets and 3,092 planetary systems. Among these confirmed exoplanets 76% were detected by using the transit method, 19.3% by using the radial velocity method, 2.3% by using the microlensing, and 1.2% by the direct imaging method.

[0.48% Transit Timing Variations, 0.38% Eclipse Timing Variations, 0.17% Pulsar Timing, 0.14% Orbital Brightness Modulation, 0.05% Pulsation Timing Variations, 0.02% Disk Kinematics, 0.02% Astrometry]

Thanks to the technological revolution that made us able to graze into the universe beyond our home planet- Earth, however not enough and is advancing day-by-day.

We have different direct and indirect exoplanet detection methods and one such (direct) is the gravitational microlensing method where the operation is almost entirely based on the gravitational force of distant objects that is responsible for bending and focusing the light coming from a star.

Gravitational Microlensing is a scaled-down version of gravitational lensing, in brief. Gravitational lensing is the process of bending of light emitted by the stars passes by a massive object.

In this (gravitational microlensing) phenomenon, the gravitational field of a star acts like a lens that magnifies the light of a distant background star and they are effectively visible when the light-emitting star and the massive object almost exactly aligned.

Image shows the optical geometry of a gravitational lens where,

α is the bending angle,

θ_s is the actual angle subtended (without lens effect) by the source at the observer,

θ₁ is the observed angle subtended (due to lens effect) by the apparent source at the observer,

D_L is the angular diameter distance to the lens,

D_S is the angular diameter distance to the source, and

D_LS is the angular diameter distance between the lens and the source.

The small screen in the above moving picture shows a graph of brightness vs time, called a light curve.

Remarkably, in 1704 Isaac Newton first suggested the idea of deflection of light by the force of gravity. In 1801 Johann Georg von Soldner, for the first time, calculated the amount of deflection of a light ray from a star under Newtonian gravity and the idea successively got hyped in 1915 from the Albert Einstein’s theory of General Relativity. 

The bending of light by the force of gravity (gravitational lensing) depends on the strength of gravitational force: strong gravitational force results in the strong bending of light and so the strong gravitational lensing.

Using the method of gravitational lensing in finding the extrasolar planet was first proposed by astronomers Shude Mao and Bohdan Paczynski in 1991 and later the principle was refined in 1992 by Andy Gould and Abraham Loeb.

Gravitational microlensing is most effective when looking for planets towards the center of the galaxy, as the galactic bulge provides a large number of background stars.

When a massive object came across the middle of the observer or the intermediary (in our real case the observer is Earth) and a star system then the light rays from the source star pass on all sides of the intermediary, or “lensing” star, creating what is known as an “Einstein ring.”

Some examples of Gravitational Microlensing Surveys are;

• Optical Gravitational Lensing Experiment (OGLE) at the University of Warsaw. OGLE is a Polish astronomical project based at the University of Warsaw that runs a long-term variability sky survey.

• Led by Andrzej Udalski, the director of the University’s Astronomical Observatory, this international project uses the 1.3 meter “Warsaw” telescope at Las Campanas, Chile, to search for microlensing events in a field of 100 stars around the galactic bulge.

• Probing Lensing Anomalies NETwork (PLANET), which consists of five 1-meter telescopes distributed around the southern hemisphere.

• A project initiated by the Korea Astronomy and Space Science Institute (KASI) in 2009, called Korean Microlensing Telescope Network (KMTNet) which relies on the instruments at three southern observatories and provide 24-hour continuous monitoring of the Galactic bulge, searching for microlensing events that will point the way towards Earth-mass planets orbiting with their stars habitable zones.

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Advantages

1. Gravitational microlensing is capable of finding the most distant and the smallest planets of any currently available method for detecting extrasolar planets. In 2006 astronomers announced the discovery of an exoplanet named OGLE-2005-BLG-390Lb orbiting a star system OGLE-2005-BLG-390L, located at 21,500 ± 3,300 light-years from Earth near the center of the Milky Way. The research work was published in the journal Nature on 26 January 2006.

2. This method is the most sensitive means of detecting low-mass planets in wider orbits that are around 1-10 astronomical units (1 AU = 1.496 10¹¹) away from Sun-like stars which make it complementary method to radial velocity and transit detection methods because radial velocity and transit method are most effective at detecting planets that orbit very close to their star.

3. Gravitational microlensing method searches are huge and can target tens of thousands of planets at once.

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Disadvantages

1. Planets that are detected by using the microlensing method will never be observed and confirmed again by using the same method because microlensing events are unique and do not repeat themselves. So, we may need other methods for confirming the existence of the planet.

2. Gravitational microlensing is all dependent on rare and random events - the passage of one-star precisely in front of another, as seen from Earth, with a planet orbiting the lensing star positioned relatively close by which makes this method both difficult and unpredictable.

3. This method does not yield accurate estimates of a planet’s orbital properties and the only orbital characteristic that this method can determine is the planet’s current semi-major axis.