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Gravitational Microlensing

Gravitational microlensing relies on chance events where from our viewpoint, one star passes in front of another star. The farther star is usually a bright star, and the near one is normally one we couldn't ordinarily see from Earth. When it passes in front of the farther star, however, its gravity causes the light from the farther star to bend and the star is magnified from our point of view. If, during the event, the background star appears to be magnified even more for a short time, that means a planet orbiting the smaller star is increasing the effect of the magnification.

Finding Exoplanets using Microlensing

Gravitational Microlensing diagram. On the left side of the image, the path of a closer star passing in front of a distant star can be seen, and as it passes in front of the distant star, the distant star's light gets bent and multiple versions of the distant star can be seen from Earth. This is gravitational lensing. On the right side of the image is a plot of Time (x-axis) vs Brightness (y-axis). This shows a curved plot that has a peak brightness halfway across at the point when the closer star is directly in front of the distant star.

The distant star's light gets bent by the closer star passing in front. This is gravitational lensing, resulting in multiple versions of the distant star being seen. Depending on the arrangement of stars, two or more versions of the distant star can be seen due to the lensing effect. Image credit: Alice Hopkinson, LCO

A planet orbiting the closer star can enhance the lensing effect, creating a small blip in the time vs brightness plot.

When a planet is orbiting the closer star, this can enhance the lensing effect briefly, creating a small blip in brightness. This is how we can use gravitational microlensing to detect exoplanets. Image credit: Alice Hopkinson, LCO

For more information, please see our Gravitational Microlensing Observing Program

Dr Yiannis Tsapras: How Gravitational Microlensing Is Used To Discover Cold Planets
Einstein predicted that the gravitational field of any massive star will act as a gravitational lens and bend the path followed by the light rays originating from any bright star that happens to pass behind the lens. The effect of lensing at cosmological distances is practically observed as multiple distorted images of the background star around the edge of the gravitational influence of the lensing star. However, lensing also occurs on smaller scales in our galaxy and then the resulting images cannot be individually resolved. We call this phenomenon microlensing. What we see in this case instead, is a brightening of the background star that can last from a few days to several weeks. Then the star fades back to it's normal brightness. If the lensing star hosts a planetary companion, there is a chance that the planet can also act as a mini-lens and thereby reveal its presence.

Microlensing is unique in its capability to rapidly survey the population of cold planets, with a sensitivity to planetary mass that goes down to just below the mass of the Earth. The population of stars that it surveys are low-mass stars, typically M-dwarfs, between here and the centre of the Galaxy. Other methods are capable of detecting planets up to a few hundred light years away but microlensing is the only method that can probe the galactic population of planets. The planets discovered by this method are typically located between 0.6 and 6 AU from the host star, which corresponds to a cold zone that is more conducive to planet formation and which nicely overlaps the colder outer edge of the Habitable Zone. This region of parameter space is still largely inaccessible to other methods.

At least 80 planets have been discovered by this method (as of October 2018). Of these planets, most are Jupiter-analogs, but a few have masses comparable to that of Neptune and below. Theoretical predictions estimate that small, cold planets are abundant and these can be detected by microlensing surveys. Microlensing is also sentitive to multiple planet systems and free-floating planets.